Book - The Elements of Embryology - Chicken 7
The changes which take place during the fourth day
ON opening an egg in the middle or towards the end of the fourth day, a number of points in which progress has been made since the third day are at once apparent. In the first place, the general growth of the embryo has been very rapid, so that its size is very much greater than on the previous day. In the second place, the white of the egg has still further diminished, the embryo lying almost in immediate contact with the shell membrane.
The germinal membrane embraces more than half the yolk, and the vascular area is about as large as a halfpenny.
Corresponding to the increased size of the embryo, there is a great increase in the quantity of blood circulating in the vascular area as a whole, though the sinus terminalis is already less distinct than it was.
The amnion becomes increasingly conspicuous. It is now seen as a distinct covering obscuring to a certain extent the view of the body of the chick beneath, and all traces of the junction of its folds are by this time lost. As yet there is very little fluid in the amniotic sac proper, so that the true amnion lies close upon the embryo.
The folding off of the embryo from the yolk sac has made great progress. The splanchnic stalk, which on the third day was still tolerably wide, inasmuch as about one third of the total length of the alimentary canal was as yet quite open to the yolk sac below, now becomes so much constricted by the progressive closing in of the splanchnopleure folds, that the alimentary canal may be said to be connected with the yolk sac by a very narrow neck only. This remnant of the splanchnic stalk we may now call the vitelline duct; though narrow, it is as yet quite open, affording still free communication between the inside of the yolk sac and the interior of the alimentary canal.
The somatic stalk, though narrowing somewhat, is much wider than the splanchnic stalk, so that a considerable ring-shaped space exists between the two.
Another very prominent feature is the increase in the cranial flexure. During the third day, the axis of the front part of the head was about at right angles to the long axis of the body; the whole embryo being still somewhat retort-shaped. On this day, however, the flexure has so much increased that the angle between the long axis of the body and that of the front segment of the head is an acute one.
The tail-fold, which commenced to be noticeable during the third day, has during this day increased very much, and the somewhat curved tail (Fig. 67) forms quite a conspicuous feature of the embryo. The general
++++++++++++++++++ FIG. 67. EMBRYO AT THE END OF THE FOURTH DAY SEEN AS A TRANSPARENT OBJECT.
The amnion has been completely removed, the cut end of the somatic stalk is shewn at S.S. with the allantois (Al.} protruding from it.
C.ff. cerebral hemisphere. F.B. fore-brain or vesicle of the third ventricle (thalamencephalon) with the pineal gland (Pn.) projecting from its summit. M.B. mid-brain. Cb. cerebellum. 1 V. V. fourth ventricle. L. lens. ch.s. choroid slit. Owing to the growth of the optic cup the two layers of which it is composed cannot any longer be seen from the surface ; the posterior surface of the choroid layer alone is visible. Cen. V. auditory vesicle, s.m. superior maxillary process. IF, %F, etc. first, second, third and fourth visceral folds. V. fifth nerve sending one branch to the eye, the ophthalmic branch, and another to the first visceral arch. VII. seventh nerve passing to the second visceral arch. G. Ph. glossopharyngeal nerve passing towards the third visceral arch. Pg. pneumogastric nerve passing towards the fourth visceral arch. iv. investing mass (basilar plate). No attempt has been made in the figure to indicate the position of the dorsal wall of the throat, which cannot be easily made out in the living embryo, ch. notochord. The front end of this cannot be seen in the living embryo. It does not end however as shewn in the figure, but takes a sudden bend downwards and then terminates in a point. Ht. heart seen through the walls of the chest. M.P. muscle-plates. W. wing. H.L. hind limb. Beneath the hind limb is seen the curved tail.
curvature of the body has also gone on increasing, and as the result of these various flexures, the embryo has become somewhat spirally curled up on itself (Fig. 67).
The distinct appearance of the limbs must be reckoned as one of the most important events of the fourth day.
Owing to the continued greater increase of depth than of breadth, the body of the embryo appears in section (Fig. 68) higher and relatively narrower than even on the third day, and the muscle-plates, instead of simply slanting downwards, come to be nearly vertical in position. Not far from the line which marks their lower ends, the somatopleure, almost immediately after it diverges from the splanchnopleure, is raised up (Fig. 68, TF..R.) into a low rounded ridge which runs along nearly the whole length of the embryo from the neck to the tail.
It is on this ridge, which is known as the Wolffian ridge, that the limbs first appear as flattened conical buds projecting outwards. They seem to be local developments of the ridge, the rest of which becomes less and less prominent as they increase in size.
+++++++++++++++++++++++++ FIG. 68. SECTION THROUGH THE LUMBAR KEGION OF AN EMBRYO AT THE END OF THE FOURTH DAY.
n. c. neural canal, p.r. posterior root of spinal nerve with ganglion, a.r. anterior root of spinal nerve. A.G.C. anterior grey column of spinal cord. A. W.C. anterior white column of spinal cord just commencing to be formed, and not very distinctly marked in the figure, m.p. muscle-plate, ch. notochord. W.R. Wolfnan ridge. A.O. dorsal aorta. V.c.a. posterior cardinal vein. W.d. Wolffian duct. W.b. Wolffian body, consisting of tubules and Malpighian corpuscles. One of the latter is represented on each side. g.e. germinal epithelium, d. alimentary canal. M. commencing mesentery. S.O. somatopleure. 8.P. splanchnopleure. V. blood-vessels, pp. pleuroperitoneal cavity.
