Part 90

The vessels which return the blood from the vitellicle are the transverse and the longitudinal vitelline veins. The first are so called because these trunks pass to the embryo at right angles to its axis. They are the largest returning canals. The longitudinal veins extend parallel with the axis of the embryo; they are of smaller size. The right anterior longitudinal vein becomes the right precaval and receives the remains of the right transverse vitelline vein, as the right vena azygos. The left anterior longitudinal vitelline vein is also persistent as the left precaval, and enters in the mature bird, as in the embryo, at the posterior or the lower part of the auricle. The left transverse vitelline vein is also subsequently reduced, by receiving only the vertebral veins of that side, to the condition of a so-called azygos vein. The main trunk of the post-caval is the result of the returning vessels from the abdominal viscera and the posterior limbs at a later stage of development. There is but one principal posterior longitudinal vitelline vein, and this anastomoses with the left transverse vein as it enters the embryo.

The auricle, which, by its dilatation of the left side, appears to be double, receives the venous blood at its right division. The left one, subsequently receives the veins from the lungs, is ultimately separated from the left precaval and the right auricle to which that vein is conducted and restricted.

The ventricular part of the heart, at the second day of incubation, is in the form of a bent tube, curving from behind downward, forward, to the right and upward, continuing insensibly into the part representing the aortic bulb, in which the septum first appears, and ultimately dividing the ventricle into two.

At this stage the piers of the maxillary arch appear as buds from beneath the eyeballs. The naso-premaxillary process is above their interspace. The piers of the mandibular arch and those of the hyoidean arch follow in close succession. The blastemal base of the scapular arch projects slightly at the sides of the fovea cardiaca; the piers, now separate, ultimately meet in front of the heart, and accompany it in its retrograde course. The mesencephalon is the largest of the segments of the brain which are connected with the eyeballs.

When the heart has assumed its form as such, distinct from the great trunks rising from it, the two arteries from the base of the ventricles appear. The artery to the right bifurcates, one division supplying the head and the wings, the other winding over the right bronchus. That to the left also bifurcates. Its left division arching over the left bronchus and anastomosing with the right arch a little below and behind the apex of the heart. Its right division arches over the back of the heart, bending to the right and anastomosing with the right aortic arch just above the outer ductus arteriosus. Each of these divisions of the left primary arterial trunks sends off a branch to its corresponding lung. As the lung expands, and especially at the beginning of the act of expansion toward the close of the period of incubation, the blood is diverted into the pulmonary vessels, and the channels below them shrink and disappear. The left primary artery is retained as the trunk of the pulmonaries, and, through the changes in the interior of the ventricle, this artery comes to discharge exclusively the ventricle corresponding to the right in mammals. The retained aorta rises from the left ventricle.

The air-sacs begin at the lower point of the lung, appearing like small hydatids, and extend further and further into the abdomen, in front of the kidneys. They are at first full of fluid. Soon after the development of the abdominal air-sacs others are developed.

The lungs are at first free, but afterward begin to be attached to the ribs and to the spine.

In the female embryo we first observe two oviducts, one on each side of the basis, or stroma of the ovarium, which appears in a relation to the primordial kidneys similar to that of the testes in the male. At the period when the permanent kidneys have sent their ureters to the cloaca, the oviducts have been developed as prolongations from that part, and, up to a certain point of development, they are of equal size and length. Subsequently the left oviduct alone continues to grow; the right remaining stationary or shrivels; occasionally it may be discerned as a rudimentary in the mature bird, but usually all trace of it has disappeared before hatching. The left oviduct expands above or at its free end into the infundibular orifice, where its parietes are very thin. As it descends, these increase in thickness, and the efferent tube gradually acquires the texture and form of an intestine. It is attached and supported by a duplicature of the peritoneum.

FIG. 82.—Embryological studies.

A. 1, The chorion and allantois. 2, The allantoic cavity. 3, The amnion. 4, The yolk sac. 5, A small quantity of remaining albumin.

B. Brain tube of a chick 25½ hours old showing partly closed brain tube with eleven folds of neuromeres.

C. The development of the alimentary tract. 1, Trachea. 2, Lung. 3, Esophagus. 4, Stomach. 5, Pancreas. 6, Bile duct. 7, V-shaped loop of midgut. 8, Cloaca. 9, Vitello-intestinal duct.

D. 1 to 6 inclusive, same as C. 7, Cæca. 8, Cloaca.

E. Kidneys, Wolffian bodies, and testes of an embryo chick. 1, The adrenals. 2, The genital. 3, Primitive oviduct. 4, Permanent kidney. 5, Ureters. 6, Duct of primitive kidney which conducts the excretion into the cloaca.

F. A transverse section of a chicks’ head 48 hours of incubation. 1, Forebrain. 2, Pigmented layer of retina. 3, Ectoderm. 4, Nervous part of retina. 5, Optic stalk. 6, Invagination of ectoderm to form the lens rudiment. ]

At first, the right and the left ovaria are similar in size, but the symmetry is soon disturbed by concentration of development in the left ovary. The right ovary remains stationary and ultimately, in most birds, completely disappears by the time the chick is ready to emerge from the shell.

