Book - Vertebrate Zoology 12 1922
Embryology - 13 May 2024    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
Beer G. Vertebrate Zoology. (1922)
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
XII The development of Rana (the Frog)
Fertilisation
The egg contains a large quantity of yolk, which is aggregated at the vegetative pole. This pole is light in colour when seen from outside, whereas the opposite animal pole, and indeed the whole animal hemisphere, is darkly pigmented. The nucleus is near the animal pole, which is determined in the ovary, probably by the orientation of the developing egg to the little arteries and veins.
The egg is surrounded by three membranes. The inner vitelline membrane is secreted by the egg itself. Outside this is a tough membrane formed by the follicle-cells which surround the egg in the ovary. Outside this again is a coating layer of jelly which is secreted by the glands of the wall of the oviduct, as the egg passes down the latter on its way to the exterior.
At the time of spawning, the males climb on to the backs of the females, and as the latter extrude the eggs from their cloacal apertures, the former shed the sperm over them. Fertilisation thus takes place in the water outside the bodies of the animals. One polar body has been extruded before the egg is laid, the second polar body is pushed out after penetration of the sperm, and the egg- and spermpronuelei then fuse.
The jelly swells out on Contact with the water, and after fertilisation the vitelline membrane becomes lifted otl' from the surface of the egg. The egg is then able to rotate, and comes to rest with the axis vertical, i.e. the vegetative pole with the heavy yolk is turned downwards.
The point of entrance of the sperm determines the median plane of symmetry of the future embryo, and, soon after fertilisation, this is indicated by the formation of the grey crescent (due to the retreat of pigment into the egg) at the point diametrically opposite to that at which the sperm entered. The egg can now be orientated with regard to the axes of the future embryo. The animal pole will become the head. and the vegetative pole the tail; the grey crescent marks the future dorsal side, and the opposite side (where the sperm entered) will be ventral.
Cleavage
Cleavage in the frog‘s egg is total, but the size of the various blastomeres is very unequal, owing to the large quantity of yolk. The cells at the vegetative pole are much larger (and fewer in number) than those at the animal pole. The blastoecel is small, and situated nearer to the animal than to the vegetative pole. The blastula is now a hollow ball, but the hollow is small and its walls are several layers thick.
Although the distinctions between them are still invisible, the demarcations between the zones which will give rise to the dififerent parts of the embryo can be projected on to the blastula as a sort of map of the presumptive fates of the different zones. This means that the materials for the whole embryo are already present at the blastula stage and require transloeation, stretching and other movements in order to reach their definitive positions. These movements constitute the processes of gastrulation and of the formation of the neurula.
Figure 73. Egg of Rana lemporaria (common frog) before and after fertilisation, showing the formation of the grey crescent. (From Jenkinson.)
A and B seen from the side; C and D seen from below; A and C before and B and D after fertilisation. The animal hemisphere is pigmented, the vegetative hemisphere is light in colour.
Gastrulation
The cells of the animal hemisphere (which are darkly pigmented) are relatively free from yolk, and therefore divide faster than the larger light-coloured yolk-laden cells of the vegetative hemisphere. One result of this is that the animal-pole cells begin to grow down over the lighter-coloured cells. This process starts by the formation of a up of overgrowth in the centre of the grey_ crescent, forming the dorsal lip of the hlastopore. Underneath this hp 1s_a groove formed by the cells tucking tn, and once they have tucked tn
Figure 74. Formation and closure of the blustopore during gastrulation m Rana, seen from below. (From Jenkinson.)
In A the dorsal lip of the blnstopore has just appeared; in B the lateral lips hnve extended, and they almost meet In C; in I) the xentral lip of the blastopore (which is now at complete circle) has been formed; and the diameter of the blustopure decreases in E and F.
the cells move towards the animal pole on the inner surface of what is now the outer layer. The lips of the blastopore extend right and left from the site of its first appearance. At the same time the edge of overgrowth moves down towards the vegetative pole, and more Figure 75. The process of gastrulation in Rana as shown by sagittal sections. (Partly after Spemann).
