LIMITS OF DIVISIBILITY OF LIVING MATTER 323 



Nussbaum's observations and conclusions were, so far as I 

 know, corroborated in every particular. 



2. In all these experiments the answer as to the quanti- 

 tative limits of the divisibility of living matter has not 

 been obtained. It is of great importance, however, to have 

 a clear idea of how large the smallest piece of nucleus and 

 protoplasm is that is capable of development. Is it of the 

 order of magnitude of two or more micellae, or is it of the 

 order of magnitude of a considerable fragment of the cell? 

 I have tried to obtain an answer to this question in the sea- 

 urchin egg. Pfltiger has stated already that the egg which 

 had been considered as a unit can give rise to many indi- 

 viduals. 1 The experiments of Driesch, which we shall men- 

 tion immediately, and my own experiments on the produc- 

 tion of double and multiple embryos from a single egg, 

 correspond with the views of Pfltiger. The question, there- 

 fore, naturally arose as to how many embryos can arise from 

 an egg, and to determine in this way the limits of the divisi- 

 bility for one kind of living matter; namely, the egg. The 

 simplest way of determining what fraction of the substance 

 of the sea-urchin egg is still able to develop into a normal 

 embryo seems at first sight to be that in which one of the 

 cells of the egg in various stages of cleavage is isolated, 

 and the last stage in which a single cell is still able to 

 develop into a pluteus is determined. (The eggs cannot 

 usually be kept beyond the pluteus stage in an aquarium.) 

 The cleavage cells become smaller as cleavage progresses 

 and the number of cells increases into which the egg divides. 

 In another connection Driesch has shown that one of the 

 cells from the four-cell stage of the sea-urchin egg is 

 still able to develop into a pluteus. 2 For our purposes, 

 however, the methods and results of Driesch cannot be un- 



1 Pflugers Archiv, Vol. XXXII, p. 562. 



2 DRIESCH, Zeitschrift fur wissenschaftliche Zoologie, Vol. LV, pp. 5ff. 



