No. 1.] THE SMALLEST PARTS OF STENTOR. 241 
There are three possible explanations of this failure of such 
small parts to develop: 
1. That the whole organization of the species cannot be 
included in so small a space. Briefly, defictent organization. 
2. That so small a volume of matter cannot fulfill the 
mechanical conditions consequent on cell-division, formation of 
a segmentation cavity, invagination, and so forth, owing per- 
haps to increased surface tension (Driesch), not to mention 
other conceivable alterations of the extrinsic factors of develop- 
ment. 
3. That such a small part “is not able to set free that 
amount of energy which would be required for its transforma- 
tion into a gastrula or a pluteus.’”’ (Loeb.) ! 
The third explanation seems to me inadequate; because such 
masses may continue to live for a considerable period of time 
and display an amount of energy in atypzcal form changes and 
rapid ciliary movement, which would suffice to produce the 
phenomena of normal development, did not other factors 
(included in the first or second of the above alternatives) 
prevent. Moreover, it is well known that exceedingly minute 
protoplasmic bodies, very much smaller than one-eighth the 
echinoderm ovum, may produce a relatively enormous amount 
of energy: e.g. bacteria and spermatozoa. Finally we do not 
know how much of developmental energy is of intrinsic and 
how much of extrinsic origin. We are limited, then, to the 
jirst two alternatives. 
Now, in the regeneration of a unicellular organism those 
mechanical conditions consequent on cell-division, formation 
of a segmentation cavity, invagination, and so forth, are not 
required to be fulfilled. Surface tension and other extrinsic 
factors of development of Metazoa have not been shown to 
exercise a controlling influence in the regeneration of such an 
organism.” It occured to me, therefore, that the cz/zate Infusoria 
1 Morgan’s explanation, that the failure to develop is due to inability to pro- 
duce a sufficient number of cells for the next ontagenetic stage, will come under 
the first or second of these alternatives, according to the general point of view. 
2 Of course it is possible for any one to maintain that extrinsic forces do 
control the regeneration. But the burden of proof rests upon the maker of such 
an assumption. 