_ In A the dorsal lip of the blastopore has just appeared, and in B a definite lpgrowth is visible resulting in the formation of the archenteron; in C the ventral lip of the blastopore has appeared, and the yolk-containing cells of the vegetative hemisphere project through the now circular blastoporc as the yolk-plug; in D the archenteron has extended greatly at the expense of the blastocoal, which is almost obliterated. E15 a transverse section showing how the wall of the archeoteron forms the notochord, mesoderm and endoderm ; F, longitudinal section through a neurula.
a, archenteron; ap, animal pole; b, blastoccel; bl, blastopore; br, brain; dl, dorsal lip of blastopore; m, mesoderm; n, notochord; nee, neurenteric canal; rag, neuropore; re,‘ spinal cord; vl, ventral lip of blastopore; yc, yolk-cells; yp, yol plus-‘
and more of the lighter-coloured cells become covered over by the overgrowing darker ones. Eventually the two horns of the lip of the blastopore meet on the ventral side, and the blastopore is then a closed ring, formed by overgrowing dark cells, and beneath which the tucking-in takes place. This tucking-in is most active on the dorsal side. The groove sinks deeper and deeper into the embryo, as the ingrowin g cells push farther and farther towards the animal pole beneath the superficial layer. The groove represents the cavity of the arehenteron, largely filled up by the yolk-cells of the vegetative pole, which are visible inside the rim of the blastopore. The cavity of the
neural crcst\ 3““T‘fil_ n.euralt1.Lbe
lateral plate-— mesoclerm
Wgmmvc site of dorsal
pole tip of blastoplmre
Figure 75a. Presumptive fates of regions in the amphibian egg. (After W. Vogt, R. (3. Harrison, and P. Ford.)
blastoccel becomes reduced and obliterated as the cavity of the arehenteron increases and gastrulation proceeds; and the yolk-laden cells of the vegetative pole come to lie on the ventral side of the archenteron.
As the blastopore approaches the vegetative pole its diameter decreases. until. when it reaches it. it is a small spherical hole with yolk-cells showing through as the so-called yolk-plug.
The processes of gastrula tion thercf ore entail overgrowth or epiboly, and invagination: and take the form of mass-movements of zones of cells. The invagination cannot take place simply, as in Amphioxus, owing go the large quantity of yolk present, and it is more in the naturcfif an ingrowth. At all events, the result of gastrulation is the same: the conversion of the single-layered hollow ball (blastula) into a doulfie-layered sac (gastrula); the outer layer (ectoderm) is formed of the cells of the animal hemisphere and those which have grown over; the inner layer (future endoderm and mesodenn) is formed of the cells which have grown in, and of the yolk-laden cells of the vegetative hemisphere. The latter form most of the ventral and the former most of the dorsal wall of the archenteron. The heaping up of the heavy yolk—cells at the ventral side causes the gastrula to rotate within its membranes, so that the former egg-axis lies more
Figure 76. Transverse sections through the closed blastopore of Rana (A) and the primitive streak of Gallus (B).
The groove between the fused lips of the blastopore of Rana is the remnant
of the blastopore, and corresponds to the primitive groove (p_.r) of Galfus. ec, ectoderm; en, endoderm; m, mesoderm; all of which are continuous with one
another at the rim of the blastoporc or primitive streak.
or less horizontal instead of vertical; the ventral side now points
downwards and the dorsal side upwards. Thus one of the results of gastrulatioii is the marshalling into position of the layers, the so-called germ-layers, out of which the
various organs are formed.
Mesoderm and Notochord
The wall of the archenteron contains the cells which are destined to give rise to the notochord and to the mesoderm as well as to the endoderm. A strip of cells running along the middle line of the roof of the archenteron is the rudiment of the notochord, on both sides of which are the sheets of mesoderm. The mesoderm gives the appearance of splitting off
Figure 77. Transverse section through an embryo of Rana showing the distinction between the mesoderm (m) and the endodermal wall (en) of the gut (g). cc, ectoderm ; y. yolk-cells. Dorsally the ectoderm is thickening to form the neural folds (nf).
Figure 78. Transverse section through an embryo of Rana slightly "blder than the previous, showing the origin of the notochord (n) from the middle line of the roof of the gut. The neural folds (m have risen up and enclose a groove between them. .oavnt.omaN'r or RAM 151 (by delamination) from the endoderm which. forms the remainder of the wall of the archenteron. But, in fact, the rudiment of the mesoderm is distinct from that of the endoderm, even in the blastula. It is only because the mesodcrm passes inwards over the rim of the blastopore at the same time as the endoderm becomes invaginated that it overlies and appears to arise from the latter.
In fact, the notochord-and-mesoderm and the endoderm form two cups, their openings facing each other, the former lying dorsally to or above the latter. Each cup then completes itself into a sphere.
Figure 79. Transverse section through an embryo of Rana slightly older than the previous, showing the complete separation of the notochord (n) from the endodermal wall (en) of the gut (g).
The coelom (c) has arisen as a split in the mesodcrm (m), which forms a somite (ms) on each side of the notochord. The neural folds (nf) have closed over the groove converting it into the nerve-tube (nc), on each side of which are the neural crests (ncr). ec, ectodcrm; y, yolk cells.
