THE EARLY DEVELOPMENT OF THE CHICK 259 



vestigial blastopore occurs after the growth of the blastoderm 

 begins, so that when the lips of the blastopore approach and 

 fuse, in the typical manner of confluence, the region of their 

 fusion is limited to the posterior region of the blastopore 

 (Fig. 96). We recognize the primitive streak, therefore, as 

 the fused halves of the blastoporal margin; the primitive 

 groove is consequently to be interpreted as a vestige of the 

 blastopore itself. Later in development we know that the 

 anus develops at the posterior end of the remains of the 

 primitive streak, while from the beginning the primitive pit, 

 ' at its anterior end, represents the vestige of the neurenteric 

 canal, as described for Amphioxus and the frog. 



The so-called head process thus represents, theoretically, 

 the earliest portion of the true embryo to be differentiated out 

 of the primitive streak. As a matter of fact, however, it is 

 difficult to observe a true genetic relation between the primitive 

 streak and the earliest portion of the head process, which 

 seems to form precociously. This theoretical distinction is 

 also the basis for the further distinction between gastral and 

 peristomial mesoderm in the chick. Really such a distinction 

 is not evident here, but it is sometimes useful to regard as 

 gastral, that mesoderm originating in the primary head proc- 

 ess, and as peristomial, that arising more posteriorly from the 

 primitive streak proper. Ultimately, of course, the "peristo- 

 mial" mesoderm becomes "gastral" in its relations to other 

 structures. 



The fact that the whole process of gastrulation is itself ves- 

 tigial, to a certain extent, offers an interpretation of the inde- 

 pendent formation of the medullary plate, which occurs 

 unusually early, and of the mesoderm, which does not appear 

 until the primitive streak is largely established. This latter 

 fact, together with the development of the mesoderm from 

 cells of ectodermal character, are probably both related to the 

 marked restriction of the endoderm; this is reduced both in 

 extent and in importance, for most of the early functions of the 

 endoderm are here , in correlation with the extreme amount of 

 yolk, taken over by the periblast and the cells of the germ wall. 



