66 BIOLOGICAL LECTURES. 



determinate and indeterminate cleavage, and that one grades 

 into the other, the apparent difference being due to insufficient 

 knowledge, as Conklin and Eisig (5) have suggested. But there 

 can be no doubt in the mind of any one as to the existence of 

 a very real difference between determinate and indeterminate 

 types of cleavage, who has compared, for instance, the cleavage 

 of the Q.gg of an annelid, possessing a perfectly definite and 

 unvarying mode of cleavage and cell-lineage of organs, with 

 that of a fish, in which slight alterations of the external condi- 

 tions cause the very greatest variations in cleavage, so that 

 often the cells of, two eggs of the same species cannot be 

 homologized, and no definite cell-lineage of organs exists. The 

 explanation of this difference, it seems to me, is a prospective 

 one. It is dependent, I believe, on the actual number of cells 

 composing the embryo at the time that the- first larval or 

 embryonic organs come into service. In other words, I would 

 think of determinate cleavage as an adaptation to a condition 

 in which the functional activity of organs begins with a rela- 

 tively small number of cells, and in which, therefore, each cell 

 is of special importance. 



Marine Biological Laboratory, 

 August, 1898. 



