182 THE DEVELOPMENT OF THE CHICK 
Anal Plate, Hind-gut, Post-anal Gut, and Allantois. At about 
the 14s stage a thickening of the ectoderm in the middle line 
just behind the primitive streak extends towards the entoderm 
which is folded up so as to nearly meet it, thus cutting off the 
extra-embryonic mesoblast from the primitive streak. The ecto- 
derm and the entoderm then come into contact here, and form 
a firm union, the anal plate (Fig. 70), which is subsequently 
perforated to form the anus. At first, however, the anal plate les 
entirely behind the embryo, and the post-anal portion of the 
embryo arises from the thickened remnant of the primitive streak 
(tuil-bud) which grows backwards over the blastoderm beyond 
the anal plate. Even before this, however, the hind-gut begins 
to be formed by a fold of the splanchnopleure directed forwards 
beneath the tail-bud, and the hind end of the tube thus formed 
ends at the anal plate (Fig. 70). The entoderm in front of the 
anal tube is fused with the substance of the tail-bud, and as the 
latter grows backwards beyond the anal plate it carries with it 
a pocket of the hind-gut, and this forms the post-anal gut (Fig. 
80). 
The formation of the tail brings the anal plate on to the ven- 
tral surface of the embryo at the junction of tail and trunk, and 
the post-anal gut then appears as a broad continuation of the 
hind-gut extending behind the anal plate, and ending in the tail 
at the hind end of the notochord (Fig. 80). The further elonga- 
tion of the tail draws out the post-anal gut into a narrow tube 
lying beneath the notochord in the substance of the tail; it 
then gradually disappears and leaves no trace. 
The formation of the hind-gut takes place prior to the for- 
mation of the embryonic body-cavity at this place. It thus 
happens that the splanchnic mesoderm, forming the floor of the 
hind-gut, is directly continuous with the somatic mesoderm. 
When the body-cavity does penetrate this region it is without 
direct lateral connections with the extra-embryonic body-cavity, 
so that the connection of the splanchnic and somatic mesoderm 
persists, forming the ventral mesentery of the hind-gut (Fig. 81). 
This is a thick mass of mesoblast binding the hind-gut to the 
somatopleure. The hind-gut is deep from the first, and its ven- 
tral division soon begins to extend into the ventral mesentery 
as a broad evagination, the allantois (see p. 148). 
