THE SPIRAL TYPE OF CLEAVAGE. 26 1 



trochoblasts " in origin may be formed without ever forming a 

 prototroch. 



It is hardly necessary to remark that the differentiation of 

 an Qgg into protoplasmic and deutoplasmic portions, which so 

 often accompanies cleavage, undoubtedly has, in most cases at 

 least, a comparatively simple explanation. 



Diffej'cntiation is then, I believe, ^^ a ftinctioti of position'' (cf. 

 Driesch). Whether the fnaterial is contained within one cell or 

 many is a -ynere incident so far as the result is concerned. The 

 comparative study of tJie spiral type of cleavage leads irresistibly 

 to this coficlnsion. 



The appearance of definite protoblasts in cleavage does not 

 necessarily imply that they contain a specific material which is 

 necessary for the formation of the organ in question. Proto- 

 blasts are, I think, to be regarded, in general, as centers of dis- 

 tribution of the material of the egg ; and their formation is 

 probably due to a condensation in the process of development, 

 or a saving of energy, as Conklin suggested. As I understand 

 Conklin's position, however, he regards the saving of energy as 

 occurring in processes of differentiation merely, for, as he 

 remarks, " it is possible to see that in an organ which reaches 

 functional activity after a dozen divisions less energy has been 

 expended than in one which reaches this stage only after one 

 hundred divisions." This is undoubtedly true, but how does it 

 explain the formation of the whole trunk ectoderm from the 

 first somatoblast, which occurs in some annelids, or the forma- 

 tion of the mesoblasts, for the descendants of these cells do 

 not, for the most part, become "functional" in the definitive 

 sense for a long time, and then chiefly with reference to the 

 adult body and not the larval .■* There is surely no reason for 

 believing that the number of cells in the organs of the adult is 

 less in those forms in which precocious segregation has occurred 

 than in others. Lillie ('95) has pointed out the fact that "almost 

 every detail of the cleavage of the ovum of Unio can be shown 

 to possess some differential significance." It is undoubtedly 

 significant as regards the distribution of the Qgg material, but 

 that is not necessarily differentiation. The material separated 

 as the result of precocious segregation may, I believe, be per- 



