1897 | THE MOVEMENTS OF DIATOMS 45 
At this point its velocity attains its maximum because of the 
sum of all the foregoing accelerations. 
After this plane is crossed the conditions are changed; the 
posterior end receives now more light than the anterior; the 
process of assimilation is going on there with more energy, and 
as the result the osmotic currents to the interior of the cell are 
strengthened. The direction of the acceleration (always being 
toward the center of light) is inverted as to the direction of 
the movement, which it now impedes. The velocity is thus 
gradually lessened, and after having reached zero is changed, 
too. The cell now runs back, repeating the same type of move- 
ment as it did when going forward, and those oscillations are 
repeated many times. 
They would go on in the same plane, were they not disturbed 
by some occasional hindrance, such as the currents produced in 
water by heat, contact with other bodies, and so on, which effect 
a deviation from the primary direction and a return at some 
angle to it. 
If we take into consideration the very low velocity of the 
diatom cells and their small mass, the slight difference in the 
quantity of light received by the two halves of the cell will not 
appear insufficient to account for its movement. 
It is not impossible that other kinds of movement could be 
reduced to this gliding type and be explained in the same way. 
The above described observations make it not improbable that 
these other kinds can be deduced from the first one by the 
action of some hindrance, such as the sticking of the mucus 
Surrounding the outside of the cell to the glass, or the friction 
when the cell is approaching one of the glasses. 
The explanation here proposed, making the periodicity of 
the movements dependent upon the artificial disposition of the 
light in the microscope, which is hardly to be found in nature, 
does not indeed exclude motion under natural conditions. For 
the matter is not essentially changed by the fact that the rays, 
instead of being divergent, are parallel, and that their direc- 
tion towards the cell is unchanged by its movement, as is the 