The dorsal edges of the endoderm grow upwards and inwards, beneath the mesodcrm and notochord, and fuse in the mid-dorsal line to form the roof of the gut-cavity. At the same time the ventral edges of the sheets of mesodcrm grow downwards, between the ectoderm and cndodcrm, and eventually meet in the midventral line. A split arises within the mesodcrm itself, dividing it into an inner splanchnic layer which covers the cndodcrm, and an outer somatic layer which lines the cctoderm. This split is the caelomic cavity. NEURULA.—AS a result of the inducing action of the underlying notochord and mesodcrm, the ectoderm along the dorsal side thickens to form the neural plate. Longitudinal ridges arise on both sides of it, called neural folds, and they enclose a groove between them. This groove is wider in front, where the brain will be, than behind in the region of the future spinal cord. Posteriorly the neural folds embrace the blastopore. As the neural folds rise up they arch over the groove which becomes converted into a tube of which the anterior four-fifths becomes the definitive nerve-tube. The posterior one-fifth of the neural folds gives rise to the mesodermal somites of the tail. During this time the embryo becomes elongated by stretching
Figure 80. Sagtttal section through an embryo of Rana.
a, anus; 1». brain: 3'. gut. Ir, hypophysis; in, heart; 1, liver; n, notochord; sc. nervc- (spinal) cord; _v, ytlllx-CIt.‘ll$.
and is now known as a neurula. A continuation of this process of stretching on behalf of the ncrxe-tube and the notochord leads to the projection backwards and upwards of the tail; the tip of the tail corresponding to the boundary between the anterior four-fifths and the posterior one-fifth of the neural folds. It used to be thought that there was a “tail-bud“ which produced the material for the tail. This is now known to be not the case; the materials of which the tail is composed are all present in the neurula and get into place by stretching.
The lateral part of the thickening which gave rise to the neural plate does not get folded into the nerve-tubeiwhen the neural folds meet. It lies just to the side of the point of fusion of the neural folds. and forms the neural crest. The cells of the neural crest have the most diverse of fates. They migrate nearly all over the body; some of them are destined to give rise to the afferent sensory nerve-cells, whose cell-bodies form the ganglia on the dorsal roots of the nerves; others give rise to the sheath-cells on the nerves; others again produce pigment-eells, mesenchyme cells in the gills, and, most remarkable of all, odontoblasts for the teeth, chondroblasts for the cartilages of the visceral arch skeleton, and osteoblasts for some of the dermal bones of the jaws. The neural crest cells from which this strange assortment of different structures arise, are collectively known as ectomesenchyme, to distinguish them from mesenchyme cells, which are of mesodermal origin.
Segmentation
The mesoderm on each side of the nervetube and notochord becomes thickened and divided into blocks, which are the somites from which the myotomes develop; they are metamerically segmented. This segmentation begins anteriorly and proceeds backwards; but it does not aifect the more ventrally situated mesoderm. Whereas the dorsal portion of the coclomie cavity (on a level with the myotomes) is interrupted by transverse septa separating the mesodermal somites from the somites in front and behind, and consists of a number of myocoels equal to the number of somites, the ventral portion of the ccnlomic cavity is continuous and uninterrupted by septa.
The segmented region of the mesoderm is called the vertebral plate, the unsegmented portion is the lateral plate. Between each somite and the lateral plate immediately below it is a small region of segmented mesoderm known as the intermediate cell-mass, or nephrotome. From these structures the tubules of the kidneys will arise, and they are therefore also segmental. Eventually, the vertebral
plate separates completely from the lateral plate, and the myotomes ;
grow down in the body-wall lateral to the splanchnocarl to give rise to the muscles of the ventral surface, of the limbs, and the hypoglossal musculature beneath the mouth.
Muscles formed from myotomes are always innervated by ventral nerve-roots, and as the myotomes are segmental, the ventral nerve‘roots which grow freely out from the nerve-tube are segmental also.
Further, the neural crest becomes subdivided into pieces corresponding to the myotomes; these are the rudiments of the dorsalroot ganglia. The cells in these ganglia develop one process which grows into the nerve-tube, and another which pushes out to its destination in the body. These dorsal nerve-roots are therefore segmental
, also.
w Medially to the myotomes, cells are proliferated by the somites
‘to form clouds of mesenchyme surrounding the nerve-tube and notochord. These cells are the sclerotomes (likewise segmental) from which later on the vertebrae are developed.
1 Another instance of segmentation will be seen in connexion with blood-vessels, which run transversely in the septa between adjacent segments. Although in the adult animal much of this segmentation
Figure 8!. Transverse section Figure 82. Transverse section through a young tadpole larva of through a tadpole larva of Rana R_rma showing the origin of the older than the previous, showing k|dfl<3YS- the formation of the kidneys and the lungs.
