Book - The Elements of Embryology - Chicken 9
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From the sixth day to the end of incubation
THE sixth day marks a new epoch in the development of the chick, for distinctly avian characters then first make their appearance.
Striking and numerous as are the features, which render the class Aves one of the most easily recognizable in the whole animal kingdom, the embryo of a bird does not materially differ in its early phases from that of a reptile or a mammal, even in the points of structure which are most distinctively avian. It may, it is true, be possible to infer, even at a comparatively early stage, from some subsidiary tokens, whether any given embryo belongs to this class or that (and indeed the same inference may be drawn from the ovum itself) ; but up to a certain date it is impossible to point out, in the embryo of the fowl, the presence of features which may be taken as broadly characteristic of an avian organization. This absence of any distinctive avian differentiation lasts in the chick roughly speaking till the commencement of the sixth day.
We do not mean that on the sixth day all the organs suddenly commence to exhibit peculiarities which mark them as avian. There are no strongly marked breaks in the history of development; its course is perfectly gradual, and one stage passes continuously into the next. The sixth and seventh days do however mark the commencement of the period in which the specialization of the bird begins to be apparent. Then for the first time there become visible the main features of the characteristic manus and pes ; the crop and the intestinal caeca make their appearance; the stomach takes on the form of a gizzard ; the nose begins to develope into a beak ; and the commencing bones of the skull arrange themselves after an avian type. Into these details we do not propose to enter, and shall therefore treat the history of the remaining days with great brevity.
We will first speak of the FOETAL APPENDAGES.
Day 6 and 7
On the sixth and seventh days these exhibit changes which are hardly less important than the events of previous days.
The amnion at its complete closure on the fourth day very closely invested the body of the chick; the true cavity of the amnion was at that time therefore very small. On the fifth day fluid begins to collect in the cavity, and raises the membrane of the amnion to some distance from the embryo. The cavity becomes still larger by the sixth day, and on the seventh day is of very considerable dimensions, the fluid increasing with it. On the sixth day Von Baer observed movements of the embryo, chiefly of the limbs; he attributes them to the stimulation of the cold air on opening the egg.
By the seventh day very obvious movements begin to appear in the amnion itself; slow vermicular contractions creep rythmically over it. The amnion in fact begins to pulsate slowly and rythmically, and by its pulsation the embryo is rocked to and fro in the egg. This pulsation is due probably to the contraction of involuntary muscular fibres, which seem to be present in the attenuated portion of the mesoblast, forming part of the amniotic fold. (Of. Chap. II. p. 45.) Similar movements are also seen in the allantois at a considerably later period.
The growth of the allantois has been very rapid, and it forms a flattened bag, covering the right side of the embryo and rapidly spreading out in all directions, between the primitive folds of the amnion, that is between the amnion proper and the false amnion (serous membrane). It is filled with fluid, so that in spite of its flattened form its opposite walls are distinctly separated from each other.
The vascular area has become still further extended than on the previous day, but with a corresponding loss in the definite character of its blood-vessels. The sinus terminalis has indeed by the end of the seventh day lost all its previous distinctness, and the vessels which brought back the blood from it to the heart are no longer to be seen.
Both the vitelline arteries and veins now pass to and from the body of the chick as single trunks, assuming more and more the appearance of being merely branches of the mesenteric vessels.
The yolk is still more fluid than on the previous day, and its bulk has (according to Yon Baer) increased.
This can only be due to its absorbing the white of the egg, which indeed is diminishing rapidly.
Day 8 to 10
During the eighth, ninth, and tenth days the aninion does not undergo any very important changes. Its cavity is still filled with fluid, and on the eighth day its pulsations are at their height, henceforward diminishing in intensity.
The splitting of the mesoblast has now extended to the outer limit of the vascular area, viz. over about three quarters of the yolk-sac. The somatopleure at this point is continuous (as can be easily seen by reference to Fig. 9) with the original outer fold of the amnion.
It thus comes about that the further splitting of the mesoblast merely enlarges the cavity in which the allantois lies. The growth of this organ keeps pace with that of the cavity in which it is placed. Spread out over the greater part of the yolk-sac as a flattened bag filled with fluid, it now serves as the chief organ of respiration.
Hence it is very vascular, the vessels on that side of the bag which is turned to the serous membrane and shell being especially large and numerous.