Each bud, roughly triangular in section, consists of somewhat dense mesoblast covered by epiblast which on the summit is thickened into a sort of cap. The front limbs or wings (Fig. 67) arise just behind the level of the heart, and the hind limbs in the immediate vicinity of the tail. The first traces of them can be seen towards the end of the third, but they do not become conspicuous till the fourth day, by the end of which the two pairs may be already distinguished by their different shapes. The front limbs are the narrowest and longest, the hind limbs being comparatively short and broad. Both are flattened from above downwards and become more so as their growth continues.
In the head, the vesicles of the cerebral hemispheres are rapidly increasing in size, their growth being enormous as compared with that of the thalamencephalon or vesicle of the third ventricle. The mid-brain is now, as compared to the other parts of the brain, larger than at any other epoch, and an indistinct median furrow on its upper surface indicates its division into two lateral halves. The great increase of the mesoblastic contents of the secondary optic vesicle or involuted retinal cup causes the two eyeballs to project largely from the sides of the head (Fig. 69, Op). The mass of mesoblast which invests all the various parts of the brain, is not only growing rapidly below and at the sides, but is also undergoing developments which result in the formation of the primitive skull. All these events, added to the cranial flexure spoken of above, give to the anterior extremity of the embryo a shape which it becomes more and more easy to recognize as that of a head.
FIG. 69. A. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY VIEWED FROM BELOW AS AN OPAQUE OBJECT. (Chromic
C.H. cerebral hemispheres. F.B. vesicle of the third ventricle or thalamencephalon. Op. eyeball, nf. naso-frontal process. M. cavity of mouth. S.M. superior maxillary process of F. 1, the first visceral fold (mandibular arch). F. 2, F. 3 second and third visceral arches. N. nasal pit.
In order to gain the view here given the neck was cut across between the third and fourth visceral folds. In the section e thus made are seen the alimentary canal al, the neural canal n.c., the notochord ch., the dorsal aorta AO., and the jugular veins V. Ao. bulbus arteriosus.
In the drawing the nasal groove has been rather exaggerated in its upper part. On the other hand the lower and shallower part of the groove where it runs between the superior maxillary process S.M. and the broad naso-frontal process has not been satisfactorily rendered. Hence the end of the superior maxillary process seems to join the inner and not, as described in the text, the outer margin of the nasal groove. A few hours later the separation of the two would have been very visible.
B. The same seen sideways, to shew the visceral folds, ot. otic vesicle. Remaining letters as before.
Meanwhile the face is also being changed. The two nasal pits were on the third day shallow depressions complete all round. As the pits deepen on the fourth day by the growth upwards of a rim round them, a deficiency or break in the ridge may be observed on that side of it turned towards the mouth; which constitutes a kind of shallow groove (Fig. 69 N) directed obliquely downwards towards the cavity of the mouth. The fronto-nasal process or median ridge (Fig. 69, nf), which on the third day rose up between the superficial projections caused by the bulging anterior extremities of the vesicles of the cerebral hemispheres, and on the fourth day becomes increasingly prominent, separates the two grooves from each other, and helps to form the inner wall of each of them, while the depth of the groove also becomes increased by the prolongation along its inner side of the rim surrounding the nasal pit. Abutting on the outer side of each groove near the mouth and so helping to form the outer wall of each, lie the ends of the superior maxillary processes of the first visceral arch (Fig. 69 B, SM), which like the fronto-nasal process are increasing in size. By their continued growth, the groove is more and more deepened, and leading as it does from the nasal pit to the cavity of the mouth, may already be recognized as the rudiment of the passage of the posterior nares.
During the latter half of the fourth day there appears at the bottom of the deep lozenge-shaped cavity of the stomodseum or primitive buccal cavity, in the now thin wall dividing it from the alimentary canal, a longitudinal, or according to Gotte a vertical slit which, soon becoming a wide opening, places the two cavities in complete communication.
The cavity of the mouth, being, it will be remembered, formed partly by depression, partly -by the growth of the surrounding folds, is lined entirely with epiblast, from which the epithelium of its surface and of its various glands is derived. In this respect, as Remak pointed out, it differs from all the rest of the alimentary canal, whose whole epithelium is formed out of hypoblast.
By the side of the hind-brain the cerebellum, through the increased thickening of its upper walls, is becoming more and more distinct from the medulla oblongata; while both in front and behind the auditory vesicle, in which the rudiments of the cochlea and recessus vestibuli are already visible, the cranial ganglia and nerves are acquiring increased distinctness and size. They may be very plainly seen when the head of the fresh embryo is subjected to pressure.
The foremost is the fifth cranial nerve (Fig. 67, F.) with its Gasserian ganglion; it lies a little way behind the anterior extremity of the notochord immediately below the cerebellum. Next to this comes the seventh nerve (Fig. 67, VII.), starting just in front of the earvesicle, and extending far downwards towards the second visceral arch. The two nerves which lie behind the earvesicle are now obviously separate from each other; the front one is the glossopharyngeal (Fig. 67, G.Ph.), and the hinder one already shews itself to be the pneumogastric (Fig. 67, Pg.).
The mesoblastic somites
Which by the continued differentiation of the axial mesoblast at the tail end of the embryo have increased in number from thirty to forty, undergo during this day changes of great importance. Since these changes are intimately connected with the subsequent development of the vertebral column, it will perhaps be more convenient to describe briefly here the whole series of events through which the somites become converted into the permanent structures to which they give rise, though many of the changes do not take place till a much later date than the fourth day.
