THE MOSAIC THEORY OF DEVELOPMENT. 13 



tion precedes or accompanies division, the latter process may 

 be in a sense qualitative. If it follows, division will be purely 

 quantitative, and in such a case we may rightly speak of differ- 

 entiation as a result of cellular interaction. The segmentation 

 of the ^%^ presents more or less of a mosaic-like character, 

 according to the period at which differentiation appears, and 

 the rate at which it proceeds, as expressed in limitations of 

 the power of development in the individual blastomeres, and 

 their differences in size and structure. 



The general interpretation of development which I have thus 

 endeavored to sketch will be found to differ widely in some 

 respects from that set forth in one of the subsequent lectures 

 of this volume, from which, through Professor Whitman's 

 courtesy, I am enabled to quote. Whitman argues that 

 "cell-orientation may enable us to infer organization, but to 

 regard it as a measure of organization is a serious error." 

 "The question as to the presence of organization," he says, 

 " is not settled by the form of cleavage. Eggs that admit of 

 complete orientation at the first or second cleavage, or even 

 before cleavage begins, are commonly supposed to reflect 

 precociously the later organization, while eggs in which such 

 early orientation is impossible are supposed to be more or less 

 completely isotropic and destitute of organization. When the 

 region of apical growth is represented by conspicuous telo- 

 blasts, the fate of which is seen to be definitely fixed from 

 the moment of their appearance, we find it impossible to doubt 

 the evidence of organization, or ' precocious differentiation ' as 

 it is conventionally called. When the same region is composed 

 of more numerous cells, among which we are unable to distin- 

 guish special proliferating cells, we lapse into the irrational 

 conviction that the absence of definitely orientable cells means 

 just so much less organization." 



It would be manifestly out of place to enter here upon any 

 of the interesting discussions suggested by the passage just 

 quoted, and I will therefore only add that Professor Whitman's 

 position seems to me to rest upon a special and peculiar use 

 of the word "organization," and that his view leads to a denial 

 of the principle of epigenesis. No one would maintain that 