The yolk now begins to diminish rapidly in bulk. The yolk-sac becomes flaccid, and on the eleventh day is thrown into a series of internal folds, abundantly supplied with blood-vessels. By this means the surface of absorption is largely increased, and the yolk is more and more rapidly taken up by the blood-vessels, and in a partially assimilated condition transferred to the body of the embryo.
By the eleventh day the abdominal parietes though still much looser and less firm than the walls of the chest may be said to be definitely established, and the loops of intestine, which have hitherto been hanging down into the somatic stalk, are henceforward confined within the cavity of the abdomen. The body of the embryo is therefore completed; but it still remains connected with its various appendages by a narrow somatic umbilicus, in which run the stalk of the allantois and the solid cord suspending the yolk-sac.
The cleavage of the mesoblast still progressing, the yolk is completely invested by the (splanchnopleuric) yolk-sac except at the pole opposite to the embryo, where for some little time a small portion remains unenclosed ; at this spot the diminished white of the egg adheres as a dense viscid plug.
The allantois meanwhile spreads out rapidly, and lies over the embryo close under the shell, being separated from the shell membrane by nothing more than an attenuated membrane, the serous membrane, formed out of the outer primitive fold of the amnion and the remains of the vitelline membrane. With this serous membrane the allantois partially coalesces, and in opening an egg at the later stages of incubation, unless care be taken the allantois is in danger of being torn in the removal of the shell membrane. As the allantois increases in size and importance, the allantoic vessels are correspondingly developed. They are very conspicuous when the egg is opened, the pulsations of the allantoic arteries at once attracting attention.
On about the sixteenth day, the white having entirely disappeared, the cleavage of the mesoblast is carried right over the pole of the yolk opposite the embryo, and is thus completed (Fig. 9). The yolk-sac now, like the allantois which closely wraps it all round, lies loose in a space bounded outside the body by the serous membrane, and continuous with the pleuroperitoneal cavity of the body of the embryo. Deposits of urates now become abundant in the allantdic fluid.
The loose and flaccid walls of the abdomen enclose a space which the empty intestines are far from filling, and on the nineteenth day the yolk-sac, diminished greatly in bulk but still of some considerable size, is withdrawn through the somatic stalk into the abdominal cavity, which it largely distends. Outside the embryo there remains nothing now but the highly vascular allantois and the practically bloodless serous membrane and amnion. The amnion, whose fluid during the later days of incubation rapidly diminishes, is continuous at the umbilicus with the body-walls of the embryo. The serous membrane (or outer primitive amniotic fold) is by the completion of the cleavage of the mesoblast and the invagination of the yolk-sac, entirely separated from the embryo. The cavity of the allantois by means of its stalk passing through the umbilicus is of course continuous with the cloaca.
In the EMBRYO itself a few general points only deserve notice.
Day 6 and 7
By the sixth or seventh day the flexure of the body has become less marked, so that the head does not lie so near to the tail as on the previous days ; at the same time a more distinct neck makes its appearance.
Though the head is still disproportionately large, its growth ceases to be greater than that of the body.
Up to this period the walls of the somatic stalk have remained thin and flaccid, almost membranous in fact, the heart appearing to hang loosely out of the body of the embryo. About this time however the stalk, especially in front, rapidly narrows and its mesoblast becomes thickened. In this way the heart and the other thoracic viscera are enclosed by definite firm chest walls, along the sides of which the ribs grow forwards and in front of which the cartilaginous rudiments of the sternum appear.
The abdominal walls are also being formed, but not to the same extent, and the stalk of the allantois still passes out from the peritoneal cavity between the somatic and the splanchnic stalks.
In the brain one of the most marked features is the growth of the cerebral hemispheres. The median division between these has in front increased in depth, so that the lateral ventricles are continued forwards as two divergent horns, while backwards they are also continued as similar divergent horns separated from one another by the vesicle of the third ventricle.
We propose to treat more fully of the development of the brain in the second part of this work, the importance of the mammalian brain rendering it undesirable to go too much into the details of the brain of the bird.
All the visceral clefts are closed by the seventh day. It will be remembered that the inner part of the first cleft persists as the Eustachian tube (p. 166).
The structures which surround the mouth are beginning to become avian in form, though the features are as yet not very distinctly marked.
The tongue has appeared on the floor of the mouth as a bud of mesoblast covered by epiblast.
Day 8 to 10
During the eighth, ninth, and tenth days the embryo grows very rapidly, the head being still especially large, and at the same time becoming more round, the mid-brain not being so prominent.
