Book - Vertebrate Zoology (1928) 11
Vertebrate Zoology G. R. De Beer (1928)
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PART II EMBRYOLOGICAL TYPES
Chapter XI Embryological Types Amphioxus
Fertilisation
The egg is surrounded by a vitelline membrane secreted by itself, and contains yolk mostly aggregated at one (the vegetative) pole. It is freed from the ovary and makes its way to the outside via the atrium and atriopore, at a stage shortly after the extrusion of the first polar body. In the water a sperm penetrates into the egg, which then proceeds to give off the second polar body ; the egg and sperm pronuclei then fuse and fertilisation is effected. The second polar body marks the animal pole of the egg, and it persists throughout cleavage until the beginning of gastrulation, when it is possible to see that the future anterior end of the embryo arises at a point near the animal pole. Actually the axis of the egg (from animal to vegetative pole) makes an angle of 30 with the antero-posterior axis of the embryo. The egg-axis is deter- mined in the ovary by the position of attachment of the egg to the germinal epithelium. The dorso-ventral median plane of symmetry of the embryo is marked by the point of entrance of the sperm.
Cleavage
The cleavage of the egg is total or holoblastic, i.e. the amount of yolk present is insufficient to prevent cell- division, but the cells of the vegetative pole are larger than those at the animal pole. Up to the 8-cell stage, the cell divisions keep pace with one another, but after that they become irregular. As a result of cleavage a ball of cells or morula is formed, and as the number of cells increases the ball becomes hollow. The central cavity is the blastocoel, surrounded by a single layer of cells which are smaller in the future anterior region of the embryo, and larger posteriorly. The embryo at this stage is a blastula.
Gastrulation
The posterior side of the blastula, where the cells are relatively larger, becomes flattened, and at one point (on the future dorsal side) actually tucked in beneath the more anterior smaller cells. In this way a lip is formed which soon extends right round the flattened region, which sinks in towards the centre of the blastula. This process of tucking-in is known as invagination, and the lip beneath which this takes place is the rim of the blastopore. At the same time as the flattened region is becoming invaginated, the rim of the blastopore is growing over towards the future posterior pole of the embryo, a process known as epiboly. Between them, the processes of invagination and epibolv result in the conversion of the hollow single-layered ball (the blastulaj into a double-layered hemispherical bowl. The original cavity of the ball (the blastoccel) has been obliterated, and the cavity of the bowl is the archenteron or primitive gut, communicating with the exterior through the blastopore. The embryo at this stage is known as a gastrula ; its outer layer is the ectoderm which will give rise to the epidermis, sense-organs, and nervous system ; its inner layer is the endoderm which is destined to give rise to the lining of the alimentary canal and its derivatives. The process of gastrulation therefore entails the separation of these germ-layers.
Fig. 68. — Amphioxus : early stages of development.
A, early blastula, showing the blastocoel (b) ; B, late blastula ; C, beginning of gastrulation, the ectoderm (ec) can now be distinguished from the endoderm {en) ; D, gastrula with primitive gut-cavity or enteron (e) ; E, late gastrula, showing the blastopore (bl) or mouth of the enteron ; F, stage in which growth in length has occurred as a result of the activity of the cells round the rim of the blastopore.
The overgrowth of the rim of the blastopore or epiboly continues as a result of the activity and division of its cells, and produces an elongation of the embryo along its antero- posterior axis. New cells are contributed to the ectoderm outside and to the endoderm inside, and the blastopore diminishes in diameter. The cells of the ectoderm develop cilia, but the embryo is still enclosed within the vitelline membrane.
