Book - Vertebrate Zoology (1928) 13

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XIII Development of Gallus (the Chick)

Fertilisation

The true egg of the hen is all that is contained within the membrane that just surrounds the yolk. It is therefore of relatively enormous size for a single cell, and this is due to the very large quantity of yolk which it contains. The pure protoplasm, of which there is comparatively little, is situated at the animal pole, which is the point at which the follicle-stalk is attached to the ovary. The egg is surrounded by the vitelline membrane which it has secreted and which thickens to form the zona radiata, perforated by numerous holes through which nutriment is passed to the egg from the surrounding follicle-cells. This is a primary membrane. The egg bursts out of its follicle into the ccelom, and the follicle is left behind. There is therefore no secondary membrane. The nucleus has grown to a very large size, and the first polar body is formed inside the mouth of the oviduct, which as it were grasps the follicle containing the egg before the latter has left the ovary (or been " ovulated ").

Sperms are introduced into the cloaca of the female during copulation, and they make their way up to the top of the oviduct where several of them penetrate an egg. After the second polar body has been formed, one of these sperm-nuclei fuses with that of the egg, and the other sperms degenerate.

The fertilised egg then begins to develop, and passes down the oviduct. The walls of the latter secrete the tertiary membranes round it in the form of a layer of albumen, an inner and an outer shell-membrane, a hard shell formed by depositing lime salts, and this in many birds is coated with a layer of pigment which gives the " egg " its characteristic colour and pattern.

The egg goes down the oviduct with its axis transverse to the long axis of the oviduct, and it is rotated as it descends, with the result that the denser albumen at the two ends, in the axis of rotation, is spirally wound and forms the chalazse.

Fig. 87. Gallus : view of the blastoderm of a hen's egg. (After Jenkinson.) A after 12 hours', B after 18 hours' incubation, as seen by transmitted light, n, notochord ; pa, proamnion ; ps, primitive streak.At the blunt end of the egg, the two shell-membranes are separated by a space full of air, the air-chamber.

Cleavage

Soon after fertilisation, the process of cleavage begins ; its early stages therefore take place while the egg is descending the oviduct, and it has proceeded some way when the egg is laid. The amount of yolk compared with that of protoplasm is so big that cleavage is incomplete, or meroblastic. While the small quantity of protoplasm at the animal pole divides into a number of cells arranged like a small disc or blastoderm on the top of the yolk, the latter is undivided. The margin of the blastoderm merges with the yolk round it, forming the periblast ; and beneath the blastoderm is a cavity, the blastoccel, which separates it from the underlying yolk. This stage, when the blastoderm is but a single layer (though of many cells), represents the blastula of Amphioxus and the frog.

Gastrulation

A layer of cells becomes split off from the under side of the blastoderm, between it and the underlying yolk. This layer soon extends over the under surface of the blastoderm and is known as the " lower layer," or secondary endoderm. It is continuous with the upper layer all round the margin, and, like it, merges into the periblast. The blastoderm extends gradually over the yolk, and in so doing it forms a margin of overgrowth. In this region, all round the edge of the blastoderm (which is called the germ- wall), the protoplasm is thicker than in the centre. When therefore a blastoderm is looked at by transparency, two zones are distinguishable. Centrally there is a relatively clear area pellucida ; and round the edge is a denser area opaca. The egg is usually at this stage when it is laid, some twenty-four hours after fertilisation,

Fig. 88. — Gallus : transverse sections through the primitive streak of the blastoderm of a hen's egg. (From Jenkinson.) A after 10 hours', B, after 15 hours' incubation. //, lateral portion of the primitive streak corresponding to the lateral lip of the blastopore (cf. Fig. 76) ; mes, mesoderm ; pd, endoderm ; yp, primitive groove.

A thickening of the upper layer of the blasto- derm appears in the centre of the area pel- lucida, in the form of a straight band stretching at right angles to the line passing through the pointed to the blunt end of the " Qgg^ This is the primitive streak, the first differentiation of the embryo, which will be formed along its axis. When an observer holds an egg in front of him with the blunt end to the left, the axis of the embryo will therefore run straight in front of him, and the embryo is so orientated that its head is away from the observer, and its tail towards him.