The section passes through the transverse septum across which the ductus Cuvieri lead from the cardinal veins to the heart. c, co;-lom dorsal to the transverse septum; er. cardinal vein: dC‘, duetus Cuvreri; g, gut; gl glottis' icm
intemtediate eell—mass or ncphrotome; I. lung; Ida, lateral dorsal a’orta; Ir ’liver:
m, mesoderm; my, myotom_e; n. notoehord: nc, nerve—cord; ncr, neurafcresti nl, nephrococl; pr, proncphne tubules; sp, splanehnocoel; y, yolk-cells.
is obscured and modified, it is important to note that in developmeng ' metamerie segmentation is as well marked as in Amp/rioxus, except for the splanehnocazl. As in invertebrates, segmentation begins with the rnesoderm and extends to the other tissues.
The Gut
The gut is a catity with an accumulation of yolkcells in the hinder part of its floor. This posterior region will become the intestine, and in front of it will develop the pharynx, (esophagus, ” and stomach. After the blastopore has closed, the anus breaks Dsvanormaxr or RANA ‘ I y i 155 through near the same’ spot, as a result of the sinking in of an5ect‘odermal pit (the proetodaeum) till it meets the endoderm, and perforation ensuing. In a similar way, the mouth perforates in front, at the bottom of an ectodermal pit (the stomodreum).
Behind the mouth, in the region of what will be the pharynx, five pouches grow out on each side from the endoderm to the ectoderm. These -are the rudiments of the visceral clefts. The first pair corresponds to the spiracles of the dogfish, but here they do not become perforated to the exterior. Their cavities persist as the Eustachian tubes. The remaining four pairs of pouches become the gill-slits, through which the pharynx communicates with the exterior.
Alternating with the visceral clefts are the visceral arches. The 1st or mandibular arch separates the mouth from the Eustachian tube (or hyomandibular cleft); the 2nd (or hyoid arch) is between the latter and the 1st gill-slit. The 6th visceral arch is behind the 4th gill-slit.
From the upper part of the 3rd, 4th, and 5th visceral arches, tufts grow out on each side which will become the external gills; bloodvessels enter them, and they serve as the first respiratory organs. The dorsal part of the 1st gill-pouch on each side proliferates to form a body which is the rudiment of the thymus gland.
In the floor of the pharynx between the 2nd gill-slits, a downgrowth is formed, which ultimately loses its connexion with the pharynx and forms the thyroid gland. Close to the point of origin of the throid gland is an elevation which will eventually give rise to the tongue. A little farther back, also in the middle line of the floor of the pharynx, the rudiment of the larynx appears as a groove. This deepens into a tube remaining in connexion with the pharynx through the glottis. From the posterior end of the larynx, the lungs develop as sacs stretching back parallel to the orsophagus on each side.
The liver arises as a ventral outgrowth of the floor of the gut, just in front of the mass of yolk-cells, and extending back beneath them. Part of the cavity of this diverticulum becomes the gall-bladder, and the open connexion with the rest of the gut persists as the bile-duct. Close to this point, the pancreas arises as a number (three) of outgrowths, which remain connected with the gut by the pancreatic duct.
The cavity of the intestine is still small owing to the presence of the yolk-cells. After hatching, this yolk becomes absorbed and the intestine elongates very much, becoming coiled like a watchspring. Behind the intestine is the region of the gut which will become the rectum and cloaca. A downgrowth from the latter gives rise to the urinary bladder.
During this time, the right and left splanchnocoelic cavities have applied their outer (or somatic) layer to the body-wall, and their inner (or splanehnic) layer to the endoderm of the gut and al1.its « derivatives. Ventrally, most of the membranes formmg the separation
Figure 83. ”()|"'l70lll‘.ll‘ScCll0n throu_uh the head of a tadpole of Rana, showing the lurmutmn of" the visceral clefts (gill-slits).
b l. h 3. b 4. b 5. 1: ts. blood-vessels running in the first, third, fourth, fifth, and snzth xiscerul ttrchcs. (the vessels In the third and fourth arches will become the carotid and h_\’$tt‘n1lt.‘ arches l‘L‘.S|'>CL‘ll\t.‘l_V')i cg, external gills; 1', infundibulum (floor of the l'orehrttin); ic. internal cctrottd artery; n, nasal sac; ac, (esophagus; pd, pronephric duct: pr, pronephtic funnel: sp, splttnchnocuclz rt‘, 1 to 5, first to fifth visceral cleft (the first mil gl\ c rise to the Eustachian tube); V, VII, IX, branches of the tngcminttl, fztcutl. and glossophztryngeal nerve, running in the first, second. and third visceral ztrchcs respectively.
between the right and left Spl:mCl1n0C0:liC cavities break down; but dorsally these walls persist. forming the dorsal mesentery. This mesentery is composed of two Closely apposed layers of coelomic epithelium spreading round the gut and suspending it. It may be noticed, therefore, that the gut is not strictly in the ctelomic cavity at all; it merely hangs in a fold of coelomic epithelium which bulges into the cozlomic cavity. From the cells of this splanchnic layer are developed the smooth muscles of the stomach, intestine, and bladder.
