BIRDS' EGGS 



265 



begins to be " folded off " from the yolk. As a result of 

 this the embryo is attached by a relatively narrow stalk to 

 the large yolk-sac, over which the extra-embryonic blasto- 

 derm spreads, showing blood-vessels before the end of the 

 first day. The yolk is gradually utilised and, just before 

 hatching, the greatly reduced sac is withdrawn into the body- 

 cavity of the embryo and the navel or umbilical opening 

 closes up. 



Of great importance are the two embryonic membranes — 

 the amnion which forms a protective hood over the embryo, 



Fig. 44. — Diagram of developing embryo of bird (after Grosser). 

 Showing the embryo (e.) ; the inner or true amnion, a protective hood 

 (am.) ; the amniotic cavity containing fluid (a.c.) ; the extra-embryonic 

 body-cavity (e.c.) ; the allantois (al.) ; the external serosa includes 

 the outer amnion and the vitelline capsule ; the yolk-sac (u.v.) full 

 of yolk. 



and the allantois which spreads as a delicate hood underneath 

 the shell. Blood-vessels from the embryo are distributed 

 abundantly on the allantois, and serve to capture the oxygen 

 which diffuses through the shell from the outer world. 

 Carbon dioxide diffuses out. If the shell be varnished, so 

 that its pores are closed, the embryo must die. The allantois 

 is thus the embryo's breathing organ ; but as it also absorbs 

 some white of egg and receives nitrogenous waste-products 

 from the embryo, it may also be reckoned as a nutritive 