Running along the middle line of the primi- tive streak is a shallow groove on the surface, the primitive groove, which runs into a small depression at the front end of the primitive streak known as the primitive pit. Immediately in front of the primitive pit is a slight rise, forming the so-called primitive knot. Now the primitive knot is the dorsal lip of the blastopore, and the primitive streak represents the lateral lips of the blastopore, fused together along their whole length. All that is left of the aperture of the blastopore is the primitive pit and the primitive groove. This is the condition of the blastopore of the frog after its aperture has become slit-like and closed by the apposition of its lateral lips. The chick therefore starts straight away from the condition reached in the frog at the close of gastrulation. The primitive streak, like the lip of a blastopore which it is, is a region of cell-proliferation, and it gives rise to mesoderm- cells which grow out laterally between the upper and lower layers. The primitive knot produces tissue which gets pushed forwards underneath the upper layer in the middle line, and gives rise to the notochord. During development the primitive streak moves back along the blastoderm, leaving in front of it a trail of cells destined to become notochord, and on each side (peristomial) mesoderm. At the extreme front end of the embryo, anterior to the region of the midbrain which is the farthest spot reached by the proliferation of the primitive pit and streak, a certain amount of mesoderm and the front end of the notochord are formed by splitting off from the lower layer. As in Amphioxus and the frog, therefore, the most anterior part of the mesoderm (gastral) and of the notochord are formed from the endoderm by delamination or splitting, while farther back they are formed as a result of the activity of the cells of the lips of the blastopore (primitive streak).

The upper layer of the blastoderm may now be called ectoderm, the lower layer endoderm, and the mesoderm extends out to the side between them. Gastrulation in the chick therefore does not involve invagination. The endoderm is formed precociously, probably so as to assist in digesting the enormous quantity of yolk.

Fig. 90. — Gallus : view of the blastoderm of a hen's egg after 20 hours' incubation, as seen by transmitted light. (After Jenkinson.) ao, area opaca ; ap, area pellucida ; av, area vasculosa ; hf, head-fold ; n, notochord ; nf, neural fold ; pa, proamnion ; ps, primitive streak.

Fig. 91. — Gallus : view of the blastoderm of a hen's egg after 24 hours' incubation, as seen by transmitted light. (After Jenkinson.) fg, foregut ; m, mesoderm ; ms, mesodermal somites ; tip, neuropore ; vv, vitelline vein. Other letters as Fig. 90.

Fig. 92. — Gallus : transverse sections through the blastoderm of a hen's egg after 24 hours' incubation. (From Jenkinson.) A, through the posterior ; B, through the anterior region of the embryo. en, endoderm ; m, mesoderm ; mg, neural groove ; mlp, unsegmented meso- derm of the lateral plate ; mvp, segmented mesoderm (somites) of the vertebral plate ; n, notochord. The posterior region of the embryo is at a less advanced stage of development than the anterior region.

Head-fold

A very important thing to notice is that the embryo in the chick will develop from only a part of the egg and blastoderm. The remainder will give rise to the membranes outside the embryo. At first, all that exists of the embryo proper is the middle line of its back represented by the primitive streak. Its sides are formed as this streak region rises and becomes folded up from the surface of the blastoderm around it. This process begins in front with the formation of the head-fold ; by this means the ectoderm, notochord, mesoderm, and endoderm are lifted off from the surface of the underlying yolk, and a cavity appears between the latter and the endoderm which represents the foregut- region of the enteron. As yet there is no floor to the gut, nor is the ventral side of the embryo formed at all. The mesoderm, lying on each side of the notochord becomes segmented into somites. That part which is nearest to the notochord will produce the myotomes ; farther laterally, a split arises in the mesoderm which becomes the ccelomic cavity, and which separates a somatic layer of mesoderm closely applied to the ectoderm from a splanchnic layer which is similarly applied to the endoderm. The ectoderm, mesoderm, and endoderm extends to the side far beyond the limits of the embryo, and so it comes about that the ccelomic cavity of the embryo is perfectly continuous with the " extra-embryonic " ccelom. As this extra-embryonic splanchnic mesoderm spreads out, blood-islands develop between it and the endo- derm. This is seen in blastoderms observed by transparency as the spreading of an area vasculosa over the area pellucida. Eventually this area vasculosa spreads over most of the blastoderm up to the germ-wall, except for a region immediately in front of the head-fold which is known as the proamnion. The peripheral extent of the area vasculosa is marked by a blood-vessel, the sinus terminalis.