Day 11 onwards
From the eleventh day onwards the embryo successively puts on characters which are not only avian, but even distinctive of the genus, species and variety.
So early as the ninth or tenth day the sacs containing the feathers begin to protrude from the surface of the skin as papillae, especially prominent at first along the middle line of the back from the neck to the rump, and over the thighs, the sacs of the tail feathers being very conspicuous. On the thirteenth day these sacs, generally distributed over the body, and acquiring the length of a quarter of an inch or more, appear to the naked eye as feathers, the thin walls of the sacs allowing their contents, now coloured according to the variety of the bird, to shine through. They are still however closed sacs, and indeed remain such even on the nineteenth day, when many of them are an inch in length.
Feathers are epidermal structures. They arise from an induration of the epidermis of papillae containing a vascular core
On the eighth day a chalky-looking patch is observable on the tip of the nose. This by the twelfth day has become developed into a horny but still soft beak.
On the thirteenth day, nails are visible at the extremities, and scales on the remaining portions of the toes. These on the sixteenth day become harder and more horny, as does also the beak.
Nails are developed on special regions of the epidermis, known as the primitive nail beds. They are formed by the cornification of a layer of cells which makes its appearance between the horny and mucous layers of the epidermis. The distal border of the nail soon becomes free, and the further growth is effected by additions to the under side and attached extremity of the nail.
By the thirteenth day the cartilaginous skeleton is completed and the various muscles of the body can be made out with tolerable clearness.
Ossification begins according to Von Baer on the eighth or ninth day by small deposits in the tibia, in the metacarpal bones of the hind-limb, and in the scapula. On the eleventh or twelfth day a multitude of points of ossification make their appearance in the limbs, in the scapular and pelvic arches, in the ribs, in the bodies of the cervical and dorsal vertebrae and in the bones of the head, the centres of ossification of the vertebral arches not being found till the thirteenth day.
The events which we have thus briefly narrated are accompanied by important changes in the arterial and venous systems.
The condition of the venous system at about the end of the third day was fully described in Chap. VI. p. 170, and the changes which have taken place between that date and the latter days of incubation may be seen by comparing the diagram Fig. 58 with the diagrams Figs. 89 and 90.
On the third day, nearly the whole of the venous blood from the body of the embryo was carried back to the heart by two main venous trunks, the superior (Fig. 58, J) and inferior (Fig. 58, (7) cardinal veins, joining on each side to form the short transverse ductus Cuvieri, both which in turn united with the sinus venosus close to the heart. As the head and neck continue to enlarge and the wings become developed, the single superior cardinal or jugular vein, as it is usually called (Figs. 89, 90, J), of each side, is joined by two new veins : the vertebral vein (Su. J. F.), bringing back blood from the head and neck, and the vein from the wing ( W).
FIG. 89. DIAGRAM OF THE VENOUS CIRCULATION AT THE COMMENCEMENT OF THE FIFTH DAT.
H. heart, d.c. ductus Cuvieri. Into the ductus Cuvieri of each side fall J. the jugular vein, W. the vein from the wing and C. the inferior cardinal vein. S.V. sinus venosus. Of. vitelline vein. U. allantoic vein, which at this stage gives off branches to the body- walls. V.C.I, inferior vena cava. l. liver.
The inferior cardinal veins have their roots in the Wolffian bodies; they become developed, pari passu, with those organs, and may be called the veins of the Wolffian bodies. On the third day they are the only veins which bring the blood back from the hinder part of the body of the embryo.
About the fourth or fifth day, however, a new single venous trunk, the vena cava inferior (Fig. 89, V.G.I.\ makes its appearance in the middle line, in a plane more dorsal than that of the cardinal veins. This, starting from the sinus venosus not far from the heart, is on the fifth day a short trunk running backward in the middle line below the aorta, and speedily losing itself in the tissues above the Wolffian bodies. When the kidneys are formed it receives blood from them, and thenceforward enlarging rapidly eventually becomes the channel by which the greater part of the blood from the hind limbs and the hinder part of the body finds its way to the heart. In proportion as this vena cava inferior increases in size, and the Wolffian bodies give place to the permanent kidneys, the posterior cardinal veins diminish. The blood originally coming to the posterior cardinals from the posterior part of the spinal cord and trunk is transported into two posterior vertebral veins; which are placed dorsal to the heads of the ribs and join the anterior vertebral veins. With the appearance of these veins the anterior part of the posterior cardinals disappears.
