258 LILLIE. [Vol. XVII. 



goes into certain definite cells which have a perfectly constant 

 origin and destiny. 



"This differential distribution of the spheres is not caused by 

 their specific weight, since their movements are the same in 

 whatever position the Q.g% may be placed. It seems to be the 

 result of a form of polarity, which, like that of the ^gg itself, 

 is not the result of gravity. 



" The centrosomes do not, apparently, arise from the sphere 

 substance of the previous division, but some distance from it, 

 and the sphere substance itself never divides, but each sphere 

 ultimately grows ragged at its periphery, and gradually fades 

 out into the general cytoplasm. 



" The differential distribution of the spheres and their sub- 

 sequent conversion into cytoplasm suggests that they may be 

 important factors in the differentiation of cleavage cells, and if 

 further investigation should establish the fact that they are in 

 part composed of the oxychromatin of the nucleus, it would 

 furnish a basis in fact for certain well-known speculations of 

 de Vries, Weismann, and Roux." 



VI. Two Recent Theories of Unequal Cleavage. 



Before proceeding to a general consideration of the organi- 

 zation of the egg of Unio let us take up two recent theories of 

 unequal cell division. Those views that seek a simple mechan- 

 ical explanation either in the arrangement of yolk or other 

 inclusions, or in extrinsic forces, have been already sufficiently 

 criticised by Jennings ('96), Conklin ('97), zur Strassen ('98), 

 myself ('95 and '99), and others. 



I . Zieglers TJicory of the Unequal Power of the Centers. 



Ziegler has suggested the theory of centers of unequal power 

 for the explanation of certain unequal cleavages in the ontogeny 

 of nematodes and the ctenophores ('96 and '98). He styles an 

 unequal cleavage in which the centers are of unequal power 

 "heterodynamic." Ziegler does not claim, himself, to have 

 observed any difference in the centers in such cases, but he 



