PHYLUM CHORDATA 



353 



cells the so-called primitive knot or primitive plate (pr. st.), and 

 on this is formed an invagination opening on the surface by an 

 aperture the blastopore (blp.) which subsequently takes the 

 form of a narrow slit running in the direction of the long axis of 



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PIG. 1018. Lacerta, longitudinal sections through the embryonic area of blastoderms 

 illustrating successive stages in the formation of the iuvaginatiou-cavity (archenteron) and 

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

 bryonic shield ; kn. primitive knot ; pr. pi. protochordal plate ; y. e. yolk-endoderm. In D 

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

 between the mvagination-cavity and the underlying space. (Modified after Wenckebach.) 



the future embryo. The cavity of the invagination corresponds 

 to the archenteron of the Frog, and the cells lining it are the 

 endodenn (primitive endoderm). The latter subsequently (Fig. 

 1018) coalesces with the yolk-endoderm below the floor of the 

 archenteron, and in this position an aperture is formed through 



