248 TELE. [VoL. XII. 
In any case this relation forms a striking contrast to that 
found in the development of the animal ovum, where a portion 
of less volume than one-fourth that of the ovum does not 
develop into an embryo (see postscript). It has been very 
generally found that a portion (of the two- or four-cell stage) 
equal to one-eighth the volume of the ovum never develops 
farther than the gastrula stage. 
In the case of the animal ovum, again, parts slightly smaller 
than the minimum necessary for the complete development may 
undergo partial development. In Stentor we have a parallel 
phenomenon: parts of less than 80 spherical diameter may 
undergo partial regeneration, but are unable to complete it. 
It seems to me that neither increased surface tension due to 
diminution of surface area, nor yet any other external factor, 
is responsible for this failure of small pieces of Stentor to 
regenerate. The cause lies within; and I do not believe that 
it is to be sought in an insufficient production of energy. 
For such small pieces may live for days, constantly producing 
and expending energy in the ordinary processes of metabolism. 
I am forced, therefore, to the conclusion that the organization 
of these parts is in some way deficient. There ts probably for 
cach species of animals a minimal mass of definite size consisting 
of nucleus and cytoplasm within which the organization of the 
species can just find its latent expression. This is the mznzmal 
organization mass. 
In the case of the Protozoa the size of this minimal mass is 
that of the smallest part capable of complete regeneration. 
But I do not believe that in the Metazoa the minimal organiza- 
tion mass is that of the smallest part of the ovum capable of 
developing into a normal embryo; for undoubtedly the in- 
fluence of external factors is of the greatest importance here. 
I would conceive then that in the Metazoa this hypothetical 
minimal organization mass is smaller than any part yet ob- 
served to develop into a normal embryo. Still, from my 
results on Stentor, I believe that it is of such a size as to be 
easily visible under a low power of the microscope. 
ZOOLOGICAL LABORATORY OF THE 
UNIVERSITY OF MICHIGAN, Dec., 1895. 
