364 



ZOOLOGY 



SECT. 



cells— the so-called frimifAve knot {pr. sL), and on this, is formed 

 an invagination opening on the surface by an aperture— the 

 blastopore (Up.)— which subsequently takes the form of a narrow 

 slit running in the direction of the long axis of the future embryo. 



Fig. 1007. — liacerta^ longitudinal seutlons throng'h the embryonic area of blastoderms 

 illustrating successive stages in the formation of the invagination-cavitj' (ai-chenteron) and 

 its com niu nidation with the segmentation-cavity, hip. blastopore ; ec. ectoderm of embryonic 

 shield; L'li. primitive knot; pr. pi. protochordal plate; i/. e. yolk-cndoderm. In D. the 

 opening below and in front of Up. points to aperture of communication established between 

 the invagination-cavity and the underlying space. (Modified after Wenckebach,) 



The cavity of the invagination corresponds to the archenteron of 

 the Frog, and the cells lining it are the endoderm (primitive endo- 

 derm). The latter subsequently (Fig. 1007) coalesces with the 

 yolk-endoderm below the floor of the archenteron, and in this 



