474 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



however is that, which commences early in the fifth day, with the 

 inner surface of the serous membrane. 



At a comparatively early period (during the fifth day) the meso- 

 derm covering the allantois becomes vascular and as the organ 

 becomes flattened its proximal or inner and its distal or outer walls 

 become strikingly different as regards their vascularity, the outer 

 wall developing an extremely rich network of capillary blood-vessels 

 with very small meshes, while the inner wall possesses merely a 

 sparse network together with the large vessels of supply. This 

 difference between the two walls of the allantois becomes conspicuous 

 about the end of the sixth day of incubation in the common Fowl. 

 The difference is associated with the fact that the distal wall of the 

 allantois is destined to become the great respiratory organ, taking- 

 over this function from the vascular area of the yolk-sac by which it 

 is performed during the early stages of development. In correlation 

 with the more efficient performance of this function the albumen, or 

 white, as it gradually shrinks in volume and acquires greater density 

 gravitates down to the lower side of the egg thus bringing the mush- 

 room-shaped allantois close up to the shell membrane on the upper 

 side. The .process is still further facilitated by the ectoderm of the 

 serous membrane becoming reduced to a very thin — hardly distin- 

 guishable — layer in the region where it is underlain by, and fused 

 with, the allantois. The capillary network thus comes into very 

 close relation with the shell membrane and the overlying porous 

 shell, and gaseous exchange can readily take place between the blood 

 circulating in the network and the external atmosphere. 



As development goes on the respiratory needs of the embryo 

 become greater and greater and these are met by the allantois 

 spreading outwards all round its periphery, so as to provide a greater 

 and greater respiratory area. During this spreading outwards of 

 the allantois the three main allantoic vessels are somewhat retarded 

 in their growth with the result that each one causes an indentation of 

 the growing edge of the allantois beyond which the allantois bulges 

 on each side. 



When the growing edge of allantois comes, after about nine days' 

 incubation, into the neighbourhood of the remaining mass of albumen, 

 a new phenomenon appears inasmuch as the allantoic margin with 

 its covering of serous membrane proceeds to grow onwards close 

 under the shell membrane as a circular fold recalling the amniotic 

 fold and enclosing the mass of albumen (Figs. 215, B, 215a, C). The 

 ectodermal lining of the cavity so formed sprouts out into the albu- 

 men in the form of irregular projections which become vascularized 

 from the allantoic mesoderm and no doubt play a part in absorbing 

 the last remains of the albumen. 



By about the end of the second week of incubation the shell 

 membrane is lined throughout the whole of its extent by the highly 

 vascular outer wall of the allantois. This remains the breathing 

 organ until — a day or two before hatching — the young chick's beak 