Fig. 93. — Gallus : embryo chick after 30 hours' incubation seen by reflected light A from the dorsal, B from the ventral side. (After Jenkinson.) aip, anterior intestinal portal ; bi, blood-islands ; fb, forebrain ; hb, hind brain ; ht, heart ; mb, midbrain. Other letters as Figs. 90 and 91.

Fig. 94. — Gallus : longitudinal section through the head-region of an embryo chick after 30 hours' incubation. (From Jenkinson.) aip, anterior intestinal portal ; en, endoderm ; pc, pericardium ; pr, proamnion ; spc, spinal cord ; st, stomodaeum. Other letters as Figs. 90, 91, and 93. The lines marked 1 to 5 indicate the planes of the transverse sections shown in Figs. 95 and 96.

Nerve-tube

The neural plate develops as a thickening of the ectoderm along the axis of the embryo, in front of the primitive knot. At its sides are the neural folds which rise up and meet forming the neural tube. The first part of the neural tube to form is the brain, which is clearly marked out into the regions which will become the fore-, mid-, and hind- brain. As the primitive knot and streak move back, the neural folds follow them and cover up the spots which they formerly occupied, and so the neural tube comes to be formed immediately above the notochord. On each side of the neural tube, the neural crests come into existence, as in the frog. Even at very early stages, the rudiments of the eyes may be seen as outpushings to each side from the fore-brain. In front, the neural tube remains open for a time at the neuropore, which is situated at that part of the brain destined to become the lamina terminalis.

Fig. 95. — Transverse sections through an embryo chick after 30 hours' incubation. (From Jenkinson.) Taken in the planes shown by lines 1, 2, and 3 on Fig. 94. / is posterior. For lettering see Fig. 96.

Fig. 96. — Transverse sections through an embryo chick after 30 hours' incubation. (From Jenkinson.) Taken in the planes shown by lines 4 and 5 on Fig. 94. 5 is anterior. a, lateral dorsal aorta ; av, auditory vesicle ; c, coelom ; cv, cardinal vein ; ec, ectoderm ; en, endoderm ; fg, foregut ; ht, heart endothelium ; htm, mus- cular wall of heart ; kt, kidney tubule ; mg, neural groove ; ms, mesodermal somite ; mt, nerve-tube ; n, notochord ; nc, neural crest ; nt, nephrotome ; pc t pericardium ; so, somatopleur ; spl, splanchnopleur ; vv, vitelline vein.

Fig. 97. — Gallus : embryo chick after 36 hours' incubation seen by trans- mitted light, A from the dorsal, B from the ventral side. (After Jenkinson.) The heart is beginning to bend to the right, and the amniotic folds are covering over the head, af, amniotic fold ; as, auditory sac ; ov, optic vesicle; va, vitelline artery. Other letters as Figs. 90, 91, and 93.

Amnion. — In front of the head of the embryo, a fold rises up from the extra-embryonic part of the blastoderm. This fold extends backwards, and soon covers over the head. It now continues growing backwards by the upgrowth of folds on each side of the embryo, and soon covers over the latter completely, much in the same manner as the neural folds previously covered over the neural tube. The folds join above the embryo, which now finds itself in a sac, the amniotic cavity, covered over by two membranes of which the inner is the amnion and the outer is the chorion. Both these membranes are of course part of the extra- embryonic ectoderm, and the fact that there are two of them is due to the amniotic fold having two layers as it rises up. At the hind end, the amnion and chorion remain in contact at their point of fusion, forming the so-called sero-amniotic connexion. Extra- embryonic mesoderm gets carried up with the ectoderm in the amniotic fold, and forms a layer on the outer side of the amnion and on the inner side of the chorion. The space between the amnion and the chorion is therefore occupied by extra- embryonic coelomic cavity. The amniotic cavity is of course lined by ectoderm, and contains fluid. Although laid on dry land, therefore, the chick embryo develops in a fluid medium which may be said to be an artificial " pond," equivalent to the pond in which the (more aquatic) ancestors developed, as the frog now does. The embryo is also pro- tected by the amnion as by a water cushion from shocks and knocks to which the shell may be subjected, and from too sudden changes of temperature.