Mesoderm, Nerve-tube, and Notochord
The cells along the middle line of the roof of the archenteron are destined to form the notochord. On each side of them is a band of cells which will give rise to the third germ-layer, or mesoderm. The cells along the mid-dorsal line of the ectoderm form a flat band which sinks in beneath the surface, and is grown over by the ectoderm on each side, which rises up to form the neural folds. This flat band is the neural plate ; it soon becomes V-shaped in section, and the two arms of the V join so as to give rise to a long tube running all the way along the back just beneath the ectoderm : the nerve-tube. In front, this tube is open at the neuropore, a place where the neural folds have not met, and which is indicated by Kolliker's pit in the adult. Behind, the neural folds rise up at the sides of and behind the blastopore. When they meet, they roof over the blastopore, which thus no longer communicates direct to
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Fig. 69. — Amphioxus : transverse sections through young embryos, showing the origin of the notochord, nerve-cord, and mesoderm. A, early stage showing the enterocoelic pouches (ep) still in communication with the gut-cavity (g) ; the roof of the gut is giving rise to the noto- chord (n) ; the nerve-cord (nc) although overgrown by the ectoderm (ec) has not yet formed a tube ; b, blastocoel ; en, endoderm ; m, mesoderm.
B, later stage showing the enterocoelic pouches nipped off from the gut.
C, stage showing the extension of the ccelom (c) between ectoderm and endo- derm, the formation of mesodermal somites (ms) ; the notochord is separate from the gut, and the nerve-cord is rolling up. D, late stage, the nerve- cord is a tube, the ccelom is divided into myocoel (ml) dorsally and splanch- nocoel (sp) ventrally, the inner wail of the latter cavity being the splanchno- pleur (sr) and its outer wall the somatopleur (so). The inner wall of the myocoel is modified into a muscle-plate or myotome (my), and ventral to the latter is the sclerocccl (se).
The notochord rises up from the rest of the roof of the archenteron and forms a solid rod of cells extending all the way down the body, just ventral to the nerve-tube. As the embryo grows in length, new cells are added on to the noto- chord rudiment from behind by the activity of the rim of the blastopore.
In each of the bands of cells which will give rise to the mesoderm, a longitudinal groove develops ; the groove opening widely into the cavity of the archenteron. The grooves deepen, and their front portions become separated from the more posterior region by a transverse partition on each side. These front portions become cut off from the archenteron altogether, and so a pair of mesodermal pouches are nipped off, each containing a portion of coelomic cavity which has been in communication with the archenteron and is therefore called an enteroccel. This pair of pouches gives rise to the first pair of somites, and it must not be mistaken for the pair of anterior head-cavities or anterior gut- diverticula, which develops farther forward and at a later stage.
Behind the first pair of somites, the grooves become nipped off from the cavity of the archenteron anteriorly, while they continue to communicate with it posteriorly. This means that the mesoderm becomes separated from the wall of the archenteron progressively from in front backwards ; and it also becomes divided up by transverse partitions into somites from in front backwards. These posterior somites (from the second inclusive) differ from the first pair only in that the mesoderm from which they are formed becomes separated from the wall of the archenteron before being broken up into somites, whereas the first pair of somites is demarcated before losing connexion with the wall of the archenteron.
The mesoderm is therefore segmented very early, and each segmental block of mesoderm or somite is separated from the ones in front and behind by a septum. The somites in the anterior region are derived from tissue which was invaginated to form the original archenteron, and consequently they are said to be formed from " gastral " mesoderm. The more posterior somites owe their substance to the production of new cells by the rim of the blastopore as the embryo elongates, and such mesoderm is called peristomial. The difference between these kinds is solely one of origin. After the separation of the mesodermal somites and of the notochord, the lateral edges of the endoderm grow over and meet to reform a roof over the gut- cavity.
The somites increase in size, and grow down between the gut and the ectoderm on each side. Eventually they meet beneath the gut and the wall separating them breaks down, so that the ccelomic cavity of each somite communicates with that of the corresponding somite on the opposite side of the body. The layer of ccelomic wall or epithelium which touches the endodermal wall of the gut is called the splanchnic layer, that touching the ectoderm of the surface of the body is the somatic layer. That part of the ccelomic wall which abuts against the nerve-tube and notochord on each side becomes thickened and gives rise to muscle-fibres forming the myotome : one myotome to each somite on each side. The more dorsal portions of the ccelomic cavity on each side, separating the myotome from the outer (or cutis) layer, are called the myocoels ; whereas the more ventral portion, into which the splanchnic layer suspends the gut from above, is the splanchnocoel. The myocoels become separated from the splanchnocoel of their somite by a horizontal partition. The myocoels retain their segmental arrangement, and remain separated by the septa from the myocoels of the somites in front and behind. The septa separating the splanchnocoels, however, break down, so that there is a continuous splanchnocoelic or perivisceral cavity from one end of the animal to the other.
