2i8 The American Naturalist. [April, 
THE POLAR DIFFERENTIATION OF VOLVOX, AND 
THE SPECIALIZATION OF POSSIBLE 
ANTERIOR SENSE-ORGANS. 
jN a recent communication upon this subject which the writer 
made to the Academy of Natural Sciences of Philadelphia, 
the fact was pointed out that in Volvox minor there are very 
distinctly differentiated anterior and posterior poles or hemi- 
spheres. The anterior or empty pole is so named here because 
it is the one which is always directed forwards when the animal 
is in motion. The posterior pole is so named because it is 
always in a posterior position when the organism is moving 
freely and normally, and it is further distinguished from the 
anterior in that it is in this hemisphere, in V. minor at least, 
in which the germs are produced which give rise to young 
Volvoces. Roughly speaking the nearly spherical csenobium 
or colony of Volvox may be divided into an anterior and a 
posterior hemisphere. Through the centres of these hemi- 
spheres there passes an imaginary axis around which the colony 
rotates in either a sinistral or dextral direction, but progressive 
locomotion is always in the direction of the anterior empty pole 
of the csnobium. This differentiation of the poles of the colonies 
of Volvox appears to have been known to Ehrenberg, who figures 
them but makes no farther mention of the fact. Hicks is 
reported in the Midland Naturalist, 1880, to have observed 
that the young leave the parent caenobium by breaking 
through the wall of the hinder or spore-bearing hemisphere, a 
fact which I can confirm. 
While these facts have been partially recorded by previous 
observers, there is another group of facts which I have noticed 
which are far more important and remarkable and serve to 
establish beyond question the polar differentiation of Volvox, 
and also raise the suspicion that this animal or plant, which- 
ever it is, is endowed with a very primitive sensory apparatus 
which is developed to an importance anteriorly, eight or ten 
times as great as at the posterior pole. It is well known that 