Fig. 98. — Gallus : embryo chick after 60 hours' incubation seen, A, from the dorsal side by transmitted light, B, from the ventral side by reflected light. (After Jenkinson.) The head-end has turned and is lying on its left side ; the heart is twist- ing ; the tail-fold has appeared, and the amniotic folds cover half the embryo, a, auricular portion of the heart ; e, eye ; /, lens ; tf, tail-fold ; v, ventricular portion of the heart ; vc, visceral cleft. Other letters as Figs. 91, 93, and 97.

The Gut

The base of the amnion grows in beneath the embryo, thus accentuating the folding off of the latter from the rest of the blastoderm. The head-fold has already been noticed, and as the base of the amnion grows in beneath it and backwards, a floor is formed for the most anterior region of the gut. In the same way at the posterior end, a tail-fold develops, and the base of the amnion growing in forms a floor for the hindmost region of the gut. The middle portion of the gut has as yet no floor, and is directly open to the surface of the yolk underneath ; its sides are formed, however, and it is known as the intestinal groove. The passage between the intestinal groove and the foregut, over the edge of the floor of the latter, is called the anterior intestinal portal. Similarly, at the hinder end a posterior intestinal portal is formed. At first the two intestinal portals are far apart, which is the same thing as saying that the formation of the floor of the gut has not yet proceeded very far back from the front end or forwards from the hinder end of the embryo. As development goes on, however, more and more of the floor of the gut and ventral wall of the embryo is formed, and the intestinal portals come close together leaving only a narrow opening between the cavity of the gut in the embryo and the yolk — the umbilicus.

Fig. 99. — Gallus : transverse sections through a chick embryo after three days' incubation through A the hinder, B, the middle, and C, the anterior regions of the trunk ; showing stages in the development of the amnion.

a, amnion ; ac, amniotic cavity ; af, amniotic fold ; c, coelom ; ch, chorion ; da, dorsal aorta ; ec, ectoderm ; eec, extra-embryonic coelom ; en, endoderm ; m, dorsal mesentery ; my, myotome ; n, notochord ; nc, nerve-cord ; pc, posterior cardinal vein ; sac, sero-amniotic connexion ; som, somatopleur ; spm, splanchnopleur ; vv, vitelline vein ; Wd, Wolffian duct.

Meanwhile, the extra-embryonic portion of the blastoderm has been extending down over the surface of the yolk, and eventually covers it completely except for a small aperture which is left open, and through which the yolk is separated from the albumen only by the vitelline membrane. The ectoderm of this region is continuous with and forms part of the chorion ; the endoderm, continuous with the endoderm of the embryo, now contains the yolk, and is known as the yolk-sac. There is a layer of mesoderm on the inner side of the ectoderm, and another on the outer side of the yolk-sac, so that the extra-embryonic coelomic cavity extends down into this region. The yolk-sac contains the store of nourishment for the developing embryo, and the yolk digested is brought into the embryo by the blood-vessels of the area vasculosa. The yolk-sac represents the heap of yolk-cells in the hinder part of the gut of the developing frog, but there is so much yolk that it cannot be accommodated inside the cavity of the gut as in that animal. Instead, it hangs in a sac beneath the gut, and gets gradually drawn up as its contents diminish, until, right at the end of development, it passes up through the umbilicus into the intestine of the embryo. Just before doing so, the albumen surrounding the chorion becomes con- tained in a sac formed by folds of the latter. This sac com- municates with the base of the yolk sac through the aperture which was left open, and it communicates also with the amniotic cavity by a reopening of the sero-amniotic connexion. The amniotic fluid and albumen are therefore able to pass into the yolk-sac and get absorbed.

