Fig. 361. Two views of the heart of a Chick upon the fifth day of incubation.
A. from the ventral, B. from the dorsal side.
l.a. left auricular appendage; r.a. right auricular appendage; r.v. right ventricle; l.v. left ventricle; b. truncus arteriosus.
As the septum grows downwards towards the heart, it finally reaches the position of these valves. One of its edges then passes between the two ventral valves, and the other unites with the prominence on the dorsal valve-ridge. At the same time the growth of all the parts causes the valves to appear to approach the heart, and thus to be placed quite at the top of the ventricular cavities. The free edge of the septum of the truncus now fuses with the ventricular septum, and thus the division of the truncus into two separate channels, each provided with three valves, and each communicating with a separate side of the heart, is complete; the position of the valves not being very different from that in the adult heart.
That division of the truncus which opens into the fifth pair of arches is the one which communicates with the right ventricle, while that which opens into the third and fourth pairs communicates with the left ventricle. The former becomes the pulmonary artery, the latter the commencement of the systemic aorta.
The external constriction actually dividing the truncus into two vessels does not begin to appear till the septum has extended some way back towards the heart.
The semilunar valves become pocketed at a period considerably later than their first formation (from the 147th to the 165th hour) in the order of their appearance.
At the end of the sixth day, and even on the fifth day ([figs. 361] and [362]), the appearance of the heart itself, without reference to the vessels which come from it, is not very dissimilar from that of the adult. The original protuberance to the right now forms the apex of the ventricles, and the two auricular appendages are placed at the anterior extremity of the heart. The most noticeable difference (in the ventral view) is the still externally undivided condition of the truncus arteriosus.
Fig. 362. Heart of a Chick upon the sixth day of incubation, from the ventral surface.
l.a. left auricular appendage; r.a. right auricular appendage; r.v. right ventricle; l.v. left ventricle; b. truncus arteriosus.