THE EARLY DEVELOPMENT OF THE CHICK 279 



pass backward and become continuous posteriorly with the 

 extra-embryonic vascular network. 



From this point onward we shall not attempt to continue 

 a description of the development of the embryo as a whole. 

 Next we shall outline the processes leading to the more complete 

 formation of the embryo, and to its well-marked separation 

 from the yolk and blastoderm on every side. Then, after a 

 consideration of the formation of the embryonic membranes 

 and appendages, we shall give, in the next chapter, a brief re- 

 sume of the more essential steps in the organogeny of the later 

 embryo. (Throughout this chapter, and more especially this 

 part of it, the student is referred for additional details to such 

 an account as Lillie's " Development of the Chick/' and to the 

 literature there cited). 



V. THE SEPARATION OF THE EMBRYO FROM THE 

 EXTRA-EMBRYONIC STRUCTURES 



The remaining primitive streak region loses its typical 

 structure soon after the fortieth hour of incubation (about 

 18 pairs of somites) (Fig. 112), and thereafter it remains, as a 

 locus of continued cell proliferation known as the tail bud (Fig. 

 109, A). Shortly after this, about the forty-sixth hour (about* 

 26 pairs of somites) a definite infolding of the blastoderm occurs 

 posteriorly, marking the hinder limit of the embryo. This fold 

 is the tail-fold, in many respects the counterpart of the head- 

 fold although appearing so much later. This fold, in which all 

 three of the germ layers are involved (Fig. 109, B) f slowly grows 

 forward forming a hind-gut, comparable with, though much 

 less extensive than, the fore-gut. The tail bud, thus separated 

 from the yolk beneath, continues to elongate slowly for a con- 

 siderable time. Mesodermal somites continue to be added pos- 

 teriorly, through the trunk region, which extends as far as the 

 thirty-fifth somite, formed about the seventieth hour, and finally 

 through the incurved tail region (Fig. 109, B). A total of 

 fifty-two pairs of somites have been counted, although some 