Allantois

Shortly after the formation of the amnion a median ventral downgrowth is developed from the floor of the hind gut. This endodermal sac, covered on the outside by a layer of mesoderm, is the allantois and it represents the bladder of the frog. The allantois grows out into the extra- embryonic coelomic cavity, and its size increases as that of the yolk-sac diminishes. It soon occupies almost the entire space within the chorion which is not filled by amnion and yolk-sac. Its outer wall becomes applied to the inner surface of the chorion, and the extra-embryonic coelomic cavity between them disappears by the fusion of the two layers of mesoderm (on the inner side of the chorion and on the outer side of the allantois). This fused layer of allantois and chorion now lies close against the inner surface of the shell, only separated from it by the shell-membranes ; it is also highly vascular being supplied by blood-vessels which run out from the embryo along the allantoic stalk. As the shell is porous, the blood- vessels of the allantois form a region where oxygen is taken into and carbon dioxide is given off from the blood. The allantois therefore functions as a respiratory organ, and is of the highest importance. The gill-slits do not function as respiratory organs, for they communicate with the amniotic cavity in which the oxygen cannot be renewed.

Fig. 100. — Gallus : view of an embryo chick after four days' incubation, from the right side. aa, arterial arches ; ac, anterior cardinal ; al, allantois ; au, auricular portion of the heart ; cf, choroid fissure of the optic cup ; da, dorsal aorta ; dC, ductus Cuvieri ; dv, ductus venosus ; e, optic cup ; ic, internal carotid artery ; /, lens ; mb, midbrain ; pc, posterior cardinal ; t, tail ; ua, umbilical artery ; uv, umbilical vein ; v, ventricular portion of the heart ; va, vitelline artery ; vv, vitelline vein.

Fig. 101. Diagrams showing the formation and relations of the amnion, chorion, yolk-sac and allantois in the chick. (From Jenkinson.) 1, transverse section ; the amniotic folds are rising up, but the gut is not yet folded off from the yolk. 2, transverse section ; the amniotic folds have met and fused ; the amniotic cavity is closed and the amnion is separated off from the chorion ; the gut is beginning to be folded up from the yolk. 3, longitudinal section ; the amniotic folds are about to fuse ; the allantois is growing out from the hind gut of the embryo. 4, longitudinal section ; the amniotic cavity is closed and the amnion is separated off from the chorion except at the sero-amniotic connexion ; the yolk is now almost enclosed in a yolk-sac which remains open beneath ; the allantois is greatly enlarged, all, allantois ; am, amnion ; ante, amniotic cavity ; c, coelom ; jam, chorion ; ham, head-fold of the amnion ; lam, lateral amniotic fold ; sac, sero-amniotic connexion ; tarn, tail-fold of the amnion ; u, side wall of the gut ; y, yolk.

Another function of the allantois is excretion, for the Wolffian ducts run into the hind gut (cloaca) near its base, and it acts as a reservoir for the excretory products accumulated during the embryonic life. At the end of this period the allantois is not drawn up into the embryo, but nipped off at the umbilicus and left behind.

The relations of the embryonic membranes are simple to make out if it is remembered that mesoderm underlies ectoderm, and mesoderm overlies endoderm. All the boundaries of any particular cavity are continuous when traced right round, and are formed from one germ-layer. They are consistent in their arrangement, so that an endodermal cavity (gut, yolk-sac or allantois) cannot communicate with a cavity lined by mesoderm (ccelom), or with one lined by ectoderm (amnion, or atrium of Amphioxus).

Vascular System

At an early stage, the blood-islands of the area vasculosa become connected up with a pair of anterior vitelline veins which run towards the embryo from each side, and pass just in front of the anterior intestinal portal where they fuse in the middle line. This vessel, lying immediately beneath the floor of the fore gut, represents the subintestinal vein of the frog, and in this region it becomes differentiated into the heart. It is suspended from the floor of the fore gut by a mesentery, the dorsal mesocardium, and that part of the coelomic cavity into which it hangs will become the pericardium. At early stages, the heart is still outside the embryo, and it is not drawn into it until the base of the amnion has grown in beneath it. The anterior vitelline veins from the yolk-sac soon become replaced by the larger pair of posterior vitelline veins.

Running forwards from the heart, the aortic arches run up round the fore gut (pharynx) on each side passing between the gill-slits in the visceral arches. The hyomandibular and three pairs of branchial pouches are developed as outgrowths from the endoderm to the ectoderm. Of these, the hyomandibular and the first two branchials actually become perforated for a time, and place the cavity of the fore gut in communication with that of the amnion.