At its first appearance the vena cava inferior may be considered as a branch of the trunk which we have called the sinus venosus, but as development proceeds, and the vena cava becomes larger and larger, the sinus venosus assumes more and more the appearance of being merely the cardiac termination of the vena cava, and the ductus venosus from the liver may now be said to join the vena cava instead of being prolonged into the sinus. While this growth of the vena cava is going on, the points at which the ductus Cuvieri enter into the sinus venosus are drawn in towards the heart itself, and finally these trunks fall directly and separately into the auricular cavities, and are henceforward known as the right and left vena cava superior (Fig. 90, V.S.R., V.8.L.). There are therefore, when these changes have been effected, three separate channels, with their respective orifices, by which the blood of the body is brought back to the heart, viz. the right and left superior and the inferior venae cavae.
FIG. 90. DIAGRAM OF THE VENOUS CIRCULATION DURING THE LATER DATS OF INCUBATION.
H. heart. V.S.R. right vena cava superior. V.S.L. left vena cava superior. The two venae cavse superiores are the original c ductus Cuvieri,' they still open into the sinus venosus and not independently into the heart. J. jugular vein. $7. V. superior vertebral vein. In. V. inferior vertebral vein. W. vein for the wing. V.C.I, vena cava inferior, which receives most of the blood from the inferior extremities, etc. D. V. ductus venosus. P. V. portal vein. M. a vein bringing blood from the intestines into the portal vein. Of. vitelline vein. U. allantoic vein. The three last mentioned veins unite together to form the portal vein. I. liver.
The remnants of the inferior cardinal veins are not shewn.
While the auricular septum is as yet unformed, the blood from these veins falls into both auricles, perhaps more into the left than into the right. As the septum however grows up, the three vessels become connected with the right auricle only while the left receives the two pulmonary veins coming from the lungs. (Compare Chap. VII. p. 228).
On the third day the course of the vessels from the yolk-sac is very simple. The two vitelline veins, of which the right is already the smaller, form the meatus venosus from which, as it passes through the liver on its way to the heart, are given off the two sets of venae advehentes and venae revehentes.
With the appearance of the allantois on the fourth day, a new feature is introduced. From the meatus venosus, a short distance behind the liver, there is given off a vein which quickly divides into two branches. These, running along the ventral side of the body from the walls of which they receive some amount of blood, pass to the allantois. They are the allantoic or umbilical veins. The single vein which they unite to form becomes, by reason of the rapid growth of the allantois, very long ; and hence it is perhaps better to speak of it as the allantoic vein (Fig. 90, U}. The right branch soon diminishes in size and finally disappears. Meanwhile the left on reaching the allantois bifurcates ; and, its two branches becoming large and conspicuous, there still appear to be two main allantoic veins uniting at a short distance from the allantois to form the single long allantoic vein. At its first appearance the allantoic vein seems to be but a small branch of the vitelline, but as the allantois grows rapidly, and the yolk-sac dwindles, this state of things is reversed, and the less conspicuous vitelline appears as a branch of the larger allantoic.
On the third day the blood returning from the walls of the intestine is insignificant in amount. As however the intestine becomes more and more developed, it acquires a distinct venous system, and the blood sent to it by branches of the aorta is returned by veins which form a trunk, the mesenteric vein (Fig. 90, Jtf ), falling into the vitelline vein at its junction with the allantoic vein.
These three great veins in fact, viz. the vitelline, the allantoic, and the mesenteric, form a large common trunk which enters at once into the liver, and which we may now call the portal vein (Fig. 90, P. F.). This, at its entrance into the liver, partly breaks up into the venae advehentes, and partly continues as the ductus venosus straight through the liver, emerging from which it joins the vena cava inferior. Before the establishment of the vena cava inferior, the venae revehentes, carrying back the blood which circulates through the hepatic capillaries, joined the ductus venosus close to its exit from the liver (Fig. 89). By the time however that the vena cava has become a large and important vessel it is found that the venae revehentes or as we may now call them the hepatic veins have shifted their embouchment and now fall directly into that vein, the ductus venosus making a separate junction rather higher up (Fig. 90).
