262 BIOLOGICAL LECTURES. 



fectly indifferent material, except as regards position, and the 

 true significance of the segregation may lie in the fact that a 

 certain amount of material is present, which is to be distributed 

 through the means of a series of perfectly definite cell divisions. 

 By "distribution " is meant the arrangement of the material in 

 such a manner that the various morphogenetic processes are 

 directly accomplished thereby, e.g., formation of the somatic 

 plate, gastrulation, which in Arenicola and some related forms 

 is probably accomplished very largely by the growth of the 

 somatic plate, formation of the mesoderm bands, beginning 

 elongation of the larva, etc. The important point in this view 

 lies, I believe, in the conclusion that these results are accom- 

 plished, at least very largely, directly by the energy of cell divi- 

 sion itself. Given a certain degree of cohesive power in the 

 protoplasmic material, a certain degree of surface tension, and 

 a certain definite series of cell divisions and the result must 

 always be the same, and the same cells will always occupy the 

 same relative positions at corresponding stages and will form 

 the same parts of the whole, i.e., will have the same fate under 

 normal conditions. This would be true even if the sesfreg-ated 

 material should remain totipotent long after its segregation. 



Since the fate of the various cells is definitely established 

 and can be foretold, it is possible that differentiation might 

 begin at an earlier stage than otherwise, but there is no reason 

 to suppose that the process of cell division is connected with it 

 even here. To take an example : why should the two lateral 

 cells of the paratroch on each side in Arenicola pass through 

 one more division than the dorsal cells before they form the 

 paratroch (Fig. 15).? If cell division is connected in these 

 eggs with differentiation, it must be either that one of these 

 divisions is "non-differential," according to Conklin's distinc- 

 tion, or else it is necessary to assume that the lateral cells 

 are more highly differentiated than the dorsal cells. As 

 regards the first assumption, there seem.s to be no definite cri- 

 terion of differential divisions, so that no decision is possible. 

 As regards the second, there is no reason to believe that it is 

 true. Taking up the matter from a different point of view, viz., 

 the relation of the constituent parts with regard to the result 



