54 
Variation in Chilomonas 
Whether the larva develops from a normal egg, a half-blastomere, or a quarter- 
blastomere, the proportionality of the three regions of the intestine so marked off is 
said to be constant. But how constant is it ? Is there really as great precision in 
the relative localisation of tlie constrictions in the embryo from a half-blastomere 
as there is in the embryo from the normal egg? For Driesch's point of view 
an affirmative answer to this question seems to me to be vitally important. 
But clearly it is a question which cannot be answered by general inspection of 
individuals, nor by the measurement of a comparativel}^ few isolated cases. Its 
answer must depend on the accurate determination of the probable errors of what 
must in the nature of the case be absolutely very small differences*. To answer 
satisfactorily such a question we must, it seems to me, turn to the biometric 
method of attack. It is, then, in connection with such problems of morphogenesis 
as these outlined that I believe much is to be gained by the application of the 
methods of biometry. 
From this general orientation we may turn to the specific problem in connection 
with which the present work was done. During the past three years I have been 
engaged on an investigation (in connection with some of the students in biology 
at the University of Michigan) of the effect of environmental conditions on the 
form of the body in the Protozoa. An experimental study of certain phases of the 
problem has been made on Paramecium, of which a preliminary report has been 
published (Pearl and Dunbar, 1905). The results of that work made it seem 
desirable to get similar data for some other protozoan, where the environmental 
differences should be such as appear in the course of the normal life of the organism, 
rather than those experimentally induced. It was desirable to compare the vari- 
ability and correlation shown by a population living under the most favourable 
natural conditions with the same characteristics of a population living under 
extremely unfavourable natural conditions. To present the results of such a 
comparison for the flagellate infusorian Chilomonas is the purpose of this paper. 
It has seemed best to publish these results in advance of the complete paper on 
Paramecium, as it is likely to be some time before that appears and the present 
results lend themselves readily to sepai'ate treatment. 
The particular protozoan chosen for the work, Chilomonas paramecium, seems 
especially well adapted for biometrical studies. It has a definite and constant 
form ; its protoplasm is relatively dense, and hence little affected by osmotic 
changes in the surrounding medium, a point of practical importance in quantitative 
work on Protozoa ; and it can be had everywhere in abundance. It may perhaps 
be well to recall very briefly some of the facts regarding the biology of the form. 
Chiloynonas is a very minute infusorian, which commonly appears in great numbers 
in cultures containing decaying plant material. The body forms an elongated ovoid 
with an asymmetrically situated depression or notch near the anterior end. From 
* Of course in the particular case cited of the proportional division of the intestine the practical 
difficulties in the way of measuring may be insuperable, but this in no way affects the point of 
principle that in this and similar cases quantitative treatment of the problems of morphogenesis is 
necessary if real advance is to be made. 