The separation of the muscle-plates (p. 187) left the remainder of each somite as a somewhat triangular mass lying between the neural canal and notochord on the inside, and the muscle-plate and intermediate cellmass on the outside (Fig. 64). Already on the third day (Fig. 65) the upper angle of this triangle grows upwards, between its muscle-plate and the neural canal, and meeting its fellow in the middle line above, forms a roof of mesoblast over the neural canal, between it and the superficial epiblast. At about the same time, the inner and lower angle of the triangle grows inwards towards the notochord, and passing both below it (between it and the aorta) and above it (between it and the neural canal), meets a similar growth from its fellow somite of the other side, and thus completely invests the notochord with a coat of mesoblast, which, as seen in Fig. 68, is at first much thicker on the under than on the upper side.
Both neural canal and notochord are thus furnished from neck to tail with a complete investment of mesoblast, still marked, however, by the transparent lines indicating the fore and aft limits of the several somites. This mesoblastic investment is sometimes spoken of as the "membranous" vertebral column.
The portions of the somites thus forming the primary vertebrae or membranous vertebral column are converted into the permanent vertebrae; but their conversion is complicated by a remarkable new or secondary segmentation of the whole vertebral column.
On the fourth day, the transparent lines marking the fore and aft limits of the somites are still distinctly visible. On the fifth day, however, they disappear, so that the whole vertebral column becomes fused into a homogeneous mass whose division into vertebrae is only indicated by the series of ganglia. This fusion, which does not extend to the muscle-plates in which the primary lines of division still remain visible, is quickly followed by a fresh segmentation, the resulting segments being the rudiments of the permanent vertebrae. The new segmentation, however, does not follow the lines of the segmentation of the muscle-plates, but is so effected that the centres of the vertebral bodies are opposite the septa between the muscle-plates.
The explanation of this character in the segmentation is not difficult to find. The primary segmentation of the body is that of the muscle-plates, which were present in the primitive forms in which vertebrae had not appeared. As soon however as the notochordal sheath was required to be strong as well as flexible, it necessarily became divided into a series of segments.
The condition under which the lateral muscles can best cause the flexure of the vertebral column is clearly that each muscleplate shall be capable of acting on two vertebrae ; and this condition can only be fulfilled when the muscle-segments are opposite the intervals between the vertebrae. For this reason, when the vertebras became formed, their centres were opposite not the middle of the muscle- plates but the inter-muscular septa.
These considerations fully explain the characters of the secondary segmentation of the vertebral column. On the other hand the primary segmentation of the vertebral rudiments is clearly a remnant of a condition when no vertebral bodies were present ; and has no greater morphological significance than the fact that the cells of the vertebrae were derived from the segmented muscle-plates, and then became fused into a continuous sheath around the notochord and nervous axis ; till finally they became in still higher forms differentiated into vertebrae and their arches.
By these changes this remarkable result is brought about, that each permanent vertebra is formed out of portions of two consecutive mesoblastic somites. Thus, for instance, the tenth permanent vertebra is formed out of the hind portion of the tenth somite, and the front portion of the eleventh somite.
The new segmentation is associated with or rather is caused by histological changes. At the time when the fusion takes place, the mesoblast, which in the form of processes from the somites surrounds and invests the notochord, has not only increased in mass but also has become cartilaginous, so that, as Gegenbaur 1 points out, there is present for a short period on the fifth day a continuous and unsegmented cartilaginous investment of the notochord.
This cartilaginous tube does not however long remain uniform. At a series of points corresponding in number to the original somites it becomes connected with a number of cartilaginous arches which appear in the mesoblastic investment of the neural canal. These arches, which thus roof in the neural canal, are the cartilaginous precursors of the osseous vertebral arches. We further find that the portions of the cartilaginous tube from which the arches spring come to differ histologically from the portions between them not connected with arches : they are clearer and their cells are less closely packed. There is however at this period no distinct segmentation of the cartilaginous tube, but merely a want of uniformity in its composition.
(1 Untersuchung zur vergleichenden Anatomic der Wirbelsaule lei Amphibien und Reptilien, Leipzig, 1862.)
The clearer portions, from which the arches spring, form the bodies of the vertebrae, the segments between them the intervertebral regions of the column.
On the fifth day a division takes place of each of the intervertebral segments into two parts, which respectively attach themselves to the contiguous vertebral regions. A part of each intervertebral region, immediately adjoining the notochord, does not however undergo this division, and afterwards gives rise to the ligamentum suspensorium.
This fresh segmentation is not well marked, if indeed it takes place at all in the sacral region.
To recapitulate: the original somites lying side by side along the notochord, after giving off the muscle plates, grow around, and by fusing together completely invest, with mesoblast, both neural canal and notochord.
This investment, of which by reason of its greater growth the original bodies of the somites now seem to be only an outlying part, becomes cartilaginous in such a way that while the notochord becomes surrounded with a thick tube of cartilage bearing no signs of segmentation, but having the ganglia lodged on its exterior at intervals, the neural canal is covered in with a series of cartilaginous arches, connected to each other by ordinary mesoblastic tissue only, but passing at their bases directly into the cartilaginous tube around the notochord.
By a process of histological differentiation the cartilaginous tube is divided into vertebral and intervertebral portions, the vertebral portions corresponding to the arches over the neural canal. Fresh lines of segmentation then appear in the intervertebral portions, dividing each of them into two parts, of which one attaches itself to the vertebra in front and the other to the vertebra behind.
Meanwhile from the fourth to the sixth day important changes take place in the notochord itself.
On its first appearance the notochord was, as we have seen, composed of somewhat radiately arranged but otherwise perfectly typical mesoblast-cells.
