204 EMBRYOLOGY 
inside of the navel and the alimentary canal, sometimes with a little 
degenerated yolk, persist in many, chiefly nidifugous, birds as the 
DIVERTICULUM cecum 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 ceca of 
Crows, Storks, and diurnal Birds-of-Prey. 
All Birds, Reptiles, and Mammals possess in their embryonic 
state an AMNION and an ALLANTOIS. The Amunion 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 with 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 lamelle 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 inner 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; while 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 wall of the outer sac, above 
the embryo, lies closely under, and fuses with, 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 wall of the 
