364 



ZOOLOGY 



cells the so-called primitive knot (pr. .<?.), and on this is formed 

 an invagination opening on the surface by an aperture the 

 blastopore (hip.") which subsequently takes the form of a narrow 

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



FIG. 1007. Laccrta, longitudinal sections through the embryonic area of blastoderms 

 illustrating successive stages in the formation of the invagination-cavity (archenteroii) and 

 its communication with the segmentation -cavity. Up. blastopore ; ec. ectoderm of embryonic 



shield ; kn. primiti 



ve knot ; pr. pi. protochordal plate ; ?/. e. yolk-endoderm. 

 in front of Mp. points to aperture of communication establish 



In D. the 



opening below and in front of Mp. 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 



