182 THE DEVELOPMENT OF THE CHICK 



Anal Plate, Hind-gut, Post-anal Gut, and Allantois. At about 

 the 14 s 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 lies 

 entirely behind the embryo, and the post-anal portion of the 

 embryo arisfes from the thickened remnant of the primitive streak 

 (tail-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. 143). 