On the third day some of the central cells become vacuolated, while the peripheral cells are still normal. The vacuolated cells exhibit around the vacuole a peripheral layer of granular protoplasm in which the nucleus lies embedded, whilst the vacuoles themselves are filled with a perfectly clear and transparent material, which in an unaltered condition is probably fluid. Towards the end of the day the notochord acquires a delicate structureless sheath which is no doubt a product of its peripheral cells.
On the fourth day all the cells become vacuolated with the exception of a single layer of flattened cells at the periphery. The vacuoles go on enlarging unti on the sixth day the vacuoles in each cell have so much increased at the expense of the protoplasm that only a very thin layer of the latter is left at the circumference of the cell, at one part of which, where there is generally more protoplasm than elsewhere, the starved remains of a nucleus may generally be detected. Thus the whole notochord becomes transformed into a spongy reticulum, the meshes of which correspond to the vacuoles of the cells and the septa to the remains of their cell-walls.
The notochord is on the sixth day at the maximum of its development, the change which it henceforward undergoes being of a retrograde character.
From the seventh day onward, it is at various points encroached upon by its investment. Constrictions are thus produced which first make their appearance in the intervertebral portions of the sacral region. In the cervical region, according to Gegenbaur, the intervertebral portions are not constricted till the ninth day, though in the vertebral portions of the lower cervical vertebrae constrictions are visible as early as the seventh day. By the ninth and tenth days, however, all the intervertebral portions have become distinctly constricted, and at the same time in each vertebral portion there have also appeared two constrictions giving rise to a central and to two terminal enlargements. In the space therefore corresponding to each vertebra and its appropriate intervertebral portion, there are in all three constrictions and three enlargements.
On the twelfth day the ossification of the bodies of the vertebrae commences. The first vertebra to ossify is the second or third cervical, and the ossification gradually extends backwards. It does not commence in the arches till somewhat later. For each arch there are two centres of ossification, one on each side.
The notochord persists for the greater part of foetal life and even into post-foetal life. The larger vertebral portions are often the first completely to vanish. They would seem in many cases at any rate (Gegenbaur) to be converted into cartilage and so form an integral part of the permanent vertebrae. Rudiments of the intervertebral portions of the notochord may long be detected in the ligamenta suspensoria.
"We may remind the reader that in the adult bird we find between each of the vertebrae of a neck and back a cartilaginous disc the meniscus which is pierced in the centre. These discs are thick at the circumference but thin off to a fine edge round the central hole. Owing to the shape of these discs there are left between each pair of vertebrae two cavities, which only communicate through the central aperture of the meniscus. Through this central aperture there passes a band called the 'ligamentum suspensorium,' connecting the two vertebrae.
In the tail the menisci are replaced by bodies known as the 'annuli fibrosi/ which precisely resemble the similarly named bodies in mammals. They differ from the menisci in being attached over their whole surface to the ends of the vertebral bodies, so that the cavities between the menisci and the vertebrae do not exist. They are pierced however by a body corresponding with the ligamentum suspensorium and known as the 1 nucleus pulposus.'
In the intervertebral regions the chorda, soon after the commencement of ossification, entirely disappears. The cartilage around it however becomes converted (in the region of the neck) into the ligamentum suspensorium, which unites the two vertebrae between which it is placed.
In the tail the cartilage becomes the nucleus pulposus, which corresponds exactly to the l ligamentum suspensorium ' of the neck and back.
Shortly after the formation of the ligamentum suspensorium the remaining cartilage of the intervertebral segments is converted into the meniscus between each two vertebrae, and in the tail into the annulus fibrosus. Both are absent in the sacrum.
We shall conclude our account of the mesoblastic somites by describing the changes which take place in the muscle-plates.
In the chick these are somewhat complicated, and have not been fully worked out.
On the third day the muscle-plates end opposite the point where the mesoblast becomes split into somatopleure and splanchnopleure. On the fourth day however (Fig. 68 mp.) they extend a certain distance into the side walls of the body beyond the point of the division into somatopleure and splanchnopleure.
Into what muscles of the trunk they become converted has been somewhat disputed. Some embryologists have stated that they only form the muscles of the back. We have, however, little doubt that all the episkeletal muscles, to use Professor Huxley's term (Vertebrates, p. 46), are their products; a view also adopted by Professors Huxley and Kolliker.
The development of the subvertebral system of muscles (hyposkeletal of Huxley) has not been worked out, but on the whole there is reason to believe that it is derived from the muscle-plates. Kolliker, Huxley and other embryologists believe however that these muscles are independent of the muscle-plates in their origin.
Whether the muscle of the diaphragm is to be placed in the same category as the hyposkeletal muscles has not been made out.
It is probable that the cutaneous muscles of the trunk are derived from the cells given off from the muscle-plates. Kolliker however believes that they have an independent origin.
The limb-muscles, both extrinsic and intrinsic, are in certain fishes (Elasmobranchii), derived from the muscle-plates, and a similar origin has been observed in Lacertilia and Amphibia.
In the Chick and other higher Vertebrata on the other hand the entrance of the muscle-plates into the limbs has not been made out (Kolliker). It seems therefore probable that by an embryological modification, of which instances are so frequent> the cells which give rise to the muscles of the limbs in the higher Vertebrata can no longer be traced into a direct connection with the muscle-plates.
At first, as is clear from their mode of origin, the muscle-plates correspond in number with the protovertebrae, and this condition is permanent in the lower vertebrates, such as fishes, where we find that the lateral muscle is divided by septa into a series of segments corresponding in number with the vertebrae.
