86 



COLLEGE ZOOLOGY 



on the first cleavage spindle (Fig. 49, j) that each daughter 

 nucleus receives half of each. This means that each daughter 

 cell will contain half of each chromosome of paternal origin and 

 half of each chromosome of maternal origin. Further mitotic 

 divisions insure a like distribution to every cell in the body. 



After nuclear division 

 comes the division of 

 the entire cells into two 

 (k and I). 



Typically the ferti- 

 lized egg divides into 

 two cells, these two 

 into four, these four 

 into eight, etc., each 

 cleavage plane being 

 perpendicular to the 

 last preceding plane 

 (Fig. 51). This is 

 known as total cleavage, 

 and is characteristic of 

 holoblastic eggs. Other 

 eggs are said to be 

 meroblastic and exhibit 

 partial cleavage ; that is, 

 only a small part of the 

 egg enters into cell 



a marine worm. C, discoidal cleavage of the division the remainder 

 egg of a squid. D, superficial cleavage of an ' . . 



insect's egg. (A-B, from Wilson; C, from Serving as nutritive 



Wilson, after Watase; D, from Korschelt and -material for the cleav- 

 Heider.) 



age cells. In all we 

 can recognize four distinct types of cleavage: (1) equal cleavage, 

 where the egg divides into two equal halves (Fig. 50, A) ; 

 (2) unequal cleavage, where the first division of the egg results 

 in one large and one small cell (Fig. 50, B) ; (3) discoidal cleavage, 

 where the entire egg does not divide, but small cells are cut off 



;'fi--t« 



. Fig. 50. — Figures illustrating four different 

 kinds of cleavage. A, equal cleavage of the sea- 

 urchin egg. B, unequal cleavage of the egg of 