This state of things continues with but slight changes till near the end of incubation, when the chick begins to breathe the air in the air-chamber of the shell, and respiration is no longer carried on by the allantois. Blood then ceases to flow along the allantoic vessels; they become obliterated. The vitelline vein, which as the yolk becomes gradually absorbed proportionately diminishes in size and importance, comes to appear as a mere branch of the portal vein. The ductus venosus becomes closed, remaining often as a mere ligament; and hence the whole of the blood coming through the portal vein flows into the substance of the liver, and so by the two hepatic veins into the vena cava (Fig. 91, HP).
Previous to these changes one of the veins passing from the rectum into the vena cava has given off a branch which effects a junction with one of the mesenteric veins. This now forms a somewhat conspicuous connecting branch between the systems of the vena cava and the portal vein (Fig. 91, Cy. M.).
All three venae cavae now fall exclusively into the right auricle, and by the closure of the foramen ovale the blood flowing through them is entirely shut off from the left auricle, into which passes the blood from the two pulmonary veins (Fig. 91, L. V.).
Such is the history of the veins in the chick. As will be seen in the second part of this work, the course of events in the mammal, though in the main similar, differs in some unimportant respects.
It remains for us to speak of the changes which have in the meantime been taking place in the arterial system. The condition of things which exists on the fifth or sixth day is shewn in the diagram (Fig. 92).
FIG. 91. DIAGRAM OF THE VENOUS CIRCULATION OF THE CHICK AFTER THE COMMENCEMENT OF KESPIRATION BY MEANS OF THE LUNGS.
W. wing vein. J. jugular vein. Su. V. superior vertebral vein. In. V. inferior vertebral vein. These unite together on each side to form the corresponding superior vena cava. L. V. pulmonary veins. V.C.L vena cava inferior. H.P. hepatic veins. P. V. portal vein. M. mesenteric veins. Gy.M. connecting vessel between the branches of the portal vein and the system of the vena cava inferior. It is called the coccygeo-mesenteric vein, and unites the cross branch connecting the two hypogastrics with the mesenteric vein. The ductus venosus has become obliterated. The three vense cavse fall independently into the right auricle and the pulmonary veins into the left auricle. Cr. crural vein. Jc. kidney. 1. liver, pp. hypogastric veins. C. V. caudal vein.
FIG. 92. STATE OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH DAY.
E.G. A. external carotid. I.C.A. internal carotid. D.A. dorsal aorta. Of. A. vitelline artery, U. A. allantoic artery.
We have already seen (Chap. vn. p. 225) that of the three aortic arches which make their appearance on the third day, the first two disappear : the first on the fourth, the second on the fifth day ; but that their disappearance is accompanied by the formation behind them of two new aortic arches, the fourth and the fifth. Thus there are generally three, never more than three, pairs of aortic arches present and functional at one time.
This statement needs some limitation ; for according to Von Baer there are four arches present both on the fourth and fifth days. In the case of the fourth day a slight remnant of the first pair of arches still persists when the fourth pair is already formed ; and on the fifth day the second pair has not entirely disappeared when the fifth pair is formed. In both of these cases however the first pair of arches of the four is only present for a very short time, and then is so diminished in size as to be of no importance.
The first pair of arches, before it entirely disappears, sends off on each side two branches towards the head. Of these, one forms the direct continuation of the bulbus arteriosus in a straight line from the point where the first aortic arch leaves it ; primarily distributed to the tongue and inferior maxillary region, it becomes the external carotid (Fig. 92, E.G. A.}. The other, starting from the point where the aortic arch of each side joins its fellow, dorsal to the alimentary canal, to form the dorsal aorta, is primarily distributed to the brain, and becomes the internal carotid (Fig. 92, I.G.A.).
When the first arch disappears, the external carotid arteries still remain as the anterior continuations of the bulbus arteriosus. And since the dorsal trunks uniting the distal ends of the first and second arches do not become obliterated at the time when the first pair of arches disappears, the internal carotids remain as branches springing from the distal ends of the second pair of arches ; they are supplied with blood from that pair, the stream in which flows chiefly towards the head instead of backwards towards the dorsal aorta, as is the case with the succeeding arches. When the second pair of arches is obliterated, the connecting branch with the next arch is again left, and thus the internal carotids appear as branches from the distal ends of the third pair of arches.