The myotomes soon begin to show the V-shape character- istic of the adult, and the alternation in position between right and left sides.
In connexion with the mesoderm, there remain to be described a pair of pouches which become nipped off from the extreme front end of the wall of the gut. These are the anterior head-cavities, or anterior gut- diverticula. They arise symmetrically, but the right one soon occupies all the anterior region of the embryo in front of the ist pair of myotomes, and becomes the head-cavity. The left anterior gut-diverti- culum remains small, and eventually acquires an opening to the outside at the bottom of an ectodermal inpushing called the preoral pit ; in the adult this opening is Hatschek's pit.
There is no mesenchyme in Amphioxus, and the connective tissue which surrounds the nerve-tube and notochord is derived from hollow ingrowths from the myocoels, forming the scleroccels, the walls of which are the sclerotomes. The fin-ray boxes are also nipped off from the myoccels.
Portions of the myoccel persist in the adult between the myotomes and the connective tissue which surrounds them. Lastly, a downgrowth from each of the myoccels in the anterior
the cells are relatively larger, becomes flattened, and at one point (on the future dorsal side) actually tucked in beneath the more anterior smaller cells. In this way a lip is formed which soon extends right round the flattened region, which sinks in towards the centre of the blastula. This process of tucking-in is known as invagination, and the lip beneath which this takes place is the rim of the blastopore. At the same time as the flattened region is becoming invaginated, the rim of the blastopore is growing over towards the future posterior pole of the embryo, a process known as epiboly. Between them, the processes of invagination and epibolv result in the conversion of the hollow single-layered ball (the blastulaj into a double-layered hemispherical bowl. The original cavity of the ball (the blastoccel) has been obliterated, and the cavity of the bowl is the archenteron or primitive gut, communicating with the exterior through the blastopore. The embryo at this stage is known as a gastrula ; its outer layer is the ectoderm which will give rise to the epidermis, sense-organs, and nervous system ; its inner layer is the endoderm which is destined to give rise to the lining of the alimentary canal and its derivatives. The process of g-astrula- tion therefore entails the separation of these germ-layers.
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Fig. 70. — Amphioxus : young embryo and larva.
A, seen from above ; B, seen from the left side, a, anterior-gut diverti- cula ; g, gut ; ?i, notochord ; nc, nerve-cord ; s, mesodermal somites.
region of the body gives rise to the gonocoels, the walls of which (gonotomes) give rise to the gonads.
It is important to notice that the whole of the mesoderm in Amphioxus is segmented, and that this segmentation is retained everywhere except in the region of the splanchnoccel.
The Gut
At the stage when there are two pairs of somites nipped off, the embryo hatches and emerges from the vitelline membrane as a larva. The gut is still a closed sac which communicates only with the nerve-tube, through the neuren- teric canal. The mouth forms on the left side by a perforation between the ectoderm and the endoderm immediately under- lying it. It is very asymmetrical and soon becomes a large opening bordered with cilia. In a similar way, the anus forms as a perforation just beneath the neurenteric canal, which becomes closed and obliterated. Behind and dorsal to the anus the tail begins to grow back.
The cells lining the cavity of the gut become ciliated, and the splanchnic layer of coelomic epithelium surrounding them gives rise to smooth muscle-fibres. The liver grows out as a diverticulum from the gut on the right side.
