CELL SIZE AND NUCLEAR SIZE 61 
escence or rejuvenescence, as Child maintains, anything which 
facilitates the nterchange between nucleus and protoplasm should 
lead to rejuvenescence, anything which decreases it should lead 
to senescence. 
During cleavage the increase in nuclear surfaces is much greater 
than the increase in nuclear volumes. While the increase in max- 
‘mum nuclear volumes up to the 32-cell stage of Crepidula is 
about 5 per cent for each division, the growth in the maximum 
nuclear surfaces during this period is about 11 per cent for each 
division. From the 2-cell to the 70-cell stage the nuclear volume 
increases only 2.24 times, while the nuclear surfaces increase 
5.30 times. In Styela the nuclear volume increases from the 2- 
cell stage to the 256-cell stage only 4.52 times, the nuclear sur- 
faces increase 13.75 times. Unquestionably this greater growth 
of nuclear surfaces as compared with nuclear volumes, facilitates 
the interchange between nucleus and protoplasm. There is also 
a considerable increase of cell membranes during cleavage, but 
most of this increase is confined to surfaces of contact between 
cells, and free surfaces show but little growth. My observations 
teach that there is little, if any, interchange of materials through 
partition walls separating cells. 
~ Another and much more efficient means of facilitating the inter- 
change between nucleus and protoplasm is found in the mitotic 
division of the nucleus. During the cycle from one division to 
the next the nucleus absorbs materials from the cell body, only 
to throw back into the cell body these and other materials when 
the nuclear membrane dissolves in mitosis. The chromatin 
is thus brought into the most intimate relations with the proto- 
plasm. There is thus a sort of ‘‘diastole and systole of the nu- 
cleus’ (Conklin, ’02), by which the interchange between nucleus 
and protoplasm is greatly hastened. Indeed in the paper just 
referred to I suggested that this function of mitosis may be quite 
as Important as the division and separation of the chromosomes, 
which is usually supposed to be the one function of mitosis. 
The hypothesis that the more rapid interchange between nu- 
cleus and protoplasm is associated with increased metabolism 
is supported by some very significant physiological work on the 