Blood Vessels
Beneath the floor of the gut, and between it and the underlying splanchnic layer of cmlomic epithelium, there are some scattered mesoderm-cells which become arranged in the form of a tube, or subintestinal vessel. In the region of the pharynx, this tube forms the endothelial lining of the heart. The ctrlomic epithelium (splanchnic layer) surrounds this tube and suspends it as it were in a little mesentery of its own from the floor of the pharynx (the dorsal mesocardium). The musculature of the wall of the heart is derived from this layer of ccelomic epithelium, and that part of the splanchnocael in which the heart finds itself is now called the pericardium. Later on, the various parts of the heart are differentiated. Posteriorly, the heart is continuous with two tubes, the vitelline veins, which run from the yolk-cells and the rudiment of the liver.
The dorsal aorta arises as a pair of longitudinal vessels, close beneath the notochord. The two remain separate anteriorly, as the lateral dorsal aorta: and their prolongations into the head, the internal carotids. Behind, they join and fuse together along the whole of the rest of the body, forming the single dorsal aorta.
Beneath the pharynx, the heart communicates forwards with the ventral aorta. In each of the 3rd to 6th visceral arches, between the gill-slits, a vessel appears which communicates below with the ventral aorta and above with the lateral dorsal aorta of its own side. In this way the series of pairs of aortic arches arise, alternating with the gill-slits. When the capillaries of the gills arise, they connect with the aortic arches which become interrupted. There are now afferent branchial arteries carrying blood from the ventral aorta to the gills. and efferent branchial arteries connecting the gills with the lateral dorsal aorta. Rudiments of aortic arches appear in the mandibular and hyoid arches.
The dorsal aorta sends arteries to the gut, which they reach by passing down between the two layers which form the dorsal mesontery.
The arteries become surrounded by coats of smooth muscle. Of the veins, the posterior cardinals arise near and parallel to the dorsal aorta. Their anterior prolongations are the anterior cardinal veins which run one on each side of the brain, and which, later on, contribute to the formation of the internal jugulars. At this period, the pericardial cavity is open posteriorly and communicates with the general perivisceral splanchnocoel. In the region of the heart, the veins develop just in this region, the tubules are as it were bathed in the venous spaces. At the same time, capillaries grow out from the dorsal aorta forming the glomus, which projects laterally towards the openings of the cmlomostomes from the mesentery, on each side.
The pronephros is the functional kidney of the embryo and early larva. Later on, however, it degenerates, and its function is taken over by another set of cmlomic funnels and tubules, which together form the mesonephros.
The mesonephros is developed from the nephrotomes of half a dozen segments, some little distance behind the pronephros. Cavities hollow out in the nephrotomes, and these connect with the splanchnocwl by coelomic ciliated funnels, and by coiled tubules with the pronephric duct. The latter loses connexion with the degenerating pronephros, and, after being tapped so to speak by the mesonephric tubules, it is known as the mesonephric or Wolflian duct.
The tubules multiply by branching, and form little chambers or Bowman's capsules which lose their connexion with the coelomic funnels. Arterioles from the dorsal aorta and venules from the posterior cardinal veins form little bunches of capillaries which project into the capsules forming glomeruli. Capsule and glomerulus together form a Malpighian corpuscle. That portion of the posterior cardinal veins which lies behind the mesonephros becomes the renal portal vein, which brings blood from the posterior regions of the body to the kidneys. The mesonephros is the functional kidney of the adult. It extracts excretory matter from the blood stream and passes it down the Woltlian duct to the cloaca, which develops a ventral outpushing, the urinary bladder.
