AMPHIBIAN GASTRULA. 20/ 



A second cavity, narrower but larger, now arises, owing 

 to an inversion of the lower pole, and to a separation in 

 the white entoderm-cells next to the cleavage-cavity (Fig. 

 32-35, jS^). This is the primitive intestinal cavity or 

 stomach-cavity of the Gastrula, the Protogaster. It was 

 first obseiwed by Rusconi in the eggs of Amphibia, and 

 is accordingly called Rusconi's " nutritive cavity." In the 

 longitudinal section (Fig. 33) it appears bent and sickle- 

 shaped, and extends from the south pole nearly to the 

 north, for it folds a portion of the inner intestinal cells 

 inward and upward — between the cleavage-cavity {F) and 

 the dorsal skin (R). The primitive intestinal cavity is so 

 narrow at first because the greater part of it is filled up 

 with the yelk-cells of the entoderm. The latter also ]Ang 

 up the entire wide opening of the primitive mouth, and 

 there form the so-caUed yelk-plug, which appears from the 

 outside as the white, circular spot at the south pole (P). 

 Round this yelk-plug the skin-layer thickens, swells, and 

 forms the lip of the primitive mouth (the properistoma. 

 Fig. 35 k, k'). Presently the primitive intestinal cavity (N) 

 extends gradually at the cost of the cleavage-cavity (P) ; 

 and, finally, the latter entirely disappears. A thin partition 

 (Fig. 34, s) alone separates the two cavities. That portion 

 of the germ in which the primitive intestinal cavity de- 

 velops, afterwards becomes the dorsal surface (B). The 

 cleavage-cavity lies in the anterior, the yelk-plug in the 

 posterior part of the body.*'^ 



When the primitive intestine is complete, the Frog- 

 embryo has reached the Gastrula stage (Plate II. Fig. 11). 

 But it is evident that this kenogenetic amphibian Gastrula 

 differs greatly from the genuine palingenetic Gastrala, which 