On the third day the dorsal aorta does not for any distance remain single in its backward course along the body, but soon divides into two trunks which run one on either side of the middle line of the body. These two trunks, as development proceeds, gradually unite along their whole length, and there is thus formed a single median aorta terminating behind in the caudal artery (Figs. 92, 94). The arteries to the kidneys, hind limbs, etc. are developed as branches of this aorta.
As the allantois grows rapidly and becomes an important respiratory organ, the allantoic or umbilical arteries increase in size. As a general, though apparently not invariable rule, the right allantoic artery gets gradually smaller and soon disappears.
The vitelline artery (Of. -A.) now leaves the aorta as a single but quickly bifurcating trunk, which at the end of the fifth day is still very large.
By the fifth day the ventricular portion of the heart (compare Chap. vn. p. 257) is completely divided into two chambers. The bulbus arteriosus is also divided by a septum into two channels, one of which communicates with the right ventricle of the heart and the other with the left.
One result of this arrangement is that all the blood which passes to the anterior extremity of the body comes from the left ventricle of the heart.
At about the seventh day an entire separation begins to take place between the arterial roots which come respectively from the right and left chambers of the heart. The root from the right chamber (Fig. 93) remains connected with the fifth pair of arches. The root from the left ventricle is connected with the third and fourth pairs of arches.
The lower part of the body still receives blood from both the right and left ventricles, since the blood which enters the fifth arch still flows into the common dorsal aorta. As the lungs however increase in size, a communication is set up between them and the fifth pair of arches in the shape of two vessels which, springing one from the arch of each side, grow downwards towards the lungs. At first small and narrow, these pulmonary arteries, for such they are, grow rapidly larger and larger, so that more and more of the blood from the right ventricle is carried to the lungs.
FIG. 93. DIAGRAM OF THE CONDITION OF THE ARCHES OF THE AORTA TOWARDS THE CLOSE OF INCUBATION.
i, 2, 3, 4, 5. the several aortic arches. E.G. A. external carotid. I.C. A. internal carotid. C.C.A. common carotid. V.a. vertebral artery. R.sc. right subclavian. L.sc. left subclavian. R.P., L.P. right and left pulmonary arteries. R.P.A. right arterial root or division of the bulbus arteriosus, or pulmonary artery ; the left root or division, constituting the aorta, is seen by its side. The system of the fifth arch is in lighter shading. The dotted lines shew the portions of the arches which have been obliterated. +++++++++++++++++++++++++++++++++
At the same time the connection between the third and fourth pairs of arches on each side grows weaker ; so that less and less of tlie blood which flows along the third pair of arches is able to pass backwards to the hind end of the body.
The fourth arch of the right side now becomes the most important of all the arches ; and nearly the whole of the blood supplying the hinder parts of the body passes through it. It is this arch which remains as the permanent aortic arch of the adult ; and it is important to notice that the arch which forms the great dorsal aorta in birds is the fourth on the right side, and not as in mammals the fourth on the left side. The fourth arch of the left side in birds, after giving off the subclavian, is continued as an exceedingly small and unimportant vessel to join the fourth right arch. It is soon obliterated.
In consequence of these changes the condition of the aortic arches during the latter days of incubation, before respiration by the lungs has commenced, is as follows (Fig. 93).
The first and second arches are completely obliterated. The third arch on each side is continued at its dorsal end as the internal carotid, I.C.A, the connection between it and the fourth arch having become entirely obliterated. From its ventral end as the direct continuation of the trunk which originally supplied the first and second arches the external carotid, E. G.A., is given off. Each pair of carotids arises therefore from a common trunk the common carotid (C.C.A.). Each of these trunks gives off near its proximal end a branch, the vertebral artery (F.a.).
The common carotid on the right side comes off from the fourth arch of the right side (the arch of the dorsal aorta), and is not as yet connected with the right subclavian, R.sc. The common carotid of the left side comes off from the fourth arch of the left side ; but since this arch becomes the left subclavian, L.sc. (the connection between the fourth and fifth left arches being obliterated), the portion of the trunk between the fourth arch and the bulbus arteriostis (or as it must now be called the common aortic root) is called the left innominate artery.
The fourth arch of the right side forms the commencement of the great dorsal aorta, and gives off the right subclavian (R.sc.) just before it is joined by the fifth arch.
The fifth arch of each side gives off branches (R.P., L.P.) to the lungs ; their distal continuations, by which these arches are connected with the. systemic circulation, though much reduced, are not obliterated.
