258 H. J. VAN CLEAVE 



marked cell constancy, the evidences of which will be brought out 

 in this paper. 



How strict an interpretation is to be placed upon this term 

 ^constancy?' Is the term to be applied with absolute precision, 

 utterly excluding the possibility of variation of any kind, or may 

 some value be placed upon instances in which there is clearly 

 a normal number and arrangement of the cells but in which a 

 few individuals depart from this condition? The former of these 

 two interpretations means the elimination of the possibility of 

 variability and thereby precludes all possibility of evolution, for 

 an absolute constancy could result in nothing else than an 

 absolute fixity of species, since variability is one of the neces- 

 sary or primary factors in evolution. The question of real im- 

 portance is how far these phenomena may vary from the normal 

 without entirely invalidating conclusions based upon the vary- 

 ing character in question. It is a very generally accepted fact 

 that the application of stimuli or the action of internal enzymes 

 may give rise to either an acceleration or a suppression of mitosis, 

 thus producing abnormal numbers of cells and resulting in a con- 

 fusing arrangement of the products. Since endoparasitism leads 

 to a practical elimination of those varying environmental condi- 

 tions which so profoundly influence the development of the free- 

 living forms, it would be quite natural to look among the endo- 

 parasites for materials adapted to the study of cell constancy. 

 Such conditions have been found to exist in members of the genus 

 Eorhynchus. 



If. Methods 



A method frequently employed in comparing the approximate 

 number of cells in a given organ or region of the body of two indi- 

 viduals consists in making a count of the number of cells present 

 in sections of the two animals at approximately the same plane. 

 Such a method may be sufficient when the problem of relation of 

 cell size to body size alone is being considered, but at best it can 

 form the basis of but the crudest sort of a comparison and is 

 valueless in determining the presence or absence of cell constancy. 

 In order to avoid the possibility of misunderstanding regarding 



