FORMS OF CLEAVAGE. 



203 



Each of these four first cleavage-cells consists of an upper, 

 darker, and of a lower, brighter half A few hours after- 

 wards a third furrow appears, perpendicularly to the two 



D 



Fig. 31. — The cleavage of a Frog's egg (10 times enlarged) : A, the 

 parent-cell ; B, the two first cleavage-cells ; C, 4 cells ; D, 8 cells (4 

 animal and 4 vegetative) ; E, 12 cells (8 animal and 4 vegetative) ; F, 

 16 cells (8 animal and 8 vegetative) ; O, 24 cells (16 animal and 8 vege- 

 tative); H, 32 cells; I, 48 cells; K, 64 cells; L, 96 cleavage-cells; M, 

 160 cleavage-cells (128 animal and 32 vegetative), 



former (Fig. 31, B). This ring-furrow is generally, but 



wrongly, called the " equatorial furrow ; " it lies north from 



the equator, and should, therefore, rather be compared to the 

 11 



