AVES 



321 



bathes the embryo throughout the entire embryonic period and pro- 

 tects it from shocks and injuries due to contacts. 



The allantois begins as an out-pouching of the hind-gut not far 

 from the yolk-stalk. It pushes outward as a thin-walled sac, lined 

 with endoderm on the inside and with mesoderm on the outside, 

 grows out between the amnion and chorion, and expands into a large 

 umbrella-shaped body until it fills the entire extra-embryonic coslom, 

 or space between the chorion and amnion. Thus the amnion is cov- 

 ered with the distal part of the al- 

 lantois and the latter is covered with 

 chorion. In the later stages these 

 three membranes fuse together in a 

 number of places into a single com- 

 pound membrane. The allantois be- 

 comes richly vascular on its outer 

 surface and acts as an embry- 

 onic lung, getting oxygen through 

 the porous shell membranes and 

 shell. 



The yolk-sac is at first nearly the 

 entire egg, but as development pro- 

 gresses it diminishes in size as the 

 yolk substance is assimilated by the 

 embryo; until finally the tiny sac 

 that remains is drawn into the body development of head, long tail wing 



J and leg nearly identical, (trom 

 cavity of the chick through the um- LiiH e , after Keibel and Abraham.) 



bilicus, and the latter closes. 



Changes in Body Form During Development. It is of interest 

 to note that the tail of the chick of four or five days' incubation is 

 comparatively long and slender, much like that of a lizard at an 

 equivalent stage of development. Also the fore and hind limbs are 

 much alike at that period, as is shown in the illustration (Fig. 165). 

 It is only after about ten days of incubation (Fig. 166) that 

 the tail becomes foreshortened into the typical avian tail and 

 the fore limbs take on the characteristic features of wings. As in 

 most other vertebrate embryos, the head is relatively enormous as 

 compared with the body during a large part of the embryonic period, 

 and it is only in the last stages of incubation that the body becomes 

 larger than the head. The feather rudiments appear about the sixth 



FIG. 165. Chick embryo at five 

 days' incubation, showing precocious 



