Development of the Third Day 171 



amnion, is directly continuous with the body cavity of 

 the embryo (Figs. 37, G-K, and 38, C). 



" On the fifth and following days the allantois grows 

 rapidly; from the first it is very vascular, and the blood 

 vessels now increase greatly in size; the arteries, which 

 lie in its superficial layer, are derived directly from the 

 aorta (Fig. 76); while the veins, V A, which lie in its 

 inner or deeper layer, join the vitelline veins from the 

 yolk-sac, and, passing through the liver, reach the heart. 

 By the seventh or eighth day (Fig. 38, D) the allantois 

 has spread all around the upper half of the egg, covering 

 over the embryo, and extending half around the yolk-sac 

 as well. It is still saccular, and its cavity contains fluid. 

 Its outer wall lies in very close contact with the outer 

 layer of the amnion, or false amnion, and soon fuses with 

 this completely, so that from this time the allantois lies 

 in close contact with the shell membrane. 



" In its further growth the allantois does not follow 

 the yolk-sac; but, keeping close to the egg-shell arid 

 carrying the somatopleure before it, it extends so as 

 gradually to enclose the mass of white, which still remains 

 on the under surface and near the small end of the egg. 

 The allantois, about the sixteenth day, completely en- 

 closes this plug of white or albumen, and from this time 

 the absorption of the plug proceeds rapidly, the albumen 

 being apparently carried by the allantoic vessels to the 

 embryo, and aiding in its nutrition. 



" Towards the close of incubation deposits of urates 

 occur in the cavity of the allantois, indicating that it 

 serves as a receptacle for the excretory matters formed 

 within the embryo itself, as well as a respiratory organ 

 in the more restricted sense of the term. 



