34 H. J. VAN CLEAVE. 



the course of nuclear and cell division is extremely difficult on 

 account of problems involved in eliminating factors until but 

 a single causal agent is operative. However, of the possible 

 factors involved in the production of mitosis and of cleavage 

 two groups are clearly recognizable, namely: (i) Environmental 

 factors, and (2) Internal factors. Any change which occurs 

 within the living organism must find explanation upon the basis 

 of one of these two groups of factors or upon the basis of a com- 

 bination of them. It is the purpose of this paper to show the 

 relationship of these two groups of factors in the production 

 of mitosis among those organisms which are made up of a fixed 



number of cells. 



i. ENVIRONMENTAL FACTORS. 



If environmental factors either caused or directly controlled 

 the phenomena of mitosis the fact of cell constancy could not 

 exist, for no two individuals, and more strongly no two groups of 

 individuals even of the same species, develop under absolutely 

 the same environmental conditions. Consequently if environ- 

 mental factors were the limiting factors in mitosis no two indi- 

 viduals would of necessity contain the same number of cells. 

 It is generally granted that the mitotic process may be accelerated 

 or retarded through the application of purely external stimuli. 

 At least in those organisms made up of a fixed number of somatic 

 cells such an acceleration or retardation of the mitotic process 

 could result in nothing beyond a modification of the normal 

 rate of the process and could in no wise be considered as a direct 

 factor in the determination of the extent of the series of mitotic 

 divisions during the cleavage stages and in subsequent develop- 

 ment. If the role of temperature, for instance, be examined as 

 a possible controlling factor in the mitotic process it becomes 

 apparent that if this alone and directly controlled the number of 

 mitotic divisions through which the developing organism should 

 pass there could be but slight possibility of any two individuals 

 having identical numbers of somatic cells. For under conditions 

 of nature no two individuals are at all times during their develop- 

 ment under absolutely identical temperature relations. A similar 

 variability of conditions for the development of different indi- 

 viduals exists in the case of almost all of the other environmental 