In the embryo, paired dorsal aortae develop, beneath the notochord. Anteriorly they connect with the aortic arches, and posteriorly at an early stage they simply spread out over the yolk-sac on each side forming the vitelline arteries. Later, the single median dorsal aorta arises by the fusion of the paired vessels (as far forwards as the pharynx), and it extends back behind the vitelline arteries into the tail as the latter is formed.

The aortic arch in the 4th visceral arch becomes the systemic (that on the left disappears), that in the 6th arch the pulmonary. The pulmonary arches also connect with the dorsal aorta by the ductus arteriosus.

The cardinal veins also arise as paired vessels, on each side of the aorta, and they communicate with the heart across a transverse septum by means of the ductus Cuvieri. Beneath the hind portion of the posterior cardinal veins, the subcardinal veins arise, in the region of the mesonephros. The sub- cardinal veins acquire connexion with the developing inferior vena cava.

As the anterior intestinal portal moves farther back in the embryo, the fusion of the two posterior vitelline, or omphale- mesenteric veins, with one another becomes more extensive. This combined vessel, which lies in the region of the developing liver and behind the heart, is known as the ductus venosus. At the hind end of the ductus venosus, the two posterior vitelline veins are separate, but farther back still they fuse together again twice : in one place dorsal to the gut, and in another place behind again, ventral to the gut. The piece on the left side between the dorsal point of fusion and the ductus venosus disappears ; the piece on the right side between the dorsal fusion and the ventral fusion behind it disappears also. The net result of all these modifications is that the vitelline veins run into the embryo on each side from the yolk- sac and join beneath the gut. From this point a single vein runs forwards and makes one complete twist round the gut in the direction of the thread of a corkscrew, and runs into the ductus venosus. Part of these posterior vitelline veins becomes the hepatic portal vein, and the ductus venosus becomes the hepatic veins and the base of the inferior vena cava.

The allantois is supplied with blood by the umbilical arteries (branches from the artery to the hind leg), and drained by the umbilical veins. These run in the side wall of the abdominal cavity and correspond to the lateral abdominal veins of the dogfish. At first they run into the ductus Cuvieri of their side, but later the right umbilical vein is reduced and the left one runs into the ductus venosus. At all events, the blood from the allantois, where it has been oxygenated, runs into the right auricle of the heart when the latter becomes subdivided by interauricular and interventricular septa. The left auricle receives the pulmonary veins. The lungs are, how- ever, not functional, and by the breaking down of the septum between the auricles, part of the blood from the allantois is able to get from the right to the left auricle, and so to the left ventricle and the systemic arch without going through the right ventricle. The remainder of the blood passes through the right ventricle to the pulmonary arches, and from them through the ductus arteriosus to the dorsal aorta. Little if any goes to the lungs. The interauricular septum is reconstituted after hatching, when the lungs are open and functional, and oxygenated blood reaches the left auricle from the lungs. The interventricular septum is complete.

Kidneys

With the exception of the urino-genital organs, the remaining organs of the chick develop in a manner very similar to that which obtains in the frog, and no useful purpose would be served by going over them in detail. The limbs may be mentioned, since they show in their development certain characters which are more primitive than the definitive adult condition ; and especially the urino-genital system is worthy of note. Not only does it differ in some respects radically from that of the frog, but in others it shows certain features more clearly.

In some half a dozen segments in the anterior region of the trunk, the intermediate cell-mass or nephrotome of each segment pushes out a rod of cells which curves backwards and meets and fuses with the similarly produced rod from the segment next behind it. Each of these rods represents a tubule of the pronephros ; they are solid instead of being hollow and opening into the ccelom by ciliated funnels, because the pronephros is degenerate in the chick, and does not even function as an excretory organ. However, the rod of cells formed by the pronephros on each side in this way grows back to the cloaca, and later becoming hollow, forms the pronephric duct.

In about a dozen segments, behind the pronephros, the mesonephric tubules develop. Like the pronephros, they are segmental in origin, and as they grow out from the intermediate cell-mass, they find the pronephric duct so to speak ready-made for them. They connect with it which now becomes known as the mesonephric or Wolffian duct. The mesonephric tubules do not as a rule open into the splanchnocoelic cavity ; they have no ccelomic funnels. The tubules give rise by branching to a number of Bowman's capsules, and each be- coming vascularised by a glomerulus becomes a Malpighian corpuscle. The mesonephros is the functional kidney of the embryo, and is therefore present in the early life of both sexes. Later, it is preserved only in the male, for it establishes con- nexion with the testis by means of the vasa efferentia, and the Wolffian duct functions as the vas deferens. It disappears in the female.

