204 EM BR YOLOG Y 



inside of the navel and the alimentary canal, sometimes with a little 

 degenerated yolk, persist in many, chiefly nidifugous, birds as the 

 Diverticulum caecum vitelli ; it is attached somewhere to the 

 middle of the small intestine, and, especially when still hollow, rather 

 closely resembles in shape, size, and colour the degraded cseca of 

 Crows, Storks, and diurnal Birds-of-Prey. 



All Birds, Eeptiles, and Mammals possess in their embryonic 

 state an AikiNiON and an Allantois. The Amnion is a peculiar 

 membrane enveloping the embryo and taking its origin from the 

 somatopleure only. Its development is closely connected with the 

 cleavage of the mesoblast. At an early period the somatopleure 

 forms a semilunar fold in front of the headfold ; it consists of a 

 very thin membrane (epiblast and somatic mesoblast), which in- 

 creases in height, and is gradually drawn backwards over the 

 developing head of the embryo. At the same time a similar fold 

 starts behind the tail and extends with its arms sidewards from the 

 embryo, meeting the corresponding lateral continuations of the 

 anterior fold. All are drawn over the body of the embryo, or 

 rather the embryo seems to sink into these folds, which ultimately 

 meet above it, and completely coalesce Avith each other, all traces of 

 their junction becoming absorbed. Thus the united folds form a 

 sac, within which the embryo lies. The sac is the amnion ; the 

 cavity between the embryo and the inner wall of the amnion is the 

 cavity of the amnion. As will be seen from the diagram (p. 200), 

 each fold of the amnion consists of two lamellae or flaps, but in one 

 the epiblast looks towards the embryo, while in the other it looks 

 away from it. The space between the two flaps or walls of the 

 folds is, according to their mode of formation, part of the cleft 

 between the somato- and splanchnopleure, and consequently continu- 

 ous with the future pleuro-peritoneal cavity. When the several folds 

 coalesce above the embryo, the double septum of their junction 

 becomes absorbed, so that the jnner flaps of each fold form a 

 continuous inner membrane or sac round the body of the embryo ; 

 this is the amniotic sac, or amnion proper ; Avhile the fluid which 

 collects in it, and in which the embryo lies, is the liquor amnii. 

 The space between this inner and the outer sac is, of course, part 

 of the general mesoblastic cleft. The Avail of the outer sac, above 

 the embryo, lies closely under, and fuses Avith, the vitelline mem- 

 brane, while marginally it is continued into the somatopleuric 

 investment of the yolk-sac, as has been described above. As the 

 white of the egg is gradually used up, the outer sac or false amnion 

 gradually approaches the inner shell membrane, and ultimately 

 lines it. 



The Allantois is a diverticulum of the alimentary canal, and 

 opens immediately in front of the anus. It forms a flattened 

 sac or bulging out of the splanchnopleure of the ventral Avail of the 



