Book - Vertebrate Zoology 14 1922

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Beer G. Vertebrate Zoology. (1922)

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XIV The development of Lepus (the Rabbit)

Fertilisation

The egg is very small, and contains very little yolk. It is surrounded by a vitelline membrane secreted by itself, and by a secondary membrane formed from the follicle-cells, the zona pellucida. The follicle-cells are several layers thick surrounding each egg-cell, which however they do not fit closely. There is a large space inside the follicle filled with fiuid and bathing the egg, which gives the characteristic appearance of the Graafian follicle, typical of mammals. One polar body is extruded in the ovary, the second is extruded after fertilisation. _

Ovulation is the process of release of the egg from the ovary, and is usually brought about during a period of “heat” or oestrus as a result of the action of the follicle-stimulating hormone of the pituitary. Other mammals, including the rabbit, only ovulate after the stimulus of copulation. The follicle vacated by the egg becomes filled by the great increase in size of the follicular cells and by the ingrowth of connective tissue and blood-vessels, and becomes a corpus lutcum under the influence of the luteinising hormone of the pituitary. Should the egg just ovulated be fertilised, the corpus luteum becomes an important structure, functioning as a gland of internal secretion producing the hormone progesterone, and among its functions are the following: it stimulates the uterus to hypertrophy and undergo “progestational proliferation" as a result of which the uterine glands secrete nutritive material for the embryo before it becomes fixed to the wall of the uterus; it prepares for the reception and fixation of the embryo; and it renders the muscles of the uterus insensitive to the osytocic hormone of the pituitary and other stimuli which would cause them to contract prematurely and produce abortion. The corpus luteum disappears at the end of the period of gestation, but if no pregnancy has ensued it disappears soon after ovulation.

During copulation sperms are introduced into the vagina, and they make their way up through the uteri to the oviducts, near the top of which they meet and fertilisc the egg. Fertilisation is therefore internal, as in the chick.

Cleavage

Cleavage is total and gives rise to a ball of cells, or morula. A cavity appears within it, and it soon becomes differentiated into an outer layer and an inner mass of cells." The former is the trophoblast, and the whole structure is known as a blastocyst.

Figure 108. Lepus: early stages in the development of the rabbit. (After Assheton.)

A, two-cell stage, enclosed by the zona pellucida (zr); B, morula (m) stage; C, blastocyst showing the dllferentiation into the trophoblast (t) and the inner mass (im); D, the inner mass has become the embryonic plate and IS dilferentiated into eetodemt (er) and endodcrm (en); E, the trophoblast overlying the embryonic plate—the cells of Rauber (cR)-—disappear; F, after the disappearance of the trophoblast over the embryonic plate; G, transverse section through the primitive streak (pr). c, ccelom; me, mesoderm.

Implantation

The blastocyst travels down the oviduct and eventually finds itself in the cavity of the uterus where it ingests through its trophoblast the nutritive substances secreted by the uterine glands. The lining of the uterus has on its mesometrial side (see p. 238) a pair of prominent folds, which project into the cavity or lumen of the uterus. To these, the blastocyst becomes attached by means of its trophoblast. This process is called implantation. In the rabbit where the blastocyst remains in the cavity of the uterus, the type of implantation is termed central. In other mammals including man, the blastocyst bores its way into the uterine wall, a’ type of implantation termed interstitial.

FORMATION or THE EMBRYO. The blastocyst enlarges and expands in the cavity of the uterus. The cells of the inner mass 196 TBMBRYOLOGICAL rvrns

become arranged in the form of a flattened disc, immediately beneath the trophoblast. This disc is known as the embryonic plate.

At the same time, the inner mass gives rise to a layer of cells which grow as an epithelium lining the inner surface of the trophoblast. This layer is endoderm (also called the “lower layer”), and the cavity which it encloses represents the yolk-sac of the chick. Here, however, there is no yolk, and the yolk-sac is consequently empty.

The cells of the trophoblast immediately overlying the embryonic plate (the cells of Rauber) disappear, and the embryonic plate thus

M

grams shon ing the formation of the amnion in the rabbit. (From Jenkinson, after van Beneden.)

The C11l'llL‘l' stage is on the left: the later stage on the right. Since the cells of Rauher have disappeared, the embryo 18 at the surface of the blastocyst until the amnion has formed. all, allantols; arr, region of the trophoblast where the allantoie placenta will he formed; c, extra-etnbryontc euclom; e, embryo; ham, head amniotic fold; arr, region of the trophoblast where the omphaloidean placenta is formett: st, sinus terminalis (blood-vessel) of the area vasculosa; mm, tall tunmottc fold. _t-s, yoll-t-sate.

Figure 109. Din

comes to the surface of the trophoblast. A primitive streak is formed in the centre of the embryonic plate, and, as in the chick, it proliferates mesodermal cells to each side, and forms the notochord in the middle line as it retreats towards the hind end of the embryo. Neural folds rise up and enclose the neural tube, and the embryo becomes folded up from the surrounding tissue by the head-fold and tail-fold. In this way, the gut begins to be formed, and, as in the chick, anterior and posterior intestinal portals arise (see p. 181).

