EMBRYOLOGY OF CLEPSINE. 301 



which had gradually acquired a separate derie and enteric cell- 

 layer in place of one cell-layer with an external deric moiety and 

 an internal enteric moiety to each cell, must have tended in their 

 individual development from the egg-cells of parent Diblastulae 

 to have established more and more early, in the course of their 

 growth, the important separation of deric and enteric cells, of 

 ectodermic and endodermic elements. In so far as the differen- 

 tiation of the two kinds of factors or molecules, the deric and the 

 enteric became dependent on heredity, and less dependent on 

 the direct adaptative causes which first brought about the differen- 

 tiation, in so far would it be possible for the differentiation, the 

 segregation of deric molecules from enteric molecules, to take 

 place at an earlier point in the embryonic development than that 

 (namely, the blastula stage), at which the direct adaptative causes 

 could come into operation. Thus, since the fertilised egg already 

 contained hereditarily-acquired molecules, both deric and enteric, 

 invisible though differentiated, there would be a possibility that 

 these two kinds of molecules should part company, not after the 

 egg-cell had broken up into many cells as a morula, but at the 

 very first step in the multiplication of the egg-cell. In fact, 

 some or all of the deric molecules might remain in one of the two 

 first cleavage- cells, and all of the enteric molecules, with or with- 

 out some of the deric molecules, might remain in the other. We 

 should not be able to recognise these molecules by sight ; the 

 two cleavage-cells would present an identical appearance, and yet 

 the segregation of deric and enteric factors had already taken 

 place. This hypothesis may be called that of Precocious Segrega- 

 tion, " precocious," since it is the acquirement of a condition in 

 the developing organism, in virtue of heredity, at an earlier period 

 of development than that at which such acquirement was attained 

 by its forefathers through adaptation." 



The principle of segregation here so clearly enunciated can 

 hardly be doubted. 



The question on the answer to which everything turns is this : 

 Is the segregation which leads to invagination more precocious 

 than that which terminates in delamination ? A negative answer 

 to this question would be inconsistent with the above explanation 

 of the transition from delamination to invagination. 



Van Beneden has reported an exceptionally sharp differentia- 

 tion of ectoplasm from endoplasm, which appears with the first 

 cleavage (rabbit). So far as yet known this case is without a 

 parallel. 



In the vast majority of cases where invagination occurs each 

 of the two first segments contain both ectoplasm and endoplasm. 

 In the case of Unio one blastomere is wholly ectodermic, but the 

 other contains both elements, and the separation of the two 



