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 
