Limits oF DivistBILITy oF Living MatrerR 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 micelle, 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. Pfluger has stated already that the egg which 
had been considered as a unit can give rise to many indi- 
viduals... 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 Pfliger. 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.” For our purposes, 
however, the methods and results of Driesch cannot be un- 
1 Pfliigers Archiv, Vol. XXXII, p. 562. 
2DRiEscu, Zeitschrift fiir wissenschaftliche Zoologie, Vol. LV, pp. 5 ff. 
Digitized by Microsoft® 
