Book - Vertebrate Zoology (1928) 15
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XV The Blastopore
The blastopore is one of the most important structures in development, for as a result of the processes which are entailed in its formation the fundamental architecture of the future embryo is laid down. Further, experimental investigations have shown that the region of the dorsal lip of the blastopore (which is the first part of the blastopore to develop) is responsible for organising the embryo. That is to say that it determines the place of formation of the various organs, and is necessary for the start of the processes of differentiation. The blastopore itself introduces the first differentiation (after the establishment of the axis of the egg in the ovary, and of the plane of bilateral symmetry by the point of entrance of the sperm) in that it converts the single-layered hollow ball (blastula) into the double-layered bowl (gastrula). In those animals where the relation of the sperm's entrance point to the blastopore is known (amphibia), it is found that the dorsal lip of the blastopore arises opposite the sperm-entrance point, and marks the dorsal side of the future embryo.
In the development of the dogfish, the egg contains so much yolk that cleavage is incomplete or meroblastic, and a disc of cells or blastoderm is formed lying on the top of the yolk. Now the important point to notice is that all round the edge of this blastoderm, cells are growing over the yolk and tucking- in underneath the upper layer of the blastoderm to form endo- derm. In fact, the edge of the blastoderm is the rim of the blastopore, and mesoderm-cells are also proliferated from it. The embryo forms in front of the posterior edge of the blasto- derm, which is the dorsal lip of the blastopore, and does not wait for the blastopore to close. Indeed, this takes a long time, for the anterior edge of the blastoderm has to grow a long way down and back under the yolk before it comes up underneath and opposite the dorsal lip to form the ventral lip of the blastopore.
The edge of the blastoderm in the dogfish corresponds to the edge of the pigmented cells of the animal hemisphere in the frog, and this is the place where the overgrowing lip which is the rim of the blastopore arises in the frog also. All round the rim of the blastopore, the ectoderm, mesoderm, and endoderm are in contact. In the case of the frog, the blasto- pore starts a little below the equator of the spherical embryo, and as it grows down to latitudes nearer the vegetative pole, the diameter of the blastopore naturally decreases. Any given point on the rim of the blastopore grows straight down along a meridional line towards the vegetative pole ; but as the diameter of the blastopore decreases, any two given points on the rim at the start will find themselves closer together at the finish of gastrulation. This process is called confluence. In the case of the dogfish, the diameter of the blastopore (edge of the blastoderm) has to increase considerably until it has grown down and passed the equator of the yolk, whereupon it decreases again.
Fig. 113. Views of a developing embryo of a dogfish. (After Jenkinson.) A from above ; B, C, and D from the left side. The lips of the blasto- pore are formed from the edge of the blastoderm, ae, anterior edge of the blastoderm ; dl, dorsal lip of the blastopore ; e, embryo ; vl, ventral lip of the blastopore ; y, yolk.
It is characteristic of these lower vertebrates (fish, frog, and newts), that the rim of the blastopore arises along the margin separating the protoplasmically-rich cells of the animal hemi- sphere from the cells rich in yolk (or the undivided yolk) of the vegetative hemisphere. Also, that in the closure of the blastopore, the yolk should be enclosed by the growth of the anterior part of this margin which becomes the ventral lip of the blastopore.
This is, however, not the case in the higher forms (reptiles, birds, and mammals), in which there is a primitive streak. In order to understand the evolution of the primitive streak from the simple blastopore of the lower vertebrates, it is necessary to consider the condition in the Gymnophiona, which is more or less intermediate. The quantity of yolk in the Gymnophionean egg brings about the formation of a blastoderm. The posterior edge of this blastoderm grows back over the yolk and tucks cells in beneath itself, like the typical dorsal lip of the blastopore which it is. Overgrowth also takes place at each side of the dorsal lip, and the blastopore becomes crescentic. Eventually the two horns of the crescent meet and the blastopore is then a closed circle. But the anterior edge of the blastoderm has not moved, it has not grown round underneath the yolk, and it takes no share whatever in the formation of the blastopore. At the same time it is to be noticed that the blastopore is a real aperture, through which the yolk can be seen from the outside. The cavity which communicates with the exterior through the blastopore is, of course, the archenteron, and the lining of this cavity is the endoderm, formed by the activity of the edge of the blastopore. In the reptiles, yolk is abundant, and cleavage leads to the formation of a blastoderm. At a place which marks the posterior end of the future embryo, cells are proliferated under the blastoderm, forming a lower layer between the blastoderm and the yolk. This lower layer is really the endoderm, which has been formed precociously, probably serving the function of digesting the large quantity of yolk. A dorsal lip of a blastopore arises (not at the extreme hind edge of the blasto- derm, but well within its margin) as a rim beneath which cells become tucked in and passed forwards beneath the blastoderm and above the lower layer. The rim of the blastopore extends to the sides, and so the lateral lips come into being. Eventu- ally the lateral lips extend backwards, and lie parallel to one another. The blastopore is now slit-like, and resembles a primitive streak. The lateral lips of the blastopore join posteriorly, and the blastopore is then closed. The cells which get tucked in by the lips of the blastopore line a cavity which is the archenteron, so that here as in fish, frogs, newts, and Gymnophiona, the blastopore is a real aperture. The archenteron extends far forwards as the result of invagination, and its roof in the middle line becomes the notochord ; on each side the roof becomes mesoderm. The floor of the archenteron fuses with the underlying lower layer and then disappears, so that the blastopore leads right down through the archenteron to the surface of the yolk. The walls of the definitive alimentary canal are formed from the lower layer, which is endoderm formed really before the blastopore proper can be said to exist.