Three fetal membranes are developed, the chorion, the amnion, and the allantois. There is also developed the yolk sac (Figs. 82 and 83). The amnion is connected to the body wall at the umbilicus. The amniotic fluid is found in this sac. The chorion is at first surrounded by the albumen, but as the albumen is absorbed the chorion comes in contact with the inner shell membrane. It is probable that the chorion consists of ectoderm on the outside and of mesoblast on the inside. The amnion, on the other hand, is formed of mesoblast on the outside and ectoderm on the inner, or embryonal, side. The allantois springs from the embryo soon after the fourth day, and develops from the ventral wall of the primitive gut. By some embryologists, the yolk sac is included in the embryonic membranes. It commences as the splanchno-pleure surrounding the mass of yolk. It becomes smaller as the yolk is absorbed. At first its outline is round but later its walls become folded in. The yolk is dissolved and absorbed by the entodermic lining of the sac, and is carried to the embryo by veins called the vitelline veins (Fig. 83, No. B, 4), which ramify on the walls of the sac.

Some time after the fourteenth day, the chick assumes a position lengthwise within the egg shell so that the head is near the broad end of the egg. The head is bent upon the chest and the beak is usually tucked under the wing. Later the head assumes a position, by a double curve of the neck, so that the beak is in contact with the air cell. About the fifteenth day, the coils of intestine, which heretofore have been outside the abdominal cavity, are withdrawn into the abdominal cavity, as is also the abdominal yolk sac. As the chick pips out of the shell, the umbilicus becomes occluded.

The outer upper part of the tip of the beak is provided with a short, stout, spike-like arrangement, called the egg tooth. On the twentieth day this part of the beak is forced against the wall of the egg and gradually breaks through the egg shell. The breathing by the lungs commences some time before hatching; this is evidenced by the chick within the shell giving chirping sounds.

FIG. 83.

A. Longitudinal section of chick, 4 days incubation. 1, Cephalic fold. 2, Caudal fold. 3, True amniotic cavity. 4, Epiblast. 5, Somatic mesoblast. 6, Visceral mesoblast. 7, Hypoblast. 8, Future anua still closed. 9. The allantoic vessicle. 10, The mesentery. 11, Intestine. 12 and 13, Yolk sac. 14, Cavity of true amnion. 15, The mouth. 16, Pleuroperitoneal cavity. 17, Fore gut.

B. Membranes of the chick at the third day of incubation. 1, Membrane surrounding albumin. 2, Amnion. 3, Allantois. 4, Vitellicle.

C. The fore part of an embryo chick at the second day. 1, Mesencephalon. 2, The eye. 3, Olfactory organ. 4, Mandibular arch. 5, Maxillary arch. 6, Maxillary arch. 7, Hyoidean arch. 8, Scapular arch. 9, Depression organ of hearing. 11, Process from encephalon for union with nerve of hearing.

D. Primitive blood-vessels at second day of incubation (Owen). 1, Mesocephalon. 2, The right anterior longitudinal vein. 3, The eye. 4, The mandibular arch. 5, The hyoidean arch. 6, Scapular arch. 7, The same. 8, The ventricular portion of the heart. 9, The right transverse vitelline vein. 10, Posterior longitudinal vitelline vein. 11, Vertebral vein. 12, Tributaries of same. 14, The auricle. 16, Aortic bulb.

E., A transverse section of chick embryo, 29 hours incubation. 1, Neural canal. 2, Neural crest. 3, Somatopleure. 4, Splanchnopleure. 5, Omphalomesenteric vein. 6, Aorta. 7, Notocord. 8, Pleuroperitoneal cavity.

F. Chick embryo at the ninth day of incubation. 1, Allantois. 2, Amnion. 3, Air cell at large end of the egg. 4, Egg shell. 5, Outer shell membrane. 6, Inner shell membrane. 7, Yolk sac. 8, Albumin (Bradley). ]

The circulation of the blood is somewhat like the circulation in the fetus of quadrupeds. The primitive tubular heart (Fig. 83, No. D, 8) is bent in S-shape; the two ends are connected with blood-vessels; and later, by the development of the septa, the cavities of the adult heart are defined. The foramen ovale is found in the median septum. This opening brings the right and the left side into communication. The septa between the cavities of the heart are completed by the end of the sixth day. The two vitelline, or omphalo-mesenteric, veins carry the blood containing the nutrients from the yolk sac to the liver, where it is mixed with the blood drained from the intestines by the portal vein, and this blood is finally carried by the posterior vena cava into the right auricle of the heart. From here it passes through the foramen ovale to the left auricle; from the left auricle it enters the left ventricle, from which it is forced into the aorta, and then out into the systemic circulation. Practically all, if not all, of the blood of the pulmonary artery is sent into the aorta through a connection with this vessel called the ductus arteriosus.

Down appears about the thirteenth day of incubation. There are two kinds of down on the chick, one long, which comes first, about two or three days before hatching; a second, or fine, down forms at the roots of the other. As the embryo develops the air cell, at the large end of the egg which is developed between the two shell membranes, or membranæ putaminæ, gradually enlarges.