882 STUDIES IN GENERAL PHYSIOLOGY 
they contained nuclear substance. So far as I was able to 
see, very small pieces developed into blastule. It is prob- 
able, therefore, that the egg can be divided much further 
when we wish only blastule to develop than when we wish 
the fragments to reach the pluteus stage. It seems prob- 
able, however, from my observations, that the blastula must 
have attained a certain size before it is able to develop into 
a gastrula, so that the limit for the amount of egg material 
necessary for development into the gastrula stage is probably 
greater than that necessary for the development of a blastula. 
7. We can here, in passing, answer a question which we 
have touched upon before, but which does not necessarily 
belong to our subject, namely: Do qualitative changes of a 
sufficiently profound nature occur in the cells during seg- 
mentation (besides the mere increase in their number), 
which interfere with the divisibility of the egg? As has 
been said, Driesch was able to convince himself that an iso- 
lated cell of the four-cell stage could still develop into a 
pluteus; but this seems not to have been possible when he 
isolated one of the cells from the eight-cell stage. He 
attributed this, very correctly, to the amount of substance 
present. Others, however, were inclined to conclude from 
such experiments that in the eight-cell stage the differentia- 
tion in the mdividual cells had progressed so far that they 
could give rise only to individual tissues, but not to entire 
embryos. But it is apparent that the limits of divisibility 
of the egg in Driesch’s experiments harmonize with those 
found in my own. If, therefore, one of the cells from the 
eight-cell stage is no longer able to develop into an entire 
embryo, this is to be attributed to the fact, as shown by my 
experiments, that the amount of material present in one cell 
in this stage does not suffice to form a pluteus. Besides 
this, of course, a differentiation might have occurred in the 
cells. I have made experiments which show that this cannot 
Digitized by Microsoft® 