RF.l"RODU(‘.TlVl"-I ORGANS.---The gonads arise as ridges which project into the splanclinoccel on each side of the dorsal mesentery. The germ-cells which they contain are derived partly from the cotlomic epithelium in sim, and partly from cells which have migrated up in the mesentery from the yolk-mass. For a long time the sexes are indistinguishable. Strings oi‘ germ-cells grow in, away from the surface of the gonads, forming the genital strands. In embryos which are going to be males these hollow out, forming the seminiferous tubules which become connected with the cavities of the tubules of the mesonephros. In this way the xasa ellerentia are formed, and they may be regarded as persistent ccelomic funnels, placing the testis in communication with the exterior (via the cloaca). The sperms therefore make their way through the tubules of the mesonephros, down the Wolffian duct or as deferens as it can also be called, to the exterior.
The Miillerian ducts develop as grooves in the roof of the splanchDEVELOPMENT or RANA 151
noctel at the side of the gonads. The sides of the groove grow over, and convert it into a tube which opens into the eeelomie cavity in front (near the place where the pronephric funnels were), and grows back to open into the cloaca behind. In males the Miillerian ducts disappear.
The kidneys and gonoducts are mcsodermal all the way, and are really coelomoducts, whose primitive function is probably to connect the ccelomic cavity with the exterior and so allow the germ-cells to escape. They take on the function of excretion as a result of the proximity of the tubules to the blood-vessels.
On the other hand, the nephridia have excretion as their primitive function; they do not occur in Chordate animals other than Amphiaxus.
Paired Sense-Organs and Brain
The eyes make their appearance as outpushings from the sides of the brain, forming the optic vesicles. Each of these vesicles grows towards the overlying ectoderm, and becomes an optic cup, with the concave side turned outwards. As a result of the inducing action of the optic cup, the lens is formed from the ectoderm overlying the optic cup. as a little vesicle which soon becomes nipped oil‘, and sinks into place at the mouth of the cup. While the cup is really part of the brain, the lens is part of the epidermis, but both are ectodermal. The outer lining of the cup forms the pigment or tapetum layer; the inner lining of the cup differentiates to form the sensitive retina, and it is inverted since the nerve-fibres run between the sensitive cells and the seen object (see p. 19). Outside the tapetum, mcsodermal tissue gives rise to the choroid and sclerotic (including the transparent cornea) layers, just as round the brain it forms the pia mater (vascular) and dura mater (protective). The superficial epidermis immediately overlying the cornea and lens becomes thin and transparent, forming the conjunctiva. The extrinsic eye-muscles arise fmm mcsodermal tissue which represents the three first somitcs of the head.
The ears arise as a pair of ingrowths from the ectoderm behind the eyes, forming the auditory vesicles. Their connexion with the ectoderm becomes severed and the remains of the connecting stalk is the duetus endolymphaticus. Each vesicle new forms a closed sac at the side of the hinder part of the brain, and above the tympanie cavity, which develops as an expansion of the hyomandibular visceral pouch (Eustachian tube). From the dorsal portion of each vesicle three shelf-like projections are formed. The centre of each shelf becomes perforated, converting the shelf into a half—ring. In this way the semicircular canals are formed. The cavity of the auditory sac contains endolymph. Between the wall of the sac and the capsule of connective tissue which surrounds it, is the perilymph. The capsule eventually becomes cartilaginous, and later on, bony; but certain apertures are left. One of these is the fenestra rotunda, and another i§ the fenestra ovalis on to which the base of the eolumella auris fits. The outer end of the columella auris is applied to the thin lateral wall of the tympanic cavity which forms the tympanic membrane.
Figure 85. Transverse sections through the head of embryos of Rana showing the development of the eyes.
A. early stage. in which the optic vesicles (or) have been pushed out on each side from the l‘oiebr.iin (fb). B, the outer walls of the optic vesicles have been pushed in, coiitcrting them into optic cups toe); the lens (I) arises opposite the mouth of the optic cup from the ectoderm tee). C‘, late stage; the cavity of the optic vesicle has been almost obliterated, the lateral layer of the optic cup is the retina (r) and the median la_vcr is the pigment layer (pl), the stalk attaching the optic cup to the l'oiehrain is the optic llt.‘l'W.‘ (on), the lens has become detached from the ectoderni.
It may be mentioned here that. remarkable as it may seem, the ears are responsible for the formation of the so-called calcigerous glands, or glands of Swaminerdamm. These glands are conspicuous objects in the trunk of the frog, lying on each side of the vertebrae, close to the points of exit of the spinal nerves. Diverticula from the DEVELOPMENT or RANA 153
auditory vesicles grow into the brain-case, and back down the canal formed by the vertebrae and which contains the spinal cord. From here, the diverticula of the auditory vesicle emerge through the font mina for the spinal nerves and give rise to the glands of Swamrnerdamm (function unknown).
Figure 86. Transverse section through an embryo of Rana showing the formation of the ears.