The final changes undergone by the arterial system after the commencement of the pulmonary respiration consist chiefly in the complete separation of the pulmonary and systemic circulations. As the branches to the lungs become stronger and stronger, less and less blood from the right ventricle enters into the dorsal aorta; and the connecting vessels become smaller and smaller.
Each of these fifth arches from the right ventricle may therefore be considered at about the sixteenth or eighteenth day as divided into two parts, an inner part which connects the heart with the lung, and an outer part which still connects the arch with the main dorsal aorta. As these outer parts become smaller they receive the name of the ' ductus or canales Botalli' or 'ductus arteriosi.' The one on the right side is short; that on the left side is much longer and narrower.
When respiration commences the blood ceases to pass through these canals, which either remain as mere ligaments or else become absorbed altogether. By this means, the foramen ovale becoming at the same time closed, a complete double circulation is established. All the blood from the right ventricle passes into the lungs, and all that from the left ventricle into the body at large.
Two other changes take place about the same time in the aortic branches. That portion of the right fourth or aortic arch which lies between the origin of the right subclavian and the common carotid becomes shortened, and is finally swallowed up in such a fashion that the right subclavian (Fig. 94, R. sc.) comes off from the right common carotid, a very short trunk being formed by the union of the two to serve as the right innominate artery.
At the same time, corresponding to the increase in the length of the neck, the common carotids are very greatly lengthened. They lie close together in the neck, and in many birds actually unite to form a common trunk.
It will of course be understood that with the disappearance of the allantois and the absorption of the yolk, the allantoic and vitelline arteries also disappear.
It may perhaps be of advantage to the reader if we here briefly summarize the condition of the circulation at its four most important epochs; viz. on the third day, on the fifth day, during the later days of incubation before respiration by the lungs has commenced, and after the chick has begun to breathe by the lungs.
FIG. 94. DIAGRAM OF THE ARTERIAL SYSTEM OF THE ADULT FOWL.
P.A. root of pulmonary artery. L.in. left innominate artery. D.A. dorsal aorta. COB. cseliac arteries, mes. mesenteric artery, ar.r. renal arteries, fern, femoral arteries. Is. ischiatic arteries, hyp. hypogastric arteries, cau. caudal artery. The other letters as in Fig. 93.
On the third day the circulation is of an exceedingly simple character.
The heart is to all intents and purposes a simple twisted tube marked off by constrictions into a series of three consecutive chambers. The blood coming from the venous radicles passes through the heart and then through the three pairs of arterial arches.
From these it is collected into the great dorsal aorta. Upon this dividing into two branches, the stream of blood passes down on each side of the notochord along the body, and thence out by the vitelline arteries, which distribute it to the yolk-sac.
In the yolk-sac it partly passes into the sinus terminalis and so into the fore and aft trunks, partly directly into the lateral trunks, of the vitelline veins. In both cases it is brought back to the two venous radicles and so to the heart.
On this day the blood is aerated in the capillaries of the yolk-sac.
On the fifth or sixth day the two auricles are present though having a common cavity. The septum of the ventricles is nearly complete, so that the blood on entering the ventricles from the auricles is divided into two streams. These two streams pass respectively from the right and left chambers of the heart into the two divisions of the bulbus arteriosus. The blood from the right ventricle passes into the fifth pair of arches and that from the left ventricle into the third and fourth pairs of arches.
From the anterior parts the blood is brought back by the anterior cardinal or jugular veins; from the hinder parts of the body, chiefly by the cardinal veins, but also in part by the now commencing vena cava inferior.
The blood from the yolk-sac and allantois, together with a small quantity from the intestine, is collected into the portal vein, and by that vessel carried to the liver. Here it becomes divided into two streams, part flowing directly by the ductus venosus into the sinus venosus, and the remainder passing through the capillaries of the liver, being brought back to the ductus venosus by the hepatic veins.
During this period the blood is aerated both by the allantois and yolk-sac, but as yet chiefly by the latter.
At a somewhat late period of incubation the blood from the ventricles passes into two entirely distinct roots. The one of these, that from the right chamber, sends the blood to the fifth pair of arches; passing through which the greater part of the blood flows into the dorsal aorta, a small portion only finding its way into the lungs through the as yet unimportant pulmonary arteries.
