AMPHIBIAN GASTKULA. 207 



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, H). This is the primitive intestinal cavity or 

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

 first observed 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 y>lug 

 up the entire wide opening of the primitive mouth, and 

 there form the so-called 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 properistowta, 

 Fig. 25 k, h'). Presently the primitive intestinal cavity {N) 

 extends gradually at the cost of the cleavage-cavity {F) ; 

 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 {R). 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 Gastrula, which 



