2/: 



CELL-DIVISION AND DEVELOPMENT 



ular succession at a fixed point, thus giving rise to long cords of cells 

 (Fig. 125). The teloblasts are especially characteristic of apical 

 growth, such as occurs in the elongation of the body in annelids, and 

 they are closely analogous to the apical cells situated at the growing 

 point in many plants, such as the ferns and stoneworts. 



Fig. 123. — The 8-ceIl stage of four different animals showing gradations in the inequality of 

 the third cleavage. 



A. The leech Clfpsinc (Whitman). B. The cheetopod Rhynchelmis (Vejdovsky). C The 

 lamellibranch Unlo (Lillie). D. Amphioxus. 



Unequal division still awaits an explanation. The fact has already 

 been pointed out (p. 51) that the inequality of the daughter-cells is 

 preceded, if not caused, by an inequality of the asters ; but we are 

 still almost entirely ignorant of the ultimate cause of this inequality. 

 In the cleavage of the animal egg unequal division is closely con- 

 nected with the distribution of yolk — a fact generaHzcd by Balfour 



