Partial Developments 109 



blastomeres. As is well known it was these very half 

 embryos that caused Roux to construct his evolutionistic 

 theory, in which he compares development, at least in 

 so far as the four quarters of the embryo come into 

 consideration, with a mosaic work. 



So long as the half formations arising from isolated 

 blastomeres are limited to the very first divisions, so 

 long for instance as one of the first two blastomeres, 

 when isolated, limits itself to giving half of the total 

 number of micromeres, or so long as the first cleavage 

 spheres arising from the isolated blastomeres succeed each 

 other and arrange themselves as if the two blastomeres 

 had remained united, so long there is still nothing to be seen 

 in these phenomena which would afford any proof against 

 simple epigenesis. For in general we can suppose that 

 the deutoplasm alone is the immediate cause of the num- 

 ber, and of the different relative sizes and disposition of 

 the first blastomeres. If then the relations to the yolk 

 of the blastomere and of the whole of the blastomeric 

 group, could not by themselves constitute any proof 

 change through the isolation of the blastomere, it is 

 clear that the first cleavages must proceed exactly as 

 though no isolation whatever had taken place. 



So for example the isolated blastomeres of the two 

 or four cell stage of the egg of the gastropod, Ilyanassa 

 obsoleta, which divide in essentially the same manner as 

 they would if they were part of the complete blastomeric 

 group, could not by themselves constitute any proof 

 whatever for or against any given developmental theory, 

 so long as the separated blastomeric group does not take 

 on any really specific form. For in this Ilyanassa obsoleta 

 the yolk is distinguished by its great mass, thickness and 