Fig. 114. Views of the blastoderm of Hypogeophis, one of the Gymno- phiona showing the origin and closure of the blastopore. (From Jenkinson, after the brothers Sarasin.) The anterior edge of the blastoderm here does not become the ventral lip of the blastopore, yp, yolk-cells seen through the blastopore.
Fig. 115. — Longitudinal sections through the blastoderm of a reptile, showing the origin of the blastopore. (From Jenkinson, after Will.) A-E, successive stages. A, precocious origin of the endoderm or lower layer (pd), which is in contact with the upper layer of the blastoderm at pp ; B, origin of the blastopore, dl, dorsal lip of the blastopore (cf. Fig. 89) ; C, invagination at the blastopore to form the archenteron ; the lower layer forms a continuous membrane separated from the yolk by the subgerminal cavity (sgc) ; mesv, mesoderm formed from the lip of the blastopore ; D, the archenteron (arch) extends a long way forwards beneath the upper layer of the blastoderm ; yp, yolk-plug (cf. Fig. 75) ; E, fusion of the floor of the archenteron with the underlying region of the lower layer, and their subsequent disappearance, so that the archenteron communicates with the subgerminal cavity.
The conditions in the reptile lead on easily to those which obtain in birds. Here, again, the endoderm is formed pre- cociously as a lower layer split off from the underside of the superficial layer of the blastoderm. The blastopore, however, never is a real aperture, because its lateral lips are fused together all along their length forming the primitive streak. The dorsal lip is the primitive knot beneath which a solid strand of cells is tucked in to form the notochord. As the primitive streak moves backwards over the blastoderm, it pays in a stream of cells into the hinder end of the notochord. The primitive streak gives off mesoderm to each side. In the bird, therefore, the blastopore is closed from the start, and its aperture is represented only by the depression of the primitive pit just behind the primitive knot, and by the primitive groove which runs along the centre of the primitive streak. The bird's blastopore begins where that of the frog leaves off, for in the latter it will be remembered that the blastopore which was spherical becomes oval, and its lateral lips become apposed to one another, forming what is in fact a short primitive streak. In the bird, there is no invagination, and no archenteron, and the walls of the alimentary canal are derived (as in reptiles) from the lower layer.
In mammals, the embryo develops from a primitive streak. In some cases the blastopore is a real aperture, or in other words, the primitive pit sinks down and opens into an archenteron beneath the superficial layer of the floor of the amniotic cavity (corresponding to the blastoderm). In others, the blastopore is reduced. The primitive streak and archen- teron give rise to the notochord and mesoderm, and the endoderm is formed from the lower layer.
In reptiles, birds, and mammals, therefore, the blastopore either closes or arises already closed without the yolk becoming enclosed. For the anterior edge of the blastoderm does not grow down under the yolk to form a true ventral lip of the blastopore.
In Amphioxus where there is little yolk, the rim of the blastopore is formed as the result of simple invagination of the vegetative hemisphere. Thereafter the rim of the blastopore grows backwards and the embryo increases in length. In Craniates, the quantity of yolk present prevents simple in- vagination, and the rim of the blastopore arises as the result of overgrowth (epiboly) accompanied by invagination or some form of ingrowth. There is an increasing tendency for the invagination to become reduced as the quantity of yolk in- creases, and the yolk ceases to become encircled in the process of closure of the blastopore. At the same time, the endoderm appears early (one might say out of its turn), and the aperture of the blastopore becomes virtual.
Amphioxus.
Frog.
Reptile.
Bird.
Yolk
Very little.
Little.
Much.
Very much.
Yolk
Enclosed.
Enclosed.
Not enclosed.
Not enclosed.
Gastrulatlon
Invagination.
Overgrowth
Primitive
Primitive
and invagi-
streak and
streak, no
nation.
invagination.
invagination.
Blastopore
Open.
Open.
Open.
Closed from start.
Gut formed
Wall of
Wall of
Lower layer.
Lower layer.
from
archenteron.
archenteron.
Literature
Assheton, R. Growth in Length. Cambridge University Press, 191 6. Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon Press, 1 91 3.
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- 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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