The origin of the structures of the pharynx is peculiar and complicated by the extraordinary asymmetry which the larva shows. A structure is formed by the downgrowth of a groove from the front of the floor of the gut, and is converted into a tube which eventually opens into the gut on the right side, and to the exterior a little to the left of the midventral line. This is the so-called club-shaped gland, which is regarded as the first gill-slit of the right side. The first gill-slit of the left side arises ventrally by a perforation between the gut and the ectoderm, and it moves up the right side of the body, opposite the mouth. Behind this slit, about a dozen more are formed ventrally, and likewise move up the right side, although they are destined to become the left gill-slits eventually. This series is known as the primary gill-slits. These slits corre- spond with the segmentation of the body at this stage ; but this correspondence is lost later on.
The definitive gill-slits of the right side, or secondary gill-slits, arise later than the primary, and above them on the right side to the number of eight. The most anterior secondary slit corresponds to the second primary slit, which is what would be expected if the club-shaped gland is really the first right gill-slit, corresponding to the first primary gill-slit.
In front of the club-shaped gland, there arises a thickening of the wall of the gut consisting of a strip of ciliated and glandular cells. This is the rudiment of the endostyle. It becomes V-shaped with the apex pointing backwards, and this apex grows backwards as a double strip along the wall of the pharynx on the right side above the primary slits and below the secondaries. It is as if the morphologically midventral line of the larva in the region of the pharynx were displaced up on to the right side. Soon the primary slits move round to the left side, the endostyle assumes a midventral position, and the secondary slits on the right side correspond more or less symmetrically with the primaries on the left. The first primary (left) gill-slit, and the club-shaped gland disappear, and the number of slits on each side is regulated to eight by the disappearance of the posterior primaries. After this stage, more and more gill-slits are formed symmetrically on both sides, and the segmental correspondence is lost.
All the gill-slits except the anterior pair become sub- divided into two by the downgrowth of the secondary or tongue-bars. The perforation of the gill-slits naturally obliterates the coelomic cavity at the place of perforation ; the ccelomic cavity is therefore restricted to the primary bars between the gill-slits, and to the dorsal coelomic canals above and the subendostylar coelom below. The tongue-bars have no coelomic cavity, being downgrowths across the openings of the gill-slits. It is because of this difference in method of formation between the primary bars and the tongue-bars, that in the adult the former contain a portion of coelomic cavity and the latter do not.
During the rearrangement of the gill-slits, the mouth moves round to the anterior end. Its aperture decreases in size as its margin grows-in all round to form the velum.
Folds of the skin give rise to the oral hood, in the roof of which the preoral pit finds itself. The latter flattens out, and its cells give rise to the wheel-organ, or ciliated organ of Miiller.
The Atrium
The atrium arises as a pair of ventral longi- tudinal folds, the metapleurs. These folds pass on each side of the region of the gill-slits, which come to be situated between them. From each fold, a median shelf or epipleur extends and meets its fellow from the opposite side, thus enclosing a part of the outside world as the cavity of the atrium. The cavity is completely closed in front ; behind it remains in communication with the exterior by the atriopore. The atrium is lined throughout by ectoderm.
The nephridia arise as little blind sacs eventually connect- ing with the exterior, at the top of each gill-slit (before the formation of the tongue-bars, so that in the adult there is a nephridium to every two gill-slits). Hatschek's nephridium arises as a small tube near the preoral pit, but in the adult its opening leads into the pharynx just behind the mouth.
Fig. 72. — Amphioxus : transverse sections through larvae showing the development of the atrium. A, early stage in which the coelom (c) is still large, and the atrial cavity (a) is small. B, later stage ; ep, epipleural folds ; fr, fin-ray box ; g, gut ; m, myotome ; ?np, rnetapleural fold ; n, notochord ; nc, nerve-cord.
Primitive features in the development of Amphioxus : Cleavage total ; Gastrulation with invagination ; All mesoderm segmented ; Enteroccelic pouches.
Literature
Kellicott, W. E. Chordate Development. Henry Holt, New York. I9I3-
MacBride, E. W. Text-Book of Embryology, Vol. I. Macmillan, London, 1914.
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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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