364 



ZOOLOGY 



SKCT. 



cells — the so-called primitive Icnot ( j)r. sL), and on this is formed 

 an invagination opening on the surface by an aperture — the 

 blastopore {Up.) — Avhich subsequently takes the form of a narrow 

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



phpl 



Fifi. 10117. -Lacerta, longitudinal scution.s through the embrvonic area of blastoderms 

 illustrating suc^fcKsive st«gus iu the fonnatioii of the inv.agiiiatioii-cavitv (archciiteron) and 

 Its cominunuratK.n with the segnieiitation-tavity. Up. blast ojun-e ; ec. ectoderm of embrvonic 

 sliield; k-n. primitive knot; ;-)•. pi. protoehonlal plate; -^ t. volk-endodirm. Tn JJ. the 

 opening below and in front of /;//'■ points to aperture of oonnnun'icatiou established between 

 the invagiuation-cavity and the underlyhig space. (Modified after Wenckebach.) 



The cavity of the invagination corresponds to the archenteron of 

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

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

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



