The ' Segmentation Cavity ' of the Egg of 

 the Frog. 



By 

 Alexander 3Ieek. 



With Plate 1 and 6 Text-figures. 



The egg of the frog, Eana temporaria, starts its career 

 of segmentation as does that of Amphioxus, but after the one- 

 celled blastula stage a delamination of cells takes place, with 

 the result that the wall becomes multicellular. The cavity is 

 excentrally situated, the anterior wall being thin and the 

 posterior wall thick. The latter consists of endoderm, and 

 while internally it is impossible to point to a distinct demarca- 

 tion between ectoderm and endoderm, externally an equatorial 

 zone is defined which marks the limits of ectoderm and endo- 

 derm, and this is emphasized when the zone becomes a ring of 

 proliferation indicating the beginning of the activities of the 

 blastopore. The proliferation is followed by the appearance 

 of a dorsal groove, the dorsal lip of the blastopore. An in- 

 vagination takes place, confined, however, to the dorsal aspect 

 of the embryo, and a horizontal slit-like cavity is produced 

 which penetrates anteriorly above the segmentation cavity and 

 is extended posteriorly as the dorsal lip of the blastopore 

 advances over the large cells of the yolk-plug. This cavity is 

 the enteron, and while it is forming the original cavity, which is 

 still called the segmentation cavity, acquires a cup shape, 

 the anterior wall being definitely resolved into ectoderm, and 

 the endodermal margin of the cup gradually narrows and 

 ultimately fuses, so that the cavity becomes enclosed in endo- 

 derm. It is almost universally believed that the segmentation 

 cavity is compressed by the expanding enteron and that 

 it finally disappears antero-ventrally in the endoderm. But 



NO. 265 D 



