56 EARLY EMBRYOLOGY OF THE CHICK 



boidalis (Fig. 21). Hensen's node and the primitive pit lie in 

 the floor of this as yet unclosed region of the neural groove and 

 subsequently are enclosed within it when the neural folds here 

 finally fuse to complete the neural tube. 



This process in the chick is homologous with the enclosure of 

 the blastopore by the neural folds in lower vertebrates. In 

 forms where the blastopore does not become closed until after 

 it is surrounded by the neural folds, it for a time constitutes an 

 opening from the neural canal into the primitive gut known as 

 the neurenteric canal or posterior neuropore. In the chick the 

 early closure of the blastopore precludes the establishment of an 

 open neurenteric canal but the primitive pit represents its 

 homologue. 



The Fate of the Primitive Streak. In embryos of about 27 

 hours the primitive streak is relatively much shorter than in 

 younger embryos (Cf. Figs. 8, n, 14, 15, and 18). This is 

 due partly to its being overshadowed by the rapid growth of 

 structures lying cephalic to it, and partly to actual decrease 

 in the length of the primitive streak itself. The cells in the 

 primitive stieak region would appear to be contributed to 

 surrounding structures. Whatever the exact fate of its cells 

 may be, the primitive streak becomes less and less a conspicuous 

 feature in the developing embryo. By the time the caudal end 

 of the body is delimited, the primitive streak as a definitely 

 organized structure has disappeared altogether (Cf. Figs. 18, 

 21, 29, 34). 



The Formation of Additional Somites. The division of the 

 dorsal mesoderm to form somites begins to be apparent in 

 embryos of about 22 hours. By the end of the first day three 

 or four pairs of somites have been cut off (Fig. 15). As develop- 

 ment progresses new somites are added caudal to those fiist 

 formed. In embryos which have been incubated about 27 

 hours eight pairs of somites have been established (Fig. 18). 



It was formerly believed that some new somites were formed 

 anterior to the first pair. The experiments of Patterson would 

 seem to indicate quite definitely that the first pair of completely 

 formed somites remains the most anterior and that all the new 

 somites are added posterior to them. The experiments referred 

 to were carried out on eggs which had been incubated up to the 

 time of the formation of the first somite. With thorough 



