THE ALLANTOIS. 295 



wall of the gut, and the allantois now appears as a saccular 

 depression of the ventral wall of the hind-gut (Fig. 114, TA). 



During the third day the allantois increases considerably in 

 size, projecting downwards and forwards, as a hollow, thick- 

 walled bud from the ventral surface of the hind-gut, into the 

 body cavity, or space between the somatic and splanchnic layers 

 of the mesoblast. 



During the fourth day, 'by its further growth, the allantois 

 passes out beyond the embryo, and turns up, along its right side, 

 into the space between the two layers of the amnion, which, from 

 the mode of formation of the amnion, is directly continuous 

 with the body cavity of the embryo (cf. Fig. 114). 



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 super- 

 ficial layer, are derived directly from the aorta (Fig. 128, AA) ; 

 while the veins, VA, 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. 101, TA), the allantois 

 has spread all round the upper half of the egg, covering over 

 the embryo, and extending half way round 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, and carrying the somato- 

 pleure before it, it extends so as gradually to inclose the mass 

 of the white, WA, which still remains on the under surface and 

 near the smaller end of the egg. The allantois, about the six- 

 teenth day, completely incloses 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 recep- 

 tacle for the excretory matters formed within the embryo itself, 

 as well as a respiratory organ in the more restricted sense of the 

 term. 