The amnion arises by the upgrowth of folds at the edge of the embryonic plate. The hinder amniotic fold develops faster than that DEVELOPMENT or LBPU8 "197

in front, and when these folds meet, the embryo is no longer at the

surface of the trophoblast, but folded away within it. The embryo is then enclosed in the amniotic cavity, just as in the chick, and the trophoblast of the rabbit corresponds to the chorion of the chick,

Figure 110. Diagram showing the relations of the embryonic membranes and placenta of the rabbit, as seen in an idealised transverse section of the uterus. (From Jenlunson, after Duval and van Beneden.)

all, allantois; c, extra-embryonic _ca:lom; ep, epithelium of the uterus; Iu, cavity of the uterus; m. mcsometrium; arr, omphaloidean trophoblast; pl, placenta (allantoic); pr am, proammon; ys, yolk-sac. the relations of which are identical (see p. 178). It may be mentioned that in some other mammals such as the mouse the amnion is not formed quite in this way, but arises precociously, even before the

embryo (see p. 219). The rabbit has been chosen for description here 198 * I aunnvonootcu. ‘nu-as

because its development is so easily comparable with that of the

The mesoderm splits into somatic and splanchnic layers with the

ccelomic cavity between them. The splanchnic layer overlies the

yolk-sac. The somatic layer grows up round the amnion and separates the latter from the trophoblast.

An area vasculosa develops in the wall of the yolk-sac, and the blood-vessels so formed extend as far as the sinus terminalis. The lower wall of the yolk-sac is not vascularised. In some mammals this lower wall of the yolk-sac with its overlying trophoblast persists for some time, and absorbs nourishment from the walls of the uterus. In the rabbit, however, this “omphaloidean” region of the trophoblast together with the lower wall of the yolk-sac disappears, and the cavity of the yolk-sac is then openly continuous with that of the lumen of the uterus. This disappearing part of the blastocyst contained neither blood-vessels nor mesoderm.

PLACENTA. Meanwhile, the upper part of the trophoblast which is in contact with the wall of the uterus on the mesometric side becomes much thicker and hollowed out by the formation of a number of lacuna: into which the trophoblast projects countless finger-like processes called pseudo-villi. The outer surface of the pseudo-villi loses its cell-boundaries forming a syncytium (or plasmodi-trophoblast). The more basal part of the trophoblast, between the syncytium and the mesoderm, retains its cell-boundaries (and is called the cyto-trophoblast). The allantois grows out from the region of the hind-gut and brings with it a covering layer of mesoderm and blood-vessels. The mesoderm covering the allantois fuses with the mesoderm underlying the cyto-trophoblast, and the allantoic bloodvessels make their way into the pseudo-villi. In this way the placenta is formed, and since it is related to the allantois, it is called an “allantoic placenta". The placenta is an organ which places the mother and embryo in physiological communication, for the interchange of substances. The epithelium of the wall of the uterus disappears where the trophoblast touches it, with the result that the trophoblast is in contact with the subepithelial tissues and bloodvessels of the uterine wall. The blood from these maternal vessels bathes the surface of the trophoblast and fills the lacuna: so that the pseudo-villi are immersed in maternal blood. This type of placenta is called hatmo-chorial. The blood of mother and embryo are never in direct communication.

The capillaries of the allantois branch in the substance of the pseudo-villi, and the blood which they contain is separated from the maternal blood only by the lining of the capillaries and the surface of the trophoblast. Across these membranes which constitute the placental barrier, substances are passed by difiusion. The maternal blood supplies not only oxygen but food—substances which may require to be digested by enzymes and broken down into simpler compounds, for the placental barrier is impassable to substances of high molecular weight. In the contrary direction the embryonic blood brings carbon dioxide and excretory products which are passed on into the maternal circulation. The placenta therefore functions as a respiratory, nutritive, and excretory organ. At the same time, a certain amount of nutriment is obtained from the glands of the uterus, and is either ingested phagocytically by the trophoblast or absorbed into the blood-vessels of the yolk-sac (the cavity of which

Figure 111. Section through a part of the haemo-chorial allantoic placenta of the rabbit.

The maternal tissue is on the right, the embryonic tissue on the left. They can often be distinguished by the fact that the red blood-corpuscles of the embryonic blood have not yet lost their nucleus. a, allantois; ct, cyto-trophoblast; eb, embryonic blood-vessels; cg, embryonic glycogen layer; I, lacuna: in the trophoblast and filled with maternal blood; mb. maternal blood-vessels; mg, maternal glycogen layer; st, syncytium or plasmodi-trophoblast; uv, umbilical vein. opens freely into that of the uterus). But the functions of the placenta do not end there, for it also serves as a store of food material for the developing embryo. In particular, glycogen is accumulated in the placenta at early stages before the embryo has a liver of its own; when the latter develops, the gycogen content of the placenta decreases. The placenta also secretes a hormone which controls and preserves the continuance of the pregnancy.