Wolffian body or mesonephros
Of all the events of the fourth day, none perhaps are more important than those by which the rudiments of the complex urinary and generative systems are added to the simple Wolffian duct and body, which up to that time are the sole representatives of both systems.
We saw that the duct arose on the second day (pp. 94, 106) as a solid ridge which subsequently became a tube, lying immediately underneath the epiblast above the intermediate cell-mass, close against the upper and outer angles of the somites, and reaching from about opposite to the seventh somite away to the hinder end of the embryo.
At first the duct occupies a position immediately underneath the superficial epiblast, but very soon after its formation the growth of the somites and the changes which take place in the intermediate cell-mass, together with the general folding in of the body, cause it to appear to change its place and travel downwards (p. 193). While the shifting is going on, the cells lining the upper end of the pleuroperitoneal cavity (the kind of bay which, as seen in sections, is formed by the divergence of the somatopleure and splanchnopleure) become columnar, and constitute a distinct epithelium. This epithelium, which is clearly shewn in Fig. 64, and is also indicated in Fig. 65, is often called the germinal epithelium, because some of its cells subsequently take part in the formation of the ovary. Soon after the appearance of the germinal epithelium, the intermediate cell-mass increases in size and begins to grow outwards into the pleuroperitoneal cavity, as a rounded projection which lies with its dorsal surface towards the somatopleure, and its ventral surface towards the splanchnopleure, but is in either case separated from these layers by a narrow chink. The Wolffian duct (Figs. 65, 68, Wd) travels down, and finally before the end of the third day is found in the upper part of this projection, near that face of it which is turned towards the somatopleure.
The tubules of the anterior part of the Wolffian body have by the end of the fourth day almost entirely disappeared; but the tubules of that part of the Wolffian body which is found behind the 16th segment undergo a further development.
Each increases in size especially that portion which proceeds from the Malpighian body and is known as the coiled tubulus (region No. 3, see p. 193). This becomes much elongated and twisted. As a consequence of this increase in size the intermediate cell-mass comes to project more and more into the pleuroperitoneal cavity.
The large size of the hinder part of the Wolffian body as compared with that of the anterior part is due to the presence of the dorsally placed secondary tubules, whose development was mentioned on p. 192. These are more numerous in the posterior than in the anterior part of the Wolffian body. At the hind end of the Wolffian body there are as many as four to each primary tubule.
The tubules, which from their contorted course are in sections (Figs. 68, 71) seen cut at various angles, possess an epithelium which is thicker than that of the Wolffian duct. From this difference it is generally easy to distinguish the sections of the tubules from those of the duct. The glomeruli of the Malpighian bodies are in sections of hardened embryos usually filled with blood-corpuscles.
Towards the hind end of the embryo, the projection of the intermediate cell-mass spoken of above becomes smaller and smaller, and the Wolffian duct is thus brought nearer to the splanchnopleure, and in the region of the hind-gut comes to lie close to the walls of the alimentary canal. On the fourth day, the two ducts meet and open into two horns, into which the side- walls of the recently formed cloaca are at that time produced, one on either side.
As we shall afterwards see, the ducts of the permanent kidneys and Miiller's duct also fall into these two horns of the cloaca.
The Wolffian bodies thus constituted perform the offices of kidneys for the greater part of embryonic life. In the chick they disappear before birth; but in most of the Ichthyopsida they remain for life as the permanent kidneys.
After the establishment of the Wolffian body there is formed in both sexes a duct, which in the female becomes the oviduct, but which in the male is functionless and usually disappears. This duct, in spite of certain peculiarities in its development, is without doubt homologous with the Müllerian duct of the Ichthyopsida.
The first rudiment of the Müllerian duct appears at the end of the fourth day, as three successive open involutions of the peritoneal epithelium, connected together by more or less well-defined ridge-like thickenings of the epithelium. It takes its origin from the layer of thickened peritoneal epithelium situated near the dorsal angle of the body-cavity, close to the Wolffian duct, and some considerable distance behind the front end of the Wolffian duct.
In a slightly later stage the ridges connecting the grooves become partially constricted off from the peritoneal epithelium, and develop a lumen. The condition of the structure at this stage is illustrated by Fig. 70, representing three transverse sections through two grooves, and through the ridge connecting them.
The Müllerian duct may in fact now be described as a short but slightly convoluted duct, opening into the body-cavity by three groove-like apertures, and continued for a short distance behind the last of these.
In an embryo not very much older than the one last described the two posterior apertures vanish and the anterior opening alone remains as the permanent opening of the Müllerian duct.
The position of these openings in relation to the Wolffian body is shewn in Fig. 71, which probably passes through a region between two of the peritoneal openings.
++++++++++++++++++++++++++ FIG. 70 SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH GIVE RISE TO THE ANTERIOR PART OF THE MlTLLERIAN DUCT (PRONEPHROS).
A is the llth section of the series.
gr 2 second groove, gr 3 third groove, r 2 second ridge, wd. Wolffian duct.
++++++++++++++++++++++++++++++ As long as the openings persist, the Mullerian duct consists merely of a very small rudiment, continuous with the hindermost of them, and its solid extremity appears to unite with the walls of the Wolffian duct.
After the closure of the two hinder openings the Müllerian duct commences to grow rapidly backwards, and for the first part of its subsequent course it appears to be split off as a solid rod from the outer or ventral wall of the Wolffian duct (Fig. 72). Into this rod the lumen, present in its front part, subsequently extends. Its mode of development in front is thus precisely similar to that of the Müllerian duct in Elasmobranchii and Amphibia.
