ORGAN-FORMING MATERIALS IN FROG S EMBRYO. 



I 2 



ability to the eight-cell stage, where it exists partly in the upper 

 four cells and partly in the upper ends of the larger lower four 

 cells. In the later segmentation stages, and in the pregastrula 

 stages, this material moves down around the sides of the egg until 

 it comes to lie some distance below the floor of the segmentation 

 cavity. Here it becomes, as described above, the germ-ring, 

 which closing over the lower hemisphere produces the " embryo." 



FIGS. 1-4. Segmentation stages of ihe Rana paltntris. 



The details of these changes, and the evidence on which these 

 statements rest, may now be considered more fully. 



