Polar Differentiation. 219 
each one of the biflagellate cells of Volvox contain superficially 
embedded a reddish lenticular refringent body known since 
Ehrenberg's time as "eyes" or "eye spots." One of these 
"eye spots" lies not very far from the base of one of the flagella in 
each cell, and produces a slight rounded projection of the thin 
layer of clear protoplasm immediately overlying and surround- 
ing it. In optic section these reddish bodies are seen to be lenti- 
cular or nearly so, the outer face being less convex than the 
inner. This is best seen in the " eye- spots " of the anterior 
pole. These " eye-spots " strange to say, bear a constant and 
definite relation to both the imaginary axis around which the 
colony revolves and the flagella of its cells. They are placed 
not quite on the extreme outer periphery of the cells as reck- 
oned from the centre of the globular colony, but nearly so. 
The anterior ones at the anterior pole consequently look for- 
ward, while the others of the rest of the cells look in all other 
directions, the hindmost ones looking directly backward. 
Now comes the most singular and interesting fact which I 
have observed, viz : that the " eye-spots " of the cells of the 
anterior pole are eight to ten times as large as those of the hinder 
pole. The passage from the large " eye-spots " of the anterior 
pole to the smaller ones of the posterior pole is very gradual, 
as can be readily observed with a moderately high power. 
These " eye-spots " diminish so much in size on passing to the 
cells of the posterior pole as to be finally visible only as a 
minute refringent reddish globule pushing out the protoplasm 
of the cell slightly in the same way as the larger anterior "eye- 
spots" push out the superficial plasma of the cells of the 
It is therefore plain that if these organs are visual or sensi- 
tive to light or any other natural agent, they are best developed 
in just the position in which they are of the most service to the 
organism, viz., at its anterior pole. These facts raise the query 
whether Ehrenberg was not after all justified in regarding the 
reddish spot in each cell of the colony as eyes. While these 
eyes are obvious to any observer it is remarkable that no one has 
hitherto called attention to their very unequal development at 