‘an, auditory nerve; av, auditory vesicle; bv, bloodfvessels running in the visceral arches; cg, external gills; g, gut; /2, heart; lib, hmdbram; n, notochord; p, pericardium; ta, truncus artenosus; vs, ventral sucker.
The olfactory organs arise as a pair of thickenings of the ectoderm, which sink in to form pits just above the mouth. The cells lining these pits will give rise to the olfactory epithelium. Behind, the pits reach the roof of the mouth and break through, forming the internal nostrils.
The various regions of the brain are roughly marked out even before the neural folds have closed over. The definitive form of the brain is soon reached by means of foldings and thickenings of its walls in certain places.
A median ectodermal inpushing arises from the epidermis of the front of the head, just above the mouth. This is the hypophysis which grows back beneath the floor of the fore-brain until it meets and fuses with the infundibular downgrowth from the brain. Hypophysis and infundibulum together form the pituitary body.
Placodes and Lateral-Line Organs
The dorsal nerves and ganglia in the region of the trunk consist of nerve-cells which have been derived entirely from the neural crests. In the region of the head, the dorsal nerve-ganglia are derived not only from the neural crest, but also from thickenings of the ectoderm at the sides of the head called plaeodes. Placodes are proliferations of the deeper layers of the epidermis which contribute cells to the underlying ganglia. The profundus, trigeminal, facial, glossopharyngeal and vagus ganglia all derive cells from the epidermis in this way, and the auditory nerve is formed from the placode which invaginates with the auditory sac. Indeed, the thickenings of the epidermis which later become pushed in to form the olfactory sacs, the lens, and the auditory sacs, may themselves be regarded as placodes.
There are two kinds of placodes: an upper row of dorso-lateral plaeodes which give rise to the lateral-line sense-organs and to the nerve-cells whose libres innervate them; a lower row of epibranchial placodes situated at the dorsal ends of the visceral slits, and which give rise to the nerve-cells whose fibres innervate the sense-organs of taste.
Sympathetic System and Adrenals
The dorsal nerveroot, formed by librcs which have grown out from cells in the dorsalroot ganglion, and the ventral nerve-root which has grown out from the spinal cord, _|0lll to form a mixed nerve. Certain cells migrate out from the spinal cord, and, lcavmg the mixed nerve, make for the side of the dorsal aorta where they form the sympathetic ganglia. These ganglia remain connected with the mixed nerve by the rami communieantcs. The sympathetic: ganglia are, like the mixed spinal nerves, segmcntally arranged. They soon become connected by fibres running to the sympathetic ganglia lll front and behind them forming the sympathetic trunks. From the sympathetic ganglia, “postganglionic" fibres are distributed to the smooth muscles of the gut, oviducts. and blood-vessels. Other cells migrate out from the sympathetic ganglia. and give rise to the medulla of the adrenal bodies. The cortex of these bodies is deriv ed from the coclomie epithelium in the region between the mcsonephric kidneys.
It may be mentioned that cells migrate out from the hind-brain along the vagus and cxentually come to he on the surface of the heart and gut, forming part of the parasympathetic system.
Skeleton
Most of the cartilaginous skeleton arises from mesenchyme cells derived from the sclerotomes, and therefore of mesodermal origin. But the eartilages of the trabeculm and of the visceral arch skeleton are formed from ectornesenchyme cells derived from the neural crest. The vertebral column arises in the form of paired cartilages beside the notochord, derived from the sclerotomes. Each vertebra arises opposite the septum separating two segments; the vertebrae are therefore intersegmental in position.
In the skull, paired trabeeula arise as struts underlying the forebrain, and, behind them, paired parachordals flank the notochord. The pterygo-quadrate or skeleton of the upper jaw arises early, and fuses on to the remainder of the skull by its ascending process. The auditory sac becomes surrounded by a cartilaginous capsule which gets attached to the paraehordals on each side. Similarly, nasal capsules surround the olfactory sacs and become attached to the front of the trabecular. The floor of the skull is established in this way, and the sides and roof develop later.
In each of the visceral arches separating the gill-slits, cartilaginous struts develop. In the mandibular arch, these are the pterygoquadrate, and Meeke1’s cartilage which forms the lower jaw. The dorsal portion of the skeleton of the 2nd or hyoid arch forms the columella auris. The cartilages of the remainder of the arches eventually form a plate beneath the floor of the mouth and pharynx, and which by raising and lowering this floor assists in the process of respiration. The skeleton of the limbs and girdles does not appear until a late stage of development.
This cartilaginous skeleton is later on partly replaced by cartilagebones, and in addition, membrane-bones are developed.
