Book - Vertebrate Zoology (1928) 14

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XIV 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 fluid and bathing the egg t 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 escape of the egg from the ovary. The follicle vacated by the tgg 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 luteum. Should the egg just ovulated be fertilised, the corpus luteum becomes an important structure, functioning as a gland of internal secretion, and among its functions are the following. It prevents ovulation of other eggs during the period of pregnancy, it stimulates the uterus to hypertrophy and so prepares for the reception and fixation of the embryo, and it stimulates the mammary glands to secrete. The corpus luteum disappears at the end of the period of gestation, but if no pregnancy has ensued it disappears soon after ovulation.


Many mammals ovulate spontaneously during periods of " heat," or oestrus, and the mouse is among them. Others, such as the rabbit, only ovulate after copulation. 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 fertilise 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

Fig. 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 differentiation into the trophoblast (t) and the inner mass (im) ; D, the inner mass has become the embryonic plate and is differentiated into ectoderm (ec) and endoderm {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 (ps). c, coelom ; me, mesoderm.


The former is called the trophoblast, and the whole structure is known as a blastocyst.



Implantation. — The lining of the uterus has on its meso- metrial side (see p. 276) a pair of prominent folds, which pro- ject into the cavity or lumen of the uterus. To these, the blastocyst becomes attached by means of its trophoblast. This process is called implantation.


Formation of the Embryo. — The blastocyst enlarges and expands in the cavity of the uterus. The cells of the inner mass 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 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. 21 1).




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Fig. 109. — Diagrams showing the formation of the amnion in the rabbit. (From Jenkinson, after van Beneden.) The earlier stage is on the left ; the later stage on the right. Since the cells of Rauber have disappeared, the embryo is at the surface of the blasto- cyst until the amnion has formed, all, allantois ; atr, region of the tropho- blast where the allantoic placenta will be formed ; c, extra-embryonic ccelom ; e, embryo ; ham, head amniotic fold ; otr, region of the trophoblast where the omphaloidean placenta is formed ; st, sinus terminalis (blood- vessel) of the area vasculosa ; tarn, tail amniotic fold, ys, yolk-sac.



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Fig. 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 Jenkinson, after Duval and van Beneden.) all, allantois ; c, extra-embryonic ccelom ; ep, epithelium of the uterus ; hi, cavity of the uterus ; m, mesometrium ; otr, omphaloidean trophoblast ; pi, placenta (allantoic) ; pr am, proamnion ; ys, yolk-sac.




The amnion arises by the upgrowth of folds at the edge of the embryonic plate. The hinder amniotic fold develops faster than that 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, the relations of which are identical (see p. 211). (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. 254). The rabbit has been chosen for de- scription here because its development is so easily comparable with that of the chick.) 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 persist for some time, and absorb 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 disappear, 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, 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 blood-vessels make their way into the trophoblast. 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 blood-vessels of the uterine wall. The blood from these maternal vessels bathes the surface of the trophoblast.





Fig. 111. - Section through a part of the 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 ; eg, embryonic glycogenic layer ; /, lacunas in the trophoblast and filled with maternal blood ; mb, maternal blood-vessels ; mg, maternal glycogenic layer ; st, syncytium or plasmodi-trophoblast ; uv, umbilical vein.



Further, the trophoblast, which in this region is now thick, becomes hollowed out by a number of spaces or lacunae ; and these lacunae become filled by maternal blood which oozes out from the wall of the uterus.


The capillaries of the allantois branch in the substance of the placenta, 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. (The blood of mother and embryo are never in direct communication.) Across these, substances are passed by diffusion. The maternal blood supplies not only oxygen but food, and 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 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 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 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 off, 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.


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 give 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 moves 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 ccelom 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 increase in the size of the smooth muscle-cells of 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.




Hair. — The development of hair starts by a thickening of the deeper layer of the epidermis, and its downgrowth into the dermis 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 this is 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, Henle's layer, and the main epidermal layer of the sheath. Surrounding 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 attach- ment to the wall of the follicle and become the arrector muscles of the hair.




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


A, early stage showing the ectodermal inpushing (ei) i nd the concentra- tion of the mesoderm to form a papilla (mp) ; B, the ectoderm forms a follicle (/) inside which the hair (h) is developing ; C, late stage after the hair has erupted from the surface, apm, arrector pili muscle ; sg, sebaceous gland.


Literature

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

Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon Press, 1913.

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

Prentiss, C. W., and, Arey, L. B. A Laboratory Manual and Text-book of Embryology. Saunders Co., Philadelphia and London, 1922.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology



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