DEVELOPMENT OF BIKDS AND MAMMALS 245 



At the hinder end of the embryo the hind-gut comes into 

 close relationship with the ectoderm at the posterior, now the 

 lower, end of the primitive streak. A short outgrowth from 

 the gut meets an invagination of the ectoderm. This is evi- 

 dent just at the time that the allantois is forming ; and later 

 the walls separating the two processes meet and fuse, thus 

 putting the alimentary canal posteriorly into communication 

 with the amniotic cavity that is to say, with what will be the 

 exterior. It is then seen that in the chick there is a postanal 

 extension of the gut, and this corresponds with the extension 

 which in reptiles is in communication with the neural canal. 

 This postanal gut degenerates into a solid mass and finally 

 disappears. 



The mesoderm during the first day extends outwards as 

 two wings from the notochord and the primitive streak. Like 

 the notochord, it is developed from the endoderm in front and 

 is indeed at first continuous with the notochord. In the region 

 of the primitive streak it is derived from the ectoderm, or rather, 

 as has been seen, from the lips of the blastopore, and forms the 

 mesenchyme of the early yolk-sac vessels. The notochord 

 separates from the endoderm below and contracts into a 

 rounded cord of cells. It is derived, therefore, at the early 

 period of the history of the bird, from the roof of the endoderm. 

 After the tail is folded off, it is continued directly from the 

 cells multiplied at the posterior end of the embryo. These 

 cells form besides in this way the ectoderm, endoderm, and 

 mesoderm, with all of which they are continuous. 



In the first day the mesoderm is split into two layers, the 

 somatopleure and splanchnopleure, the former applied to the 

 ectoderm and the latter to the endoderm. The cavity between 

 them is the coelom. The layers remain connected at their 

 outer edge, and again along each side of the notochord. The 

 cavity is thus a closed cavity and, with the fusion which takes 

 place in front of the embryo, the extra-embryonic portion of it 

 forms a cavity completely encircling the embryo. 



The formation of the medullary canal i% associated with 

 an expansion of the mesoderm on each side, and, beginning 

 anteriorly in the head region, this parachordal mesoderm 

 is split up into a gradually increasing series of myotomes, 