This mode of development only occurs however in the anterior part of the duct. In the posterior part of its course its growing point lies in a bay formed by the outer wall of the Wolffian duct, but does not become definitely attached to that duct. It seems however possible that, although not actually split off from the walls of the Wolffian duct, it may grow backwards from cells derived from that duct.
FIG. 71. SECTION OF THE INTERMEDIATE CELL-MASS ON THE FOURTH DAY. (From Waldeyer.) Magnified 160 times.
m. mesentery. L. somatopleure. a', portion of the germinal epithelium from which the involution to form the duct of Muller (z) takes place, a. thickened portion of the germinal epithelium in which the primitive ova O and o are lying. E. modified mesoblast which will form the stroma of the ovary. WK. Wolman body. y. Wolman duct. +++++++++++++++++++++
++++++++++++++++ FIG. 72. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID PORTION OF THE MULLERIAN DUCT WITH THE WOLFFIAN DUCT.
In A the terminal portion of the duct is quite distinct ; in B it has united with the walls of the Wolffian duct.
md. Müllerian duct. Wd. Wolffian duct.
The Müllerian duct finally reaches the cloaca though it does not in the female for a long time open into it, and in the male never does so.
The anterior part of the commencing Müllerian duct with its three openings into the body-cavity is probably homologous with the head kidney or pronephros of the Ichthyopsida.
Permanent kidney or metanephros
Between the 80th and 100th hour of incubation, the permanent kidneys begin to make their appearance, and as is the case with the Wolffian bodies, the first portion of them to appear is their duct. Near its posterior extremity the Wolffian duct becomes expanded, and from the expanded portion a diverticulum is constricted off which in a transverse section lies dorsal to the original duct, and the blind end of which points forwards, that is, towards the head of the chick. This is the duct of the permanent kidney or ureter. At first the ureter and the Wolffian duct open by a common trunk into the cloaca, but this state of things lasts for a short time only, and by the sixth day the two ducts have independent openings.
The ureter thus beginning as an outgrowth from the Wolffian duct grows forwards, and extends along the outer side of a mass of mesoblastic tissue which lies mainly behind, but somewhat overlaps the dorsal aspect of, the Wolffian body.
This mass of mesoblastic cells may be called the metanephric blastema. It is derived from the intermediate cell-mass of the region reaching from about the thirty-first to the thirty-fourth somite. It is at first continuous with, and indistinguishable in structure from, the portion of the intermediate cell-mass of the region immediately in front of it, which breaks up into Wolffian tubules. The metanephric blastema remains however quite passive during the formation of the Wolffian tubules in the adjoining blastema; and on the formation of the ureter breaks off from the Wolffian body in front, and, growing forwards and dorsalwards, becomes connected with the inner side of the ureter in the position just described.
In the subsequent development of the kidney collecting tubes grow out from the ureter, and become continuous with masses of cells of the metanephric blastema, which then differentiate themselves into the kidney tubules.
The formation of the kidneys takes place before the end of the seventh day, but they do not become of functional importance till considerably later.
From their mode of development it clearly follows that the permanent kidneys are merely parts of the same system as the Wolffian bodies, and that their separation from these is an occurrence of a purely secondary importance.
The generative ridge
Before describing the subsequent fate of the Wolffian and Müllerian ducts, it will be necessary to give an account of the formation of the true sexual glands, the ovaries and testes.
At the first appearance of the projection from the intermediate cell-mass, which we may now call the genital ridge, a columnar character is not only visible in the layer of cells covering the nascent ridge itself along its whole length, but may also be traced for some little distance outwards on either side of the ridge in the cells lining the most median portions of both somatopleure and splanchnopleure. Passing outwards along these layers, the columnar cells gradually give place to a flat tesselated epithelium. As the ridge continues to increase and project, the columnar character becomes more and more restricted to cells covering the ridge itself, over which at the same time it becomes more distinct. On the outer side of the ridge, that is on the side which looks towards the somatopleure, the epithelium undergoes, as we have seen, an involution to form the commencement of the duct of Muller, and for some little time retains in the immediate neighbourhood of that duct its columnar character (Fig. 71, a'), though eventually losing it.
The median portion of the ridge is occupied by the projection of the Wolffian body, and here the epithelium rapidly becomes flattened.
On the inside of the ridge however, that is on the side looking towards the splanchnopleure, the epithelium not only retains its columnar character, but grows several cells deep (Fig. 71, a), while at the same time the mesoblast (E) underlying it becomes thickened. In this way, owing partly to the increasing thickness of the epithelium, and partly to the accumulation of mesoblast beneath it, a slight eminence is formed, which when viewed from below, after opening the abdominal cavity, appears in direct light as a fusiform white patch or streak, in its early stages extending along the whole length of the Wolffian body and genital ridge, but subsequently restricted to its anterior portion. Its appearance under these circumstances has been well described by Von Baer.
This 'sexual eminence' is present in the early stages of both sexes. In both the epithelium consists of several layers of short cylindrical cells, a few of which are conspicuous on account of their size and their possessing a highly refractive oval nucleus of considerable bulk; in both, the underlying thickened mesoblast consists as indeed at this epoch it does generally in all parts of the body of spindle-shaped cells.
The larger conspicuous cells of the epithelium which appear to have quite a common origin with their fellow cells and to arise from them by direct differentiation, and which are seen at the first in male as well as female embryos, are the primordial ova or primitive germinal cells (Fig. 71, o). Thus in quite early stages it is impossible to detect the one sex from the other.
