BODY SIZE AND CELL SIZE 185 



suits of Levi ('05). On the other hand Berezowski ('10) finds that 

 the size of intestinal epithelial cells is smaller in young mice than 

 in older ones, and that with the general growth of an animal there 

 occurs a growth in the height of these cells. However this obser- 

 vation does not contradict the conclusion reached in this paper; 

 indeed it is true of Crepidula, as of the mice studied by Berezow- 

 ski, that younger animals have smaller cells than older ones, as 

 will be seen by comparing the size of larval cells given in table 6 

 with that of adult tissue cells given in tables 2 and 7. It is well 

 known that the size of cells depends to a certain extent upon the 

 rate of cell division and the length of the resting period, and the 

 rate of division is slower and the resting periods longer in mature 

 animals than in young ones. In all my measurements I have, 

 so far as possible, compared animals of the same stages, so that the 

 developmental changes in the size of cells does not materially in- 

 fluence my results. 



But while tissue cells maintain a very uniform size in Crepidulae 

 of all species and sizes, provided they are of corresponding ages, 

 the sex cells differ enormously in size and number in the different 

 species, the species of small body size having in general larger and 

 fewer eggs than species of larger size. On the other hand within 

 the same species the sex cells are of approximately the same size 

 in all individuals, but they differ in number in animals of different 

 body size, just as the tissue cells do. 



The larger eggs of C. convexa and C. adunca are larger in every 

 respect, having more cytoplasm as well as more yolk than the 

 eggs of C. plana and C. fornicata. Even in the oogonia and early 

 oocytes, before yolk begins to form, the eggs of the former species 

 are larger than those of the latter. The spermatozoa and sperma- 

 tocytes of C. convexa are also larger than those of C. plana or 

 C. fornicata. Presumably the sex cells of C. convexa are larger 

 from the time of their first appearance, and it is possible that this is 

 due to their being derived from larger blastomeres, as well as to 

 the fact that the primitive sex cells divide less often in species 

 with large eggs than in those with small ones. It seems probable 

 also that the oogonia or oocytes ingulf a larger number of oogonia 

 and follicle cells in C. convexa than in C. plana. 



