CELL SIZE AND NUCLEAR SIZE 85 
9. The average nuclear growth during cleavage is not more 
than 5 per cent to 9 per cent for each division up to the 32-cell 
stage and it may fall as low as 0.3 per cent to 1 per cent for each 
division after that stage; and in every case it falls far short of a 
doubling, or increase of 100 per cent, for each division (pp. 36- 
44, 54, 55). 
10. Both nuclear sap and linin belong to the cytoplasm as well 
as to the nucleus. The chromatin is the most distinctive nuclear 
substance. All of these constituents are more abundant in large 
cells than in small ones. The mitotic spindle is of both nuclear 
and cytoplasmic origin and its size depends upon the volume of 
both nucleus and cytoplasm (pp. 44-47, 55). 
11. The average growth in volume of chromatin from the 2- 
cell to the 32-cell stage is about 8 per cent for each division period, 
being about the same as the growth of the nucleus as a whole 
(pp. 47-48, 55). 
12. The chromosomes become individually smaller as cleavage 
progresses, and in general small nuclei give rise to smaller chromo- 
somes than do large nuclei (pp. 48-51, 55). 
13. The size of the nucleoli (plasmasomes) depends upon the 
size of the nucleus and the length of the resting period; the larger 
the nucleus and the longer the resting period, the larger the plas- 
masomes become (pp. 51-53, 55-56). 
14. Centrosomes and spheres of large cells are larger than those 
of smaller ones (pp. 53, 56). 
15. The rate of growth of chromatin during the early cleavages 
of Crepidula (8 per cent for each division) harmonizing with the 
slight rate of increase of the oxidative power of the egg as de- 
termined by Warburg (p. 56). 
16. My observations do not support the view that senescence 
is due to a decrease (Minot), or an increase (Hertwig) of nuclear, 
as compared with protoplasmic material; nor that rejuvenescence 
is accomplished during cleavage by the great increase of nuclear 
material relative to the protoplasm. On the other hand senes- 
cence seems to be associated with a decrease, rejuvenescence with 
an increase of metabolism (Child). Anything which decreases 
the interchange between nucleus and cytoplasm, such as products 
