220 EMBRYOLOGY. 



through the ventral wall. It is here employed to help in the closure 

 of the intestinal wall. 



The amnion undergoes regression, inasmuch as the fluid diminishes 

 and almost entirely disappears, until the membrane is again closely 

 applied to the body of the embryo. The albumen, too, is almost 

 entirely consumed. The allantois alone continues to increase, and 

 finally grows around so completely on the entire inner surface of the 

 serosa that its edges come together and fuse with one another into 

 a sac entirely enclosing the embryo and the amnion. It adheres so 

 firmly to the serosa that a separation is no longer possible. 



The urine likewise diminishes toward the end of incubation, and 

 finally, like the amniotic fluid, has entirely disappeared. As the 

 result of this, there are found in the allantois precipitates of uric 

 salts, which become more and more abundant. 



Amnion and allantois finally undergo complete retrogressive meta- 

 morphoses. Inasmuch as the Chick, shortly before hatching, breaks 

 through the surrounding membranes with its bill, it begins to take 

 in directly the air contained in the air chamber, which has become 

 larger. A result of this is that the circulation in the allantois 

 is retarded and finally ceases altogether. The afferent umbilical 

 vessels disappear. Amnion and allantois die away, dry up, and then 

 separate from the dermal umbilicus, which closes on the last day 

 before hatching, and when the Chick leaves the egg-shell they are 

 stripped off with it as useless remains. 



SUMMARY. 



1. In Reptiles and Birds the embryo during its development sinks 

 into the underlying yolk, which has become liquefied, and becomes 

 enveloped by folds of the extra-embryonic area of the somatopleure, 

 the anterior, posterior, and lateral folds of the amnion (cephalic 

 sheath, caudal sheath, lateral sheaths). 



2. As the result of the folding processes two sacs arise around 

 the embryonic body, the amnion and the serous membrane (serosa). 



3. The amnion is united at the dermal umbilicus with the belly 

 of the embryo. 



4. The dermal umbilicus encloses an opening through which the 

 embryonic and extra-embryonic portions of the body-cavity are in 

 connection. 



5. The stalk of the yolk-sac passes through the dermal umbilicus 

 in order to attach itself to the intestine at the intestinal umbilicus. 



