266 



THE BIOLOGY OF BIRDS 



and excretory organ. This is interesting in connection with 

 the fact that in ordinary mammals the allantois combines 

 with part of the amnion to form the placenta — an intricate 

 structure that binds the unborn mammal into partnership 

 with its mother. Now the placenta enables the embryo 

 mammal to capture oxygen from the maternal blood and to 

 get rid of CO^ ; it also serves for the capture of fluid food 



AS 



uv 



Fig. 45. — Diagrammatic vertical section of bird embryo (after 

 Grosser). Showing the embryo (e.) ; the amniotic cavity (am.) ; the 

 allantoic cavity (al.) ; the yolk-sac (uv.) ; the albumen (w.) ; the extra- 

 embryonic body cavity (ec.) ; the serous membrane (as.). 



from the same source and to get rid of nitrogenous waste. 

 In other words, it functions like the allantois of birds. 



Eventually the young bird, often provided with an " egg- 

 tooth " (a hardening of horn and lime at the tip of the upper 

 jaw), perforates the membrane of the air-chamber at the 

 broad end of the egg, and gets its first breath. The air fills 

 the hitherto functionless, but steadily developing, lungs. 

 Important changes take place very rapidly in the circulation ; 



