180 EDWIN G. CONKLIN 



to take food, is due to absorption of water; and in the embryos 

 and larvae of Crepidula it seems probable that varying rates of 

 growth may be due to this same factor. This increased growth 

 of the embryo of C. plana as compared with those of the other 

 species, continues through the larval and post-larval stages, so 

 that in the end the adults of the first named species become much 

 larger than those of the latter. On the other hand the number 

 of cell divisions during embryonic stages is much greater in C. 

 convexa and C. adunca than in C. plana, so that the embryos 

 ' and larvae of the former species are composed of a much greater 

 number of cells than in the case of the last named species. 

 But in post-larval stages cell divisions become much more 

 numerous in C. plana than in C. convexa or C. adunca, so that in 

 the adult condition the first named species contains a very much 

 greater number of cells than either of the others. 



Considering only the intrinsic factors of growth, one may say 

 that the size of an embryo is the result of the initial size of the egg, 

 but the size of an adult individual is due for the most part to the 

 duration and rate of cell growth and cell division. The size of the 

 germ cells is of little significance in determining the body size of 

 adult individuals in different species of Crepidula, though it may 

 possibly be of some importance in determining the body size of 

 different individuals of the same species and race, when all other 

 conditions are equal. In different races and species the important 

 factor in determining body size is the duration and rate of cell 

 growth and cell division. Many extrinsic factors are known to 

 limit or promote such growth and division, but in the case of 

 species which differ greatly in size and in which body size is a very 

 characteristic feature it would seem necessary to suppose that 

 size is inherited, — that some of its causes must be intrinsic 

 ones. What these intrinsic factors are which limit cell growth 

 and division in some cases and which promote such processes in 

 others, is at present entirely unknown. However it seems prob- 

 able that upon such factors depends in the main the difference 

 between a mouse and an elephant, while the initial size of the 

 sex cells is of only minor importance. 