In the female the primordial ova enlarge and become more numerous, the whole epithelium growing thicker and more prominent, and the spindleshaped cells of the underlying mesoblast also increase rapidly and thus form the stroma of the ovary. The primordial ova after undergoing some further changes, into which it is not within the scope of this work to enter, become surrounded by a number of the ordinary epithelium cells. These form a distinct layer, the follicular epithelium, round the ovum. After a time there appear numerous vascular ingrowths from the stroma, which penetrate through all parts of the germinal epithelium and break it up into a sponge-like structure formed of trabeculse of germinal epithelium interpenetrated by vascular strands of stroma. The trabeculae of the germinal epithelium form the egg-tubes of Pfliiger.
In this way each ovum becomes invested by a capsule of vascular connective tissue lined internally by a layer of epithelium; the whole constituting a Graafian follicle.
The large nucleus of the primordial ovum becomes the germinal vesicle, while the ovum itself remains as the true ovum ; this subsequently becomes enlarged by the addition of a quantity of yolk derived from a differentiation of its protoplasm.
The first traces of the testes are found in the dorsal and inner side of the intermediate cellmass, and appear about the sixth day. From the first they differ from the rudimentary ovaries, by coming into somewhat close connection with the Wolffian bodies; but occupy about the same limits from before backwards. The mesoblast in the position we have mentioned begins to become somewhat modified, and by the eighth day the testis is divided by septa of connective tissue into a number of groups of cells; which are the commencing tubuli seminiferi. By the sixteenth day the cells of the tubuli have become larger and acquired a distinctly epithelial character.
The history of the primordial cells in the male has not been so thoroughly worked out as in the female. The spermatozoa appear to arise by the division of the primitive ova (present, as we have stated, in the early stages of both sexes), which probably migrate into the adjacent stroma, accompanied by some of the indifferent epithelial cells. Here the primitive germinal cells increase in number and give rise to the cells lining the secretory tubules of the testes.
Outgrowths from the Malpighian bodies of the Wolffian body appear to be developed, which extend into the testis and come into connection with the true seminiferous stroma.
It is evident from the above account that the male and female generative products are homodynamous, but the consideration of the development of the products in the two sexes shows that a single spermatozoon is not equivalent to an ovum, but rather that the whole of the spermatozoa derived from a primordial ovum are together equivalent to one ovum.
We have now described the origin of all the parts which form the urinary and sexual systems, both of the embryo and adult. It merely remains to speak briefly of the changes, which on the attainment of the adult condition take place in the parts described.
The Wolffian body, according to Waldeyer, may be said to consist of a sexual and urinary part, which can, he states, be easily distinguished in the just-hatched chick. The sexual part becomes in the cock the aftertestes or coni vasculosi, and consists of tubules which lose themselves in the seminiferous tubules. In the hen it forms part of the epoophoron 1 of Waldeyer, and is composed of well-developed tubes without pigment. The urinary part forms in both sexes a small rudiment, consisting of blindly ending tubes with yellow pigment ; it is most conspicuous in the hen. This rudiment has been called by Waldeyer parepididymis in the male and paroophoron in the female.
The Wolffian duct remains as the vas deferens in the male. In the female it becomes atrophied and nearly disappears.
The duct of Miiller on the right side (that on the left side with the corresponding ovary generally disappearing) remains in the female as the oviduct. In the male it is almost entirely obliterated on both sides.
We may return to the changes which are taking place in the circulation.
On the fourth day, the point at which the dorsal aorta divides into two branches is carried much further back towards the tail.
A short way beyond the point of bifurcation, each vessel gives off a branch to the newly-formed allantois.
(1 This is also called parovarium (His), and Bosenmiiller's organ.)
It is not, however, till the second half of the fourth day, when the allantois grows rapidly, that these allantoic y or, as they are sometimes called, umbilical, arteries acquire any importance, if indeed they are present before.
The vitelline arteries are before the end of the day given off from the undivided aortic trunk as a single but quickly bifurcating vessel, the left of the two branches into which it divides being much larger than the right.
During this day, the arterial arch running in the first visceral fold becomes obliterated, the obliteration being accompanied by the appearance of a new (fourth) arch running in the fourth visceral fold on either side.
The second pair of arterial arches also becomes nearly (if not entirely) obliterated; but a new pair of arches is developed in the last (fifth) visceral fold, behind the last visceral cleft; so that there are still three pairs of arterial arches, which however now run in the third, fourth and fifth visceral folds, the last of these being as yet small. If we reckon in the slight remains of the second pair of arches we may consider that there are in all four pairs of arches. When the first and second arches are obliterated, it is only the central portion of each arch on either side which absolutely disappears. The ventral portion connected with the bulbus arteriosus, and the dorsal portion which joins the dorsal aorta, both remain, and are both carried straight forward towards the head. The ventral portions of both first and second arches unite on each side to form a branch, the external carotid (Fig. 73, E.C.A.), which runs straight up from the bulbus arteriosus to the head.
+++++++++++++++++++++++ FIG. 73. STATE OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH DAY.
E.G. A. external carotid. I.C.A. internal carotid. D. A. dorsal aorta. Of. A. vitelline artery. U.A. allantoic arteries.
In the same way the dorsal portions form a branch, the internal carotid, which takes its origin from the dorsal or far end of the third arch.
In the venous system important changes also occur.
As the liver in the course of its formation wraps round the common trunk of the vitelline veins, or meatus venosus, it may be said to divide that vessel into two parts : into a part nearer the heart which is called the sinus venosus (Fig. 74, S. V.), and into a part surrounded by the liver which is called the ductus venosus. Beyond, i.e. behind the liver, the ductus venosus is directly continuous with the vitelline veins, or, as we may now say, vein, for the right trunk has become so small as to appear a mere branch of the left (Fig. 74, 0/0.