The kidney of the adult is the metanephros. It is formed after the mesonephros, and at a time when the segmental arrangement of the intermediate cell-mass has been lost. A diverticulum develops from the base of the Wolffian duct, near the cloaca ; this is the ureter. The ureter grows forwards dorsal to the mesonephros, and branches repeatedly forming a large number of collecting tubules. The metanephric tubules, or Bowman's capsules, arise from an indistinct heap of cells of the intermediate cell-masses belonging to one or two segments of the hinder region of the body, and called simply the metanephrogenous tissue. These capsules hollow out and connect with the collecting tubules which were formed by the branching of the ureter. Each capsule is vascularised by a glomerulus, and forms a Malpighian corpuscle. The meta- nephric tubules never have ccelomic funnels.

So long as the mesonephros functions as an excretory organ, there is a renal portal system formed by the hinder region of the posterior cardinal veins. These veins filter through the mesonephros, and are collected into the so-called subcardinal veins, which contribute to the formation of the inferior vena cava. The renal portal system disappears with the excretory function of the mesonephros ; meanwhile the metanephros has developed and renal veins connect its glomeruli to the inferior vena cava. The posterior cardinals then run into these renal veins.

The Mullerian ducts develop in both sexes as grooves in the coelomic epithelium which become closed over to form tubes, and these tubes grow back to the cloaca. In the male, both these ducts disappear ; in the female the right duct is lost and that on the left side persists as the definitive oviduct.

Limbs

The limbs appear as buds at a stage relatively earlier than that at which they arise in the frog. Their interest lies in the fact that the cartilaginous skeleton of the limbs in larvas reflects the primitive condition, before the modifications of the wings and the mesotarsal joints arose. In the wrist, the distal carpals and the metacarpals are at first separate ; there is as yet no carpo-metacarpus. There are also vestiges of the i st and 5th digits, so that the wing at this stage resembles a more normal pentadactyl fore limb. Similarly in the hind limb, the proximal tarsal cartilages are not yet fused on to the tibia to form a tibio-tarsus, neither are the distal tarsals yet joined on to the metatarsals to give rise to a tarso-metatarsus.

The early stages of the pelvic girdle are of great interest, for the pubis when it first arises in cartilage points forwards and downwards ; it is only later that it extends back beneath the ischium.

Fig. 102. Gallus : diagram showing the final relations of the embryonic membranes. (From Jenkinson, after Duval and Lillie.) ach, air-chamber ; all, allantois the outer wall of which is closely- pressed against the shell-membrane ; amc, amniotic cavity which com- municates with the (as) albumen sac through the reopened (sac) sero- amniotic connexion ; ast, allantoic stalk ; c, extra-embryonic ccelom ; sh y shell ; y, yolk in the yolk-sac which still communicates with the albumen- sac ; x, point at which the yolk-sac will eventually close.

Fig. 103. Gallus : view of the face of a chick after four days' incubation. cf, choroid fissure of the optic cup ; e, optic cup ; fnp, fronto-nasal process ; ha, hyoid arch ; /, lens ; Ing, lachrymo-nasal groove ; Inp, lateral nasal process ; m, mouth ; ma, mandibular arch ; mp, maxillary process ; ns, nasal sac.

Fig. 104. Longitudinal section through an embryo showing the develop- ment of the metanephros.

(Actually, this section is of a mammal, not a bird, but the difference is immaterial.) The ureter arises as an outgrowth from the Wolffian duct. ce, ccelomic epithelium ; m, myotomes ; me, metanephrogenous tissue ; mt, mesonephric tubules ; u, ureter ; Wd, Wolffian duct.

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Fig. 105. — Longitudinal section through a metanephric kidney.

Showing the Malpighian corpuscles (Mc) in communication with the ureter (w) by means of the collecting tubules (ct) ; ra, renal artery.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Fig. 106. — Gallus : views of the (cartilaginous) skeleton of the limbs and girdles of embryo chicks, A and B, after 5 days' C and D after 9 days' incubation, as seen from the left side.