Teeth arise late. In their formation, an ingrowth of ectodcrm takes place inside the margin of the mouth forming the enamel organs of the teeth. These secrete a cap of enamel beneath which the odontoblasts (derived from the neural crest) produce the body of the tooth which is composed of dentine. hventually the tooth is pushed up through the surface of the mouth and its base is attached to the bone of the jaw.
EXTERNALS.— By the time that differentiation and the formation of organs have proceeded as far as has just been described, the embryo emerges from its membranes and hatches into a freeswimming larva which is familiarly known as the tadpole. Its ectodcrm is ciliated, and just beneath the mouth it has a V-shaped sucker by means of which it can attach itself to objects. Its tail elongates and develops dorsal and ventral extensions or fins, which make it a very efficient organ for swimming. Its food consists of vegetable matter, its stock of yolk being by now used up. Food is seized by the edges of the mouth or lips which are assisted by horny epidermal teeth, which have of course nothing to do with the true teeth.
From the sides of the head, folds grow back which cover over the gill—slits. The external gills disappear, and so-called internal gills develop in the walls of the gill-slits and subserve the function of respiration. The folds just mentioned form the operculum, which leaves only a small hole on the left side through which the water which passes through the gill-slits may escape.
The organisation of the larva is just like that of a fish, and there is’ little indication of the frog into which it will develop. The changes which take place in the conversion of the tadpole into the frog are known as metamorphosis.
Metamorphisis. —The chief differences between the organisation of the tadpole and that of the frog concern the limbs, lungs and pulmonary respiration, intestine, tongue and tail.
The limbs arise as buds in tadpoles about half an inch long, and muscles grow into them from the myotomes. The buds of the forelimbs are, however, concealed beneath the operculum, and are therefore invisible. Those of the hind-limbs are situated at the base of the tail, on each side of the cloaca. In time. the fore-limbs grow out through the operculum, making use of the opening on the left side and making a new one on the right. Soon the limbs become visibly jointed and the toes appear.
Meanwhile. the lungs are dexeloping, and to each of them there runs,a blood-vessel which is formed as a branch from the efferent artery of the last or 6th arch. This vessel is the rudiment of the pulmonary artery. l"rom time to time, the tadpole takes in a gulp of air at the surface of the water and hlls its lungs. A certain amount of oxygenation of the blood now begins to take place in the lungs, and the gill-circulation becomes reduced by the establishment of direct connexions between the afferent and efferent branchial arteries. The gills therefore become “short-circuited", and left out of the circulation gradually as more and more of the blood goes to the lungs to be oxygenated, and returns to the heart by the pulmonary veins. The now continuous vessel in the 3rd visceral arch becomes the carotid, that in the 4th becomes the systemic arch, that in the 5th disappears, and the 6th as already seen becomes the pulmonary. The lateral dorsal aorta between the dorsal ends of the carotid and systemic arches (the ductus caroticus) disappears, as also does the connexion between the pulmonary artery and the lateral dorsal aorta (ductus arteriosus. or Botallil. After this change, the organism is perfectly adapted to breathe in air after the manner of landanimals.
The gills disappear; the gill-slits close up; the animal ceases development or feeding, and the horny teeth drop off. The month becomes wider and its angle moves farther back. The tongue develops,‘ and the eyes become more prominent and bulge out from the top of the head. The lateral-line organs disappear and the skin is shed. Glands appear which will keep it moist on land. Internally, great changes take place in the intestine, which loses its watchspring-like coils, and becomes relatively much shorter. This is an adaptation to the carnivorous habits of the frog, for less surface is required for the digestion of a meal of animal food. Lastly, the tail becomes reduced and finally completely absorbed, its debris being ingested by wandering white blood-corpuscles, or phagocytes.
This astonishing and comparatively rapid change is brought about by the secretion of the thyroid gland, which has been increasing until it reaches a size sufficient to “pull the trigger" of metamorphosis. During the process of change, the weight of the body actually decreases, but after coming out on land and recommcncing to feed, the size of the young frog increases.
In some newts, metamorphosis fails to take place. and the animals become mature in a larval condition with open gill-slits, median fins, etc., living in water. This condition is known as neoteny.
Literature
JENKINSON, J. W. Vertebrate Embryology. Oxford, at the Clarendon Press, 1913. “
KELLICOTT, W. E. C /10) date Development. Henry Holt, New York, 1913. MORGAN, T. H. T he Development of the Frog‘s Egg. Macmillan, New York, 1897.
SPEMANN, H. Embryonic Development and Induction. Yale University Press, 1938.
Cite this page: Hill, M.A. (2024, May 13) Embryology Book - Vertebrate Zoology 12 1922. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Vertebrate_Zoology_12_1922
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
