370 



CELL-DIVISION AND DEVELOPMENT 



mechanical conditions ; it is beyond this " a builder which lays one 

 stone here, another there, each of which is placed with reference to 

 future development."^ 



The third class of modifications, due to unequal division of the cells, 

 not only leads to the most extreme types of cleavage but also to its 



Fig. 172. — The eight-cell stage of four different animals showing gradations in the inequality of 

 the third cleavage. 



A. The leech Clepsine (Whitman). B. The oh^Xo'^o^ Rhynchelmis (Vejdovsky). C. The 

 lamellibranch Unto (Lillie). D. Amphioxus. 



most difficult problems. Unequal divisions appear sooner or later 

 in all forms of cleavage, the perfect equality so long maintained 

 in Synapta being a rare phenomenon. The period at which the in- 

 equality first appears varies greatly in different forms. In Polygordius 

 (Fig. 170) the first marked inequality appears at the fifth cleavage; 



1 Lillie, '95, p. 46. 



