20 



INTRODUCTION. 



iiTegnlar manner in wliicli the several clefts appear ; while Fig. 7 

 rtical section of the germinal disc, with the 

 T 



represents a vert 



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

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



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

 two equal blastomeres. II, stage with eight blastomeres ; au 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 ['nrtlier increase in the number of 

 the blastomeres, witli great inequality in their size. B, scgmcutatiou cavity or blastocoel. 

 U , nucleus. 



