CLEAVAGE AND DIFFERENTIATION. 43 



after the complete differentiation of all the tissues and organs, 

 the number of cells may vary greatly in different individuals of 

 the same species or in the same individual at different times. 

 In adult Crepidulas the number of cells varies directly as the 

 body size varies, the cell size remaining practically constant. 

 These later divisions, in the main, are non-differential, and like- 

 wise it is probable that in the later stages of cleavage many non- 

 differential and inconstant divisions occur. Not only is there 

 greater variation in the number and size of cells in later as 

 compared with earlier stages of cleavage, but there is also 

 greater variation in the direction and time of division ; all of 

 which goes to prove that the earlier cleavages are more con- 

 stant, more frequently differential, and therefore morphologi- 

 cally more important. This view, though reached by a different 

 line of reasoning, is in entire agreement with Watase's l con- 

 clusions, and is opposed to those of Wilson. 2 



At first thought it may seem strange and improbable that 

 the earlier cleavages should be more important than the later 

 ones. It is generally, and I think truly, believed that processes 

 of differentiation increase in extent as we approach the end 

 stage. However, the greater differentiations of later stages are 

 dependent upon the lesser differentiations of earlier ones, which 

 are therefore causally the more important. Moreover, the later 

 differentiations in general are not phenomena of individual 

 cells, but of cell aggregates, whereas the differentiations of 

 cleavage are primarily differentiations of individual cells. The 

 mosaic character of cleavage is, therefore, most pronounced in 

 early stages, whereas the cellular phenomena of differentiation 

 become less prominent as development advances. 



1 op. tit., p. 38. 



2 op. dt., p. 36. 



