I o8 CONK LIN. [Vol. X 1 1 1 . 



The repeated division of small cells like the apical and tip 

 cells, when others like the turrets, ten or twenty times as large, 

 remain undivided, suggests an inquiry into the cause or stimu- 

 lus of cleavage in a normal egg. The difference between the 

 turret and apical cells, for example, is not to be found in the 

 fact that one is laden with yolk or food material, while the other 

 is not. Both are protoplasmic cells derived from the first 

 quartette of ectomeres, lying on the same side of the egg, for 

 a long time in close contact, with apparently the same condi- 

 tions of nutrition, growth, and external environment, the dif- 

 ferences of size in the early stages being the reverse of those 

 in the later; and yet the smaller cell grows continually and does 

 not divide at all, and the larger cell, while growing no more than 

 the other, divides repeatedly, producing, at the stage shown in 

 Diagram 15, twelve cells, whose total mass scarcely exceeds 

 that of a single posterior turret. What the normal stimulus to 

 cleavage may be is not definitely known, but to any one who 

 will attentively study any definite and regular cleavage it will 

 be abundantly evident, I think, that the stimulus is not to be 

 found in external environment alone, but rather in internal 

 conditions. How any one can follow the history of the blasto- 

 meres of an ovum like that of Crepidula, and still maintain that 

 the peculiarities of each cell are due entirely to external condi- 

 tions or to intercellular relations, is more than I can under- 

 stand. To me it seems absolutely necessary to believe that 

 between cells with such different histories there must be sotne 

 internal or constitutional difference. 



The cause of the small number of divisions of these cells 

 and of their large size in both annelids and mollusks is cor- 

 related with their prospective destiny. And, at least among the 

 mollusks, I believe that a law might be formulated to the effect 

 that the size of cells in general, the frequency and direction of 

 their divisions, and the size of the resulting cell products a7'e all 

 correlated with the ultimate tises to which these cells are put} 



1 Lillie ('95) has advanced a similar view in the case of Unio, and supports it 

 by a number of observations in which he shows conclusively that there is a 

 close relation between the size of a blastomere and the size of the part to which it 

 gives rise. The ground here taken is merely an extension of Lillie's proposition. 

 It is not always true that the size of a blastomere when first formed is proportional 



