ORGAN-FORMING MATERIALS IN FROG S EMBRYO. 



in the frog's egg. A section through an eight-cell stage is 

 shown in Fig. 22. The thickness of the roof and the sides of 

 the blastocoel, and the extension of the blastoccel far down into 

 the lower hemisphere is very noticeable. In the next stage, Fig. 

 23, the roof is thinner, and the segmentation cavity extends out 

 more horizontally. The same changes are seen in the next 

 figure, Fig. 24, which is an egg in the same stage of develop- 

 ment, but compressed, in cutting, in a plane at right angles to 

 that of the last figure. An older stage is seen in the next figure, 



FIGS. 22-27. Segmentation and gastrulation (Fig. 27) stages of Bufo lentiginosus. 



Fig. 25, where the conditions are much the same. The small 

 embryo-forming cells still lie above the equator. In the next 

 stage, Fig. 26, the segmentation cavity is greatly enlarged, its 

 roof is much thinner, and now the embryo-forming cells lie at 



