198 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Umbrella) while it may (in Limax and Umbrella) at the same 

 time apparently hasten the cleavage of those cells of the organ- 

 ism in which it is more abundant. This apparent conflict of 

 statements has its foundation in the hypothesis that the amount 

 of yolk alone is the decisive factor in the determination of the 

 rapidity of cleavage. But there are other factors to be consid- 

 ered, especially the quality of both yolk and protoplasm, and in 

 these there may ultimately be found some solution of the diffi- 

 culty. In the case of the presumably undifferentiated blasto- 

 meres of the cleavage stages of Limax (and Umbrella) the differ- 

 ence in the rapidity of cleavage is apparently correlated with the 

 greater or less absolute amount of protoplasm in the individual 

 cells. The amount of protoplasm, in turn, is dependent on both 

 the quality of the yolk and the activity of the protoplasm. The 

 yolk, by contributing to the amount of protoplasm in the larger 

 cells, may thus indirectly hasten their division. In this, how- 

 ever, the appropriation of the yolk by the protoplasm is the im- 

 portant factor, for in case the protoplasm fails to appropriate the 

 yolk with sufficient rapidity, the division of the yolk-laden cell 

 may be delayed as in the frog's egg. This delay may depend 

 on either one or both of the two factors, — quality of protoplasm 

 and quality of yolk. These same factors probably determine the 

 differences in the rates of cleavage of different eggs. While this 

 does not afford a solution of the difficulties encountered in at- 

 tempting to harmonize the facts here presented with Balfour's 

 law of cleavage as influenced by yolk, future inquiries in the 

 direction suggested may lead to a better understanding of the 

 factors determining the nature of cleavage.* 



Cambmdge, Mass., December 22, 1893. 



* The substance of this paper was presented at the annual meeting of the 

 American Morphological Society, in New Haven, Conn., December 29, 1893. 