+++++++++++++++++++++++ FIG. 74. DIAGRAM OF THE VENOUS CIRCULATION AT THE COMMENCEMENT OF THE FIFTH DAY.
H. heart, d.c. ductus CuvierL Into the ductus Cuvieri of each side fall J. the jugular vein or superior cardinal vein,
W. the vein from the wing, and c. the inferior cardinal vein. &. V. sinus venosus. Of. vitelline vein. U. allantoic vein, which at this stage gives off branches to the body-walls.
V.C.I, vena cava inferior. L liver.
The hepatic circulation, which was commenced on the third day, becomes completely established. Those branches which come off from the ductus venosus soon after its entrance between the liver lobes carry blood into the substance of the liver and are called vence advehentes, while those which join the ductus venosus shortly before it leaves the liver (i. e. nearer the heart) carry blood away from the hepatic substance into the ductus and are called vence revehentes. As a result of this arrangement there is a choice of paths for the blood in passing from the vitelline vein to the sinus venosus; it may pass through the capillary net- work of the liver, going in by the venae advehentes and coming back again by the vense revehentes, or it may go straight through the ductus venosus without passing at all into the substance of the liver.
As the alimentary canal by its continued closing in becomes on the fourth day more and more distinct from the yolk-sac, it gradually acquires veins of its own, the mesenteric veins, which first appear as small branches of the vitelline vein, though eventually, owing to the change in the relative size and importance of the yolksac and intestine, the latter seems to be a branch of one of the former.
Corresponding to the increase in the size of the head, the superior cardinal veins (Fig. 74, J.) become larger and more important and are joined by the wing veins (Tf.). As before, they form the ductus Cuvieri (d.c.) by joining with the inferior cardinal veins (c.).
The latter are now largely developed, they seem to take origin from the Wolffian bodies, and their size and importance is in direct proportion to the prominence of these bodies. They might be called the veins of the Wolffian bodies.
As the kidneys begin to be formed a new single median vein makes its appearance, running from them forwards, beneath the vertebral column, to fall into the sinus venosus (Fig. 74, V.C.I.). This is the vena cava inferior.
As the lungs are being formed the pulmonary veins also make their appearance and become connected with the left side of the auricular division of the heart.
The blood carried to the allantois by the allantoic arteries is brought back by two veins, which very soon after their appearance unite, close to the allantois, into a single trunk, the allantoic vein, which, running along the splanchnopleure, falls into the vitelline vein (Fig. 74, K).
Meanwhile the heart is undergoing considerable changes. Though the whole organ still exhibits a marked curvature to the right, the ventricular portion becomes directed more distinctly ventralwards, forming a blunted cone whose apex will eventually become the apex of the adult heart.
The concave (or dorsal) walls of the ventricles become much thicker, as did the convex or ventral walls on the third day.
Well-marked constrictions now separate the ventricles from the bulbus arteriosus on the one hand, and from the auricles on the other. The latter constriction is very distinct, and receives the name of canalis auricularis (Fig. 75, C. A.) ; the former, sometimes called the /return Halleri, is far less conspicuous.
++++++++++++++++++++++++++ FIG. 75. HEART OF A CHICK ON THE FOURTH DAY OF INCUBATION VIEWED FROM THE VENTRAL SURFACE.
La. left auricular appendage. C.A. canalis auricularis. v. ventricle. 6. bulbus arteriosus.
The most important event is perhaps the formation of the ventricular septum. This, which commenced on the third day as a crescentric ridge or fold springing from the convex or ventral side of the rounded ventricular portion of the heart, now grows rapidly across the ventricular cavity towards the concave or dorsal side. It thus forms an incomplete longitudinal partition, extending from the canalis auricularis to the commencement of the bulbus arteriosus, and dividing the twisted ventricular tube into two somewhat curved canals, one more to the left and above, the other to the right and below. These communicate freely with each other, above the free edge of the partition, along its whole length.
Externally the ventricular portion as yet shews no sign of the division into two parts.
The bulbus arteriosus (Fig. 75, 6.) has increased in size, and is now very conspicuous.
The venous end of the heart is placed still more dorsal, and to the left of the arterial end; its walls are beginning to become thicker.
The auricles are nearly if not quite as far forward as the ventricles, and the auricular appendages (Fig. 75, La.), which were visible even on the third day, are exceedingly prominent, giving a strongly marked external appearance of a division of the auricular portion of the heart into two chambers; but Von Baer was unable to detect at this date any internal auricular septum.
Summary Day 4
The chief events then of the fourth day are :
(1) The increase of the cranial and body flexure.
(2) The increase in the tail-fold.
(3) The formation of the limbs as local thickenings of the Wolffian ridge.
(4) The formation of the olfactory grooves.
(5) The absorption of the partition between the mouth and the throat.
(6) The vacuolation of the cells of the notochord.
(7) The formation of the ureter.
(8) The formation of the duct of Muller.
(9) The appearance of the primitive ova in the germinal epithelium.
(10) The development of a fifth pair of arterial arches, and the obliteration of the first, and partial obliteration of the second pair.
(11) The development of the 'canalis auricularis,' the growth of the septum of the ventricles and of the auricular appendages.
The Elements of Embryology - Volume 1 (1883)
The History of the Chick: Egg structure and incubation beginning | Summary whole incubation | First day | Second day - first half | Second day - second half | Third day | Fourth day | Fifth day | Sixth day to incubation end | Appendix
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