3 H. J. VAN CLEAVE. 



number of cells and so upon attaining maturity will have a 

 fixed number of somatic cells which are unable to divide farther 

 on account of the presence of the inhibiting materials. It must 

 be remembered that throughout this discussion principles are 

 being laid down for the determination of the mitotic process in 

 organisms or parts of organisms displaying cell constancy. The 

 objection involving the variability in the number of cells arising 

 from the early blastomeres, cited in connection with the dis- 

 cussion of the influence of environmental factors, finds no 

 grounds here. In fact it lends its support to this second hypothe- 

 sis. If the ultimate check to the mitotic process comes as the 

 result of an accumulation of metabolic by products, then the 

 rate of the nuclear division, which unquestionably may be in- 

 fluenced by environmental factors, would tend but to be directly 

 proportional to the rate of the accumulation of the inhibiting 

 elements, though the actual amount of the inhibitors necessary 

 to terminate the series of mitotic divisions would remain the 

 same. Evidently the accumulation of the inhibitors proceeds 

 unequally in various tissues of the metazoan body. The degree 

 of the differentiation of the cell and, in all probability connected 

 with this, the nature of the cell membranes, determines the rate 

 of the accumulation of the inhibiting factors. 



The impossibility of explaining the phenomena of cell con- 

 stancy on the basis of mitotic control by environmental factors 

 and the facts from experimental embryology in their bearing 

 upon the control of mitotic division by internal factors, lead to 

 the evident conclusion that during the development of cell 

 constant forms the mitotic process is controlled and determined 

 from within the cell but its rate may be regulated by factors 

 which are considered purely environmental. 



CONCLUSIONS. 



1. At least in those forms displaying a high degree of uni- 

 formity in the number and arrangement of their component cells 

 and nuclei, environmental factors such as ordinarily influence 

 physical and chemical changes play no direct part in the deter- 

 mination of the number of mitotic divisions of the somatic nuclei. 



2. The only influence of normal environmental factors in these 



