No. 2.] ORGANIZATION OF THE EGG OF UNIO. 259 



believes that in many cases of unequal cleavage there is no 

 other explanation possible. The theory certainly is an advance 

 over the blind faith in invisible yolk that plays so prominent 

 a part in Hertwig's theorizing ('92 and '98) in such matters, 

 inasmuch as it squarely meets the conditions of the problem 



unequal cleavage in a mass of protoplasm, in which yolk 



may be absent or uniformly distributed. But it appears to me 

 to be without satisfactory foundation in fact. If the postulated 

 difference really exists, it should be evident from the start in 

 unequal mastery of the cytoplasm by the two centers ; but if we 

 take the case that Ziegler himself instances as the most marked 

 example of " heterodynamic " cleavage, the formation of the 

 polar globules, we find that in the first maturation spindle, 

 when first formed and lying horizontally, there is no discov- 

 erable difference in the size or appearance of the centrosomes 

 or asters at the ends of the spindle. It is only after the spindle 

 has rotated, and one end has moved out to the surface, that a 

 difference is to be noted ; and this is a secondary difference, 

 due not to unequal force but to unequal opportunity. The 

 same is true of every unequal cleavage of the entire ontogeny. 

 As Conklin has pointed out, the centrosomes accommodate 

 themselves to the size of the mass of cytoplasm in which they 

 come to lie, being at first equal. Inequality of centers and 

 asters is an effect, not a cause, of unequal cleavage. 



But one has a right to expect of a theory meant to be 

 purely formal, that it should be at least far-reaching ; here, 

 however, we are told simply, the cause lies not in the proto- 

 plasm but in the centrosomes. How the centrosomes become 

 of unequal power, and how this is adapted to embryo forma- 

 tion, of this not the slightest clue. 



It goes without saying, then, that I cannot find in this 

 theory any explanation of the unequal cleavage under discus- 

 sion. I have looked in vain, though with great care, for any 

 difference in the two centers of the first cleavage, while the 

 spindle still occupies the center of the egg. A difference 

 becomes noticeable only after the spindle has become excentric, 

 and this is plainly due to the fact that the centers are now 

 operating in areas of unequal extent. 



