6o 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 Morphologie " 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 q%^. 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 q%^ 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 



