20 



INTRODUCTION. 



irregular manner in which the several clefts appear ; while Fig. 7 

 represents a vertical section of the germinal disc, with the 



I 



FIG. 3. Segmentation of the Frog's Egg. The second figure is a surface 

 view, the remaining four figures represent the egg in section, x 20. 



I, the ovum just before the completion of the first cleft, by which it is divided into 

 two equal blastomeres. II, stage with eight blastomeres : an upper tier of four small 

 ones, and a lower tier of four much larger ones. Ill, the same stage, with eight blas- 

 tomeres, in section. IV, V, later stages, showing further increase in the number of 

 the blastomeres, with great inequality in their size. B, segmentation cavity or blastoccel. 

 U, nucleus. 