Through the other aortic root, viz. that from the left ventricle, the blood flows into the third and fourth pairs of arches. That part of the blood which flows into the third pair, passes almost entirely to the head and upper extremities by the external and internal carotids ; that which flows into the right arch of the fourth pair is chiefly brought to the dorsal aorta, but some of it passes to the right wing ; that, on the contrary, which goes into the left fourth arch is for the most part sent to the left wing, a small part only reaching the dorsal aorta. There is still a mixture of the blood from the two chambers of the heart, so that the blood in the dorsal aorta is composed partly of blood from the left, and partly from the right chambers.
The blood of the upper (anterior) end of the body comes entirely from the left ventricle.
The blood of the dorsal aorta passes to the yolksac and allantois, and to all the hinder parts of the body. It is brought back from the yolk-sac, from the allantois, and to a certain extent from the intestines, by the portal vein, part of the blood from which passes to the inferior vena cava by the direct course (ductus venosus), and part indirectly by the more circuitous course of the capillaries of the liver and hepatic veins.
The blood from the generative and urinary organs, and from the hinder extremities, is brought back to the heart by the vena cava inferior ; that from the upper extremities and head by the jugular, vertebral and wing veins into the two venae cavae of the right and left side, and so to the heart. Of these three venae cavae, the right superior and the inferior join the auricle by a common entrance, but the left superior has an entrance of its own. All of these open into the cavity of the right auricle, but the opening of the inferior vena cava is so directed (vide Chap. vill. p. 263) that the blood carried by this vessel flows chiefly through the foramen ovale into the left auricle. The blood from the two superior venae cavae enters the right auricle only. Now the blood of the inferior vena cava has been partly aerated by the allantois; and, since it is this blood which passing through the left auricle and ventricle is distributed to the third aortic arch, unmixed by any blood from the right ventricle (the mixture with the blood from the fifth arch reaching only as far as the fourth arch), it happens that the blood which flows to the anterior extremities and head is more aerated than that in any other part of the body.
From the anterior extremities the blood is to a great extent returned by the left superior cava, and goes into the right auricle, whence, by the right ventricle, it is distributed through the fifth pair of arches over the body, after joining the more aerated blood passing through the fourth pair of arches.
The blood from the lungs is brought back by two small veins into the left auricle.
The characteristics of the circulation at this time are that the blood is aerated by the allantois, and that there is a partial double circulation. (Vide Chap. vm. p. 263.)
As soon as respiration commences the canals leading to the dorsal aorta from the fifth pair of arches, which communicate only with the right ventricle, become closed. The blood passing along the fifth arch now flows only into the lungs, through the pulmonary arteries. The blood from the left ventricle owing to the cessation of the circulation of the yolk-sac and of the allantois is distributed exclusively to the body of the chick, from whence it is all brought back into the right auricle by the three now independently opening vena cavae.
The portal veins henceforward receive blood from the intestines only, and the ductus venosus is obliterated, so that all the blood of the portal vein passes through the capillaries of the liver.
The partition between the auricles is rendered complete by the closure of the foramen ovale; into the right auricle the veins of the body enter, and into the left the pulmonary veins.
There is thus a completely double circulation formed, in which all the blood of the left ventricle is arterial, and all the blood of the right ventricle venous, and there is at no part of the circulation a mixture of venous and arterial blood.
As early as the sixth day movements, as we have said, may be seen in the limbs of the embryo upon opening the egg. We may conclude that after this epoch spontaneous movements occur from time to time in the unopened egg. They cannot however be of any great extent until the fourteenth day, for up to this time the embryo retains the position in which it was first formed, viz. with its body at right angles to the long axis of the egg.
On the fourteenth day a definite change of position takes place ; the chick moves so as to lie lengthways in the egg, with its beak touching the chorion and shell membrane where they form the inner wall of the rapidly increasing air-chamber at the broad end (Chap, i. p. 3).
On the twentieth day or thereabouts the beak is thrust through these membranes, and the bird begins to breathe the air contained in the chamber. Thereupon the pulmonary circulation becomes functionally active, and at the same time blood ceases to flow through the umbilical arteries. The allantois shrivels up, the umbilicus becomes completely closed, and the chick piercing the shell at the broad end of the egg with repeated blows of its beak, casts off the dried remains of allantois, amnion and chorion, and steps out into the world.
The Elements of Embryology - Volume 1 (1883)
The History of the Chick: Egg structure and incubation beginning | Summary whole incubation | First day | Second day - first half | Second day - second half | Third day | Fourth day | Fifth day | Sixth day to incubation end | Appendix
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