186 EDWIN G. CONKLIN 



The larger eggs give rise to larger blastomeres and to larger 

 embryos and larvae than do the smaller eggs. The sizfe of tissue 

 cells is nearly the same in the larvae of the different species, and 

 this size is less than that of adult tissue cells; but the number of 

 cells in the larvae of different species differs greatly being approx- 

 imately proportional to the body volume of the various larvae. 

 Finally cell growth and division continue for a longer period in 

 species of Crepidula which have small eggs than in those which 

 have large ones, with the result that the former give rise to larger 

 adults than the latter. In the different species of this genus the 

 size of the germ cells does not determine the size of the adult 

 (Popoff, Chambers). 



Within the same species differences in body size are due in the 

 main to differences in cell number, the cell size being approxi- 

 mately constant. Small individuals are as complete and perfect 

 as large ones, all the typical differentiations and organs being 

 present in the former the same as in the latter. But in parts which 

 are reduplicated, such as the gill filaments, lobules of liver, kid- 

 ney, ovary and testis, etc., these parts are more numerous in 

 large individuals than in small ones. In the parts which are 

 reduplicated, whether they be organs or cells, there is practically 

 no differentiation between the different members. Increase in 

 size is due to an increase in the number of these parts or cells, 

 without any increase in the total number of the various kinds of 

 differentiations. 



On the other hand, differential cell divisions, such as are found 

 in the early cleavages of the egg do not vary in number in eggs 

 or embryos of different size. The study of the cell lineage of these 

 gasteropods shows that the cleavage is cell for cell the same in 

 eggs and blastomeres of all sizes and species until ectomeres, ento- 

 meres and mesomeres are completely separated, and until differ- 

 ential divisions have given place to non-differential ones. So far 

 as cell division is associated with differentiation and morphogene- 

 sis in the cleavage period, the number and character of these 

 divisions do not vary in different species or individuals; so far 

 as it is associated with growth, and the ' vegetative duplication of 

 parts,' but not with differentiation, it may vary enormously. Pro- 



