EMBRYOLOGY OF THE CHICK 327 



texture and form of an intestine. It is attached and supported by a 

 duplicature of the peritoneum. 



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 mem- 

 branes. 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 vitelHne 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 heretofore have been outside the abdominal cavity, are withdrawn 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. 