The vascular system of the embryo rabbit resembles that of the chick, but the posterior cardinals persist as the azygos and hemiazygos veins. The blood from the placenta arrives in the umbilical veins, of which the right disappears and the left runs into the ductus

venosus and so to the right auricle. As in the chick, the septum 200 H i éyaunnvorocrcxs types, between the auricles in the heart is perforated, and the oxygenated

. blood from the placenta can get through to the left auricle, left ventricle, and so to the carotids and brain, which requires the

purest blood in the body. The pulmonary artery connects with the aorta on the left side by the ductus arteriosus, so that the remainder of the venous blood in the right auricle passes through the right ventricle, pulmonary artery, and ductus arteriosus to the aorta below the place where the carotids come oil", and does not have to go through the lungs. The ductus arteriosus degenerates and the perforation of the interauricular septum is closed at birth when the lungs begin to function. The right systemic arch disappears.

A feature of great interest for the respiration of the intra-uterine embryo is the fact that the haemoglobin of the embryonic red blood corpuscles is different from that of the adult in having a greater affinity for oxygen. Indeed, it is obvious that the embryonic blood in the placenta must be able to take up oxygen at an oxygen-tension at which the maternal blood parts with it.

As in lower forms, the fore-gut and the hind-gut remain blind for a long time. In these regions the endoderm becomes apposed to the overlying ectoderm forming the oral plate and cloacal plate respectively. Perforation of these gives rise to the mouth and cloaca, which latter is divided into anus and urino-genital aperture. The bladder forms from the base of the allantois.

The urino-genital ducts develop much as in the chick, except that the right oviduct persists, and the testis descends into the scrotum.

From the fact that the perforation of the mouth does not occur at the extreme front end, but in the centre of the oral membrane, a small pocket is formed morphologically in front of the mouth. This is the so-called preoral gut. In a similar way, a post-anal gut is left after perforation of the anus.

In the region of the pharynx, the gill-pouches arise as outpushings from the gut to the ectoderm. They do not, however, become perforated.

Several structures enter into the formation of the diaphragm. The transverse septum moxcs backwards a considerable distance during development, and it is followed in its course by the phrenic nerve. The transverse septum forms the ventral portion of the diaphragm, and the wall which separates the pericardium from that part of the perivisceral cavity into which the lungs extend. The dorsal portion of the diaphragm separates this pleural corlom from the abdominal cavity behind, and it is formed by the growth of the mesenteries associated with the liver (which enlarges), kidneys, lungs, and gut.

As the placenta and the embryo increase in size, the uterus becomes enlarged to accommodate them. This is effected by a great , nnvanormanr or Laws . 201

increase in the‘ size’ of the smooth miiscle-cellsiiof which the wall of the uterus is composed, without any increase in their number.

When the period of gestation is accomplished, the amnion breaks and the embryo is expelled by the contractions of the muscular walls of the uterus. The umbilical cord is torn. The placenta also becomes detached from the wall of the uterus, and, together with clots of blood and debris, is expelled as the after-birth. 4."

HAIR. The development of hair starts by a thickening of the deeper layer of the epidermis, and its downgrowth into the dermis

Figure 112. Sections through the skin of mammalian embryos showing stages in the development of the hairs.

A, early stage showing the ectodennal inpushing (er) and the concentration of

the mesoderm to form a papilla (mp?; B, the ectoderm forms a follicle (f) inside

which the hair (/2) is developing; C, ate stage after the hair has erupted from the surface. apm, arrector plli muscle; sg, sebaceous gland.

forming a little cylinder. At its base a papilla is formed, and just above this, the epidermal cells proliferate and give rise to the shaft of the hair. This elongates as more material is added to it from beneath, and it finally emerges from the follicle and grows freely out. The centre of the hairshaft is composed of the medulla; surrounding thistis the cortex, and round this again is the cuticle. The outer wall of the follicle forms a sheath round the base of the hair, and the following layers can be made out in it. In contact with the cuticle of the hair is the cuticle of the sheath, and next outside that are Huxley’s layer. Healers myegfirmd the ‘inain epidermal layer of the sheath. ,,S1g»rt'ounding this again is the dermal sheath of the follicle. The epidermis of the wall of the follicle gives rise to little pouches which become the sebaceous glands. Some mesenchyme cells outside the ‘follicle become differentiated into smooth muscle-fibres; they gain

, attachment to the wall of the follicle and become the arrector finiuscles of the hair.

LITERATURE

BONNET, R. Lehrbuch der Entwicklungsgeschichte. Parey, Berlin, 1920.

CORNER, G. W. Hormones in Human Reproduction. Princeton University Press, 1946.

HAMMOND, J., and MARSHALL, F. H. A. Reproduction in the Rabbit. Oliver and Boyd, Edinburgh, 1925.

Ji!NK1l;§t;N, J. W. Vertebrate Embryology. Oxford, at the Clarendon Press, I .

KELUCOIT, W. E. Chordate Development. Henry Holt, New York, 1913.

PRENTISS, C. W., and AREY, L. B. A Laboratory Manual and T ext-book of Embryology. Saunders Co., Philadelphia and London, 1922.