THE SPIRAL TYPE OF CLEAVAGE. 235 



upon fixed material, but since these were made a very inter- 

 esting series of observations upon living cells have appeared 

 (G. F. Andrews, '97; A. E. Andrews, '97a, '97b, '98a, '98b, '980), 

 which must, I believe, lead us to the conclusion that intercel- 

 lular protoplasmic continuity between animal cells is at least 

 of very wide occurrence, if not universal, during developmental 

 stages (Figs, i and 2). This being the case, we have, it seems 

 to me, very strong evidence upon both the botanical and zoologi- 

 cal side for the correctness of 

 the so-called organism stand- 

 point, as distinguished from 

 any cell theory. And I be- 

 lieve we must admit that 

 intercellular • protoplasmic 

 continuity is of the greatest 

 importance in our interpreta- 

 tion of the organism. 



... , . r Fig. 2. — Two-cell stage of the egg of a fresh-water 



AltnOUgll my study 01 nemertean, showitig the ectoplasmic activity in 



cleavage was beo'Un from a ''^^ cleavage furrow and the rounded form of the 



^ ... cells immediately after division. 



point of view coinciding fully 



with the cell theory, my observations have led me to the belief 

 that any explanation of the spiral cleavage is impossible upon 

 that basis. The developing egg of the turbellarian, annelid, 

 and mollusk is, I believe, more than a cell mosaic ; it is dis- 

 tinctly an organism at every stage. 



I desire, then, to discuss the spiral type of cleavage, not as 

 a series of self-differentiations or cellular interactions, but as a 

 series of processes, each of which has its cause in the organism, 

 as distinguished from this or that cell. From this point of view 

 alone, as I believe, is it possible to explain the various phenom- 

 ena observed, and especially the lack of correspondence in ori- 

 gin and fate of the blastomeres, which is not explicable by any 

 strictly cellular theory of development. 



In his classical paper upon Nereis, Wilson ('92) has given us 

 a most valuable discussion of the origin of the spiral form of 

 cleavage, and it is desirable at this point to review this briefly. 

 In general, his view is as follows : " The ficndainental forms of 

 cleavage are primarily due to mechattical co7iditions, and are only 



