ADAPTATION IN CLEAVAGE. 65 



Thus, putting Conklin's results 1 on the movements of the 

 sphere-substance and my own together, it would appear that, 

 whereas in the first two cleavages this substance is divided 

 between the cells in proportion to their size, in the formation 

 of the generations of ectomeres the substance enters special 

 cells. This would coincide very closely with the differential 

 value of the cleavages, the first four cells possessing ectoblastic, 

 entoblastic, and, in part, mesoblastic materials, while the three 

 subsequent divisions of the macromeres separate ectoblastic 

 portions. This tends, it seems to me, to strengthen Conklin's 

 conclusion that the sphere-substance may be an important 

 factor in the differentiation of cells. 



Finally, I do not believe that the process of nuclear or cell- 

 division is ever in itself an act of differentiation. That it is 

 not, in certain cases at any rate, is shown beyond the possibility 

 of any doubt by examples of non-determinate cleavage, such as 

 that of the fish-egg, in which the cleavage-planes bear no con- 

 stant relation to each other or to the embryonic parts, and, 

 still more strikingly, in the case of ciliate Infusoria, where the 

 entire process of development takes place without any cell- 

 division. If my observations are correctly interpreted in 

 what has preceded, the essential process in early embryo- 

 formation proceeds on the basis of a definite orientation and 

 organization of the egg-substance, carried forward and elab- 

 orated by certain intercellular processes, in which the produc- 

 tion of special substances which have been acted on by the 

 chromatin may play an important role. Now the distribution 

 of these substances is not dependent on cell-division, though by 

 this they may be isolated in separate cells ; but it is conceiv- 

 able that the cleavage-planes may, so to speak, ignore the lines 

 of orientation of the egg and of distribution of specific parts of 

 it ; thus it may be that determinism in the cleavage is no 

 measure of the degree of organization of the egg, as Whitman 

 has so ably argued. 



It is quite possible that there is no sharp distinction between 



1 In Crepidiila, apparently, the substance of the spheres is not divided in the 

 second cleavage, but passes into special cells. See lecture by Dr. Conklin in this 

 volume. 



