288 OUTLINES OF CHORDATE DEVELOPMENT 



tinues in the manner previously described, the differentiation 

 of the mesoderm and the subsequent formation of exoccelom, 

 somatopleure, and splanchnopleure, slowly proceeding centri- 

 f ugally . During the fifth day the blastoderm extends completely 

 over the yolk, save for a narrow circular space, opposite the 

 embryo, which remains uncovered until toward the close of 



FIG. 116. Ninth day of incubation. The yolk-sac umbilicus has become 

 much narrowed, it is surrounded by the mesodermal connective-tissue ring, 

 and by the free edges of the ectoderm and endoderm. The vitelline membrane 

 still covers the yolk-sac umbilicus and is folded into the albumen. The allantoia 

 has expanded around the amnion and yolk-sac and its outer wall is fused with the 

 chorion. It has pushed a fold of the chorion over the sero-amniotic connection, 

 into which the mesoderm has penetrated, and thus forms the upper fold of the 

 albumen-sac. The lower fold of the albumen-sac is likewise formed by a dupli- 

 cation of the chorion and allantois; it must be understood that lateral folds are 

 forming also, so that the albumen is being surrounded from all sides. The stalk 

 of the allantois is exaggerated so as to show the connection of the allantois 

 with the embryo; it is supposed to pass over the amnion, and not through the 

 cavity of the latter, of course. (For explanation of lettering see Fig. 117.) 



embryonic life (Fig. 115). The mesoderm and associated 

 structures of the blastoderm push around more slowly, but 

 finally arrive at the blastodermal margin, which becomes a 

 narrow ring called the yollt-sac umbilicus. The yolk-sac itself 

 is the splanchnopleural portion of the extra-embryonic blasto- 

 derm, one surface applied directly to the surface of the yolk, the 



