€ELL CONSTANCY IN THE GENUS EORHYNCHUS 281 

 GENERAL PROBLEMS RELATING TO CELL CONSTANCY 



1. Cell size vs. body size 



A problem such as the relationship existing between body size 

 and cell size is often approached only from a teleological point of 

 view. Given as the end result the adult body form, that becomes 

 the goal toward which the development of the organism is di- 

 rected. Thus the possibility that this final body form and size 

 may be a mere incidental result attained through precisely de- 

 termined series of processes of development is entirely ignored. 

 In other words, a common conception of body form and size might 

 be crudely expressed in the idea that nature establishes a mold for 

 each particular type of body form and that, given a zygote of a 

 definite size, the development of the individual must consist 

 in attaining the boundaries of size and form set for that species, 

 regardless of the number of cell divisions that may be required 

 for that purpose. According to this interpretation, the size of 

 the body is the constant factor while the number of cells which 

 go to make up the organism is purely a matter of circumstance. 



The problem of the origin of the adult body and the relation- 

 ship between body size and size of the component cells is one of 

 long standing. WTiile the following discussion does not attempt 

 to bring all the literature upon the subject into relationship, 

 it will serve to give some idea of the nature of the contentions on 

 the various sides of the problem. Conklin is one of the staunch 

 supporters of the view that not only is cell size fixed but also 

 that it bears no direct relation to body size. In Crepidula forni- 

 cata, in which large and small individuals occur, he has shown ('98) 

 that the difference is due entirely to the larger number of cells 

 in the larger individuals: 



In Crepidula therefore the cell size is constant, and variations in the 

 size of the body are due to variations in the number of cells present. The 

 dwarfs are what they are by reason of external conditions and not be- 

 cause of inheritance; they are in short a physiological and not a morpho- 

 logical variety. In such a case the shape and size of the body as well as 

 the number of cells in the entire organism, are greatly modified- by the 

 direct action of the environment. 



