BODY SIZE AND CELL SIZE 171 



easy to get the average length of the head and middle piece in all 

 three species. These measurements show that the male cells at 

 all stages are larger in C. convexa than in the other species named, 

 thus forming a parallel case with the egg cells of this species. 



Popoff ('08) maintains that there are small variations in the 

 sizes of the sex cells of different species, caused by inequalities of 

 division and by unequal growth during the growth period. He 

 supposes that when a large egg is fertilized by a large spermato- 

 zoon a large individual, composed of large cells, results; whereas 

 if the sex cells are smaller than usual the individual developing 

 from them will also be smaller. Applying this hypothesis to the 

 case of Crepidula, we should expect to find that C. convexa, 

 which has larger eggs and spermatozoa than the other species 

 considered in table 2, would show a larger body size and cell size 

 than the other species; on the contrary the size of tissue cells 

 is not greater, and the body size is much less in C. convexa than 

 in C. plana and C. fornicata. In this case it is evident that the 

 egg size does not determine the body size nor the cell size of the 

 adult, but that differences in body size are due to varying rates of 

 growth and cell division in the different species. It is true that 

 I am here dealing with different species, whereas Popoff's hypoth- 

 esis applied to different individuals of the same species, but it 

 would be a remarkable fact if so general a proposition as Popoff's 

 should be completely reversed in two closely allied species. We 

 have not generally regarded specific differences as so fundamental. 



Popoff admits that body size may be the result of conditions 

 favorable or unfavorable to growth. A study of the conditions 

 which lead to the production of dwarfs or giants in Crepidula 

 plana shows that here these factors are environmental, and not 

 germinal; I have discussed elsewhere (Conklin '98) this case and 

 will summarize it later in this paper. Undoubtedly environ- 

 mental conditions have much to do with body size. But in the 

 case of different species, each with characteristic body size, the 

 factors which determine size cannot be merely environmental. 



In all animals there is a limit, and in most cases a clearly defined 

 one, to body size and consequently to cell growth and cell division. 

 This hmit may be imposed by unfavorable environment, or by 