A and C, pectoral girdle and limb ; B and D, pelvic girdle and limb. Note that in the earlier stage the pubis points forwards, c, coracoid ; ca, carpals ; dt, distal tarsals (which will fuse with the metatarsals to form the tarso-metatarsus) ; fe, femur ; fi, fibula ; h t humerus ; i, ilium ; is, ischium ; mc, metacarpal ; mt, metatarsal ; p, phalanx ; pt, proximal tarsals (which will fuse with the tibia to form the tibio-tarsus) ; pu, pubis ; r, radius ; s, scapula ; t, tibia ; u, ulna.

Hatching

Meanwhile the mouth and the anus have broken through into the gut as the result of the sinking in of the stomodaeum and proctodeum. As the chick grows during its development, its position changes to accommodate it to the surrounding membranes. At an early stage it turns and lies on its left side, and later, its body lies along the long axis of the egg with its head near the blunt end of the shell, which is where the air-chamber is situated. Its beak pierces the inner shell-membrane, and it begins to breathe the air in the air-chamber into its lungs, often making the characteristic " peep peep " sound. The connexions between the pulmonary arteries and the dorsal aorta (ductus arteriosus) disappear, and more and more blood passes through the lungs. The yolk-sac has been completely absorbed within the body. The beak of the upper jaw bears a sharp projection, the egg-tooth, with which the chick pierces the shell, and soon after, it emerges, having severed its connexion with the allantois. The septum between the auricles of the heart is reformed, and the chick now lives in the same manner as the adult bird.

Feathers

Feathers begin developing at about the seventh day of incubation and the first sign of their appearance is in the form of a thickening of the epidermis overlying a con- densation of the dermis, and forming a papilla. The rudiment of the feather soon becomes conical and eventually takes the form of an elongated cylinder. The papilla at its base becomes sunk beneath the general level of the skin forming a follicle. The deepest layer of the epidermis differentiates into a number of longitudinal thickened ridges, two of which will become the rachis, and the remainder will give rise to the barbs which come off from the rachis. At this stage they are still rolled up inside the cylinder and covered by the outer layer of epidermis forming the feather-sheath. The central dermis is nutritive in function, and eventually degenerates. The vane of the feather is formed by the shedding of the sheath, the splitting of the cylinder on the side opposite the rachis, and the flattening out of the barbs which have thus been released, on each side of the rachis. The former cylindrical nature of the feather is betrayed by the presence of a hole at the base of the quill — the inferior umbilicus — and another at the bottom of the vane (between it and the aftershaft), the superior umbilicus. The aftershaft represents the thickenings of deeper layers of the epidermis on the side of the cylinder opposite the rachis, and below the lowermost barbs belonging to the rachis.

The feathers of the adult bird, pennae, plumulae, and filoplumes, are typically preceded by " nestling-down " in the form of prepennae, preplumulae, and prefiloplumes respectively. There are two generations of prepennae, but in the majority of birds it is the first generation which forms the nestling- down, and the second is reduced. Both generations are present in the penguins.

The nestling- down feathers are carried out on the tip of as their predecessors, and when the adult feathers are properly formed, the nestling-down is worn off.

Fig. 107. — Sections and diagrams showing the development of feathers.

A, section through an early papilla ; F, slightly later stage in which the sides of the papilla are beginning to sink beneath the surface ; C, longitudinal section through a young feather in which the epidermal thickenings are present, but the feather-sheath has not yet disappeared ; D, transverse section of C ; E, the feather-sheath has been shed from the end of the feather and the barbs are freed ; F, diagram showing the relations of the rachis, barbs, and aftershaft ; G, view of an adult feather, a, aftershaft ; b, barbs ; c, calamus ; e, ectoderm ; fs, feather-sheath ; iu, inferior um- bilicus ; m, mesoderm ; p, papilla ; r, rachis ; su, superior umbilicus ; t, ectodermal thickenings (which will give rise to the barbs) ; v, vane.

the adult feathers, for the latter grow from the same papillae

Literature

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

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

Lillie, F. R. The Development of the Chick. Henry Holt, New York, 1919.

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Cite this page: Hill, M.A. (2024, April 28) Embryology Book - Vertebrate Zoology (1928) 13. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Vertebrate_Zoology_(1928)_13

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