1. Surface movement of the dividing spermatocyte of the grasshopper, Acrida lata, was followed by a marking method.
2. Throughout the division process of the spermatocytes, incipient daughter cells maintain spherical contours.
3. By direct observation of markers and calculation using the condition given in item 2, the following points are established.
(a) As in the sea-urchin egg, there are a pair of circular zones on a grasshopper spermatocyte surface which retain their respective radii unchanged while the cell undergoes a division.
(b) In the grasshopper spermatocyte, unlike the sea-urchin egg, the surfaces of these circular zones do change their positions and move towards the poles during division.
(c) As a spherical cell goes through a constricted form to become two daughter cells, not only is the radius of curvature of the surface everywhere uniform (item 2), but both axial length and surface area increase uniformly everywhere except in the region of the furrow.
4. From the findings of item 3 it is inferred that the prevailing surface stress is uniform and isotropic, like surface tension, and that the force causing division must be derived from some other parts of the cell such as the furrow cortex or the endoplasm.
5. Basically, the nature of the surface of sea-urchin eggs is similar to that of the spermatocyte. That the circular zones of the former are stationary while those of the latter move steadily during cleavage is tentatively explained in terms of the speed of advance of the furrow in relation to the relaxation time of the cortex.