332 STUDIES IN GENERAL PHYSIOLOGY 



they contained nuclear substance. So far as I was able to 

 see, very small pieces developed into blastulsB. It is prob- 

 able, therefore, that the egg can be divided much further 

 when we wish only blastulse 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 individual 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 



