THE EARLY DEVELOPMENT OF THE CHICK 281 



ten of these disappear again during the eighth clay of embryonic 

 life, leaving a total of forty-two pairs. 



The formation of the tail-fold is but one phase in the general 

 process of the separation of the embryo from the remainder of 

 the blastoderm, and at the same time from the yolk. The for- 

 mation of the head-fold we have already described. We may 

 recall that this extended backward along each side of the head. 

 Posteriorly this now becomes continued either side of the trunk, 

 by a somewhat similar lateral fold, which finally reaches to the 



la 



a 





^2 



vv 



FIG. 110. Diagrammatic transverse section through the region where the 

 gut is open out over the yolk (yolk-stalk umbilicus), in a chick of about forty- 

 eight hours (about twenty-eight pairs of somites). After Duval. a, Dorsal 

 aorta; c, ccelom; ebc, exoccelom; ig, intestinal groove; la, lateral folds of amnion; 

 vv, vitelline vein. 



tail-fold. These folds involve both somatopleure and splanch- 

 nopleure (Fig. 110). The lateral folds gradually extend inward 

 toward the mid-line beneath the embryo, converting the flat 

 sheet of endoderm into a median fold above the yolk; this is 

 the intestinal groove, the rudiment of the true intestine (Fig. 

 110). The head- and tail-folds continue to extend posteriorly 

 and anteriorly, respectively, increasing the extent of the fore- 

 and hind-gut. These connect with the intestinal groove by 

 the anterior and posterior intestinal portals. 



Gradually these folds all approach beneath the embryo, cut- 

 ting it off from the yolk, and marking a clear distinction 

 between embryonic and extra-embryonic regions. Finally, 

 about the fifth day, the lateral folds of the splanchnopleure fuse 

 together extensively, and the gut is entirely closed off except 

 in a restricted region in the hinder part of the trunk (Fig. 115). 



