60 BIOLOGICAL LECTURES. 



originated indeed from a fragment whose mass was not less 

 than one-eighth of the fertilized ovum. As I mentioned before, 

 after the blastula stage is reached the small fragments of an 

 ovum which has been made to burst develop as a rule more 

 slowly than the larger pieces. Now I showed in my " Unter- 

 suchungen zur physiologischen Morphologic " that processes 

 of growth and of organization are within certain limits func- 

 tions of the same variables. Therefore we have reason to 

 believe that small fragments of an egg grow more slowly than 

 larger pieces. If, therefore, in such experiments we find a 

 Pluteus whose volume is only one-eighth that of a normal 

 Pluteus, we may be certain that this small Pluteus comes from 

 a fragment that under no circumstances was less than one- 

 eighth the mass of the normal egg. I will not deny the possi- 

 bility that a later observer may find still smaller Plutei, but as 

 the number of my experiments is very large I feel pretty con- 

 fident that the reduction of this limit cannot be considerable. 



5. I am not yet able to tell where the limit of divisibility 

 lies, if we require only that the fragments go into the blastula 

 stage. The smallest pieces of protoplasm that I observed 

 segmented if they contained nuclear substance, and so far as 

 I could ascertain most of them reached the blastula stage. 

 Hence the part of an egg able to develop as far as the blastula 

 stage is much smaller than the part necessary to produce a 

 Pluteus. Moreover, it seemed to me that in order that blas- 

 tulae may become gastrulae the size must reach a certain limit. 

 If this be the case, it is obvious that more substance is neces- 

 sary for the formation of a gastrula than of a blastula. 



6. We are now able to decide a question which does not 

 belong strictly to our subject, namely, whether aside from the 

 mere increase in the number of cells any qualitative differen- 

 tiation takes place through the first segmentation. As I men- 

 tioned above, Driesch found that an isolated cell of the four-cell 

 stage could develop into a Pluteus, but that the same was not 

 possible for a cell of the eight-cell stage. One might conclude 

 from this that such a differentiation in the single cells of the 

 eight-cell stage had taken place that they could produce now 

 only single tissues or parts of a Pluteus, but no longer a whole 



