FACTORS CONCERNED IN THE PRODUCTION OF MITOSIS. 35 



factors which might be considered as directly influencing tin- 

 process of mitosis. 



Furthermore if environmental conditions could directly control 

 and determine the mitotic process, cell division would continue 

 indefinitely through the life of the organism as long as the 

 external conditions remained favorable for it. Again the data 

 brought from the field of cell-constancy show the impossibility 

 of such factors being operative in a controlling manner. In the 

 development of any organism or part of organism the powers 

 of reproduction of most of the somatic cells are restricted to 

 definite periods in the early stages of development, usually 

 preceding the introduction of histological differentiation. Thus 

 in the genus Eorhynclius the writer ('14: 280) has shown that in 

 no instance did he find any adult worm displaying abnormal 

 numbers of somatic nuclei or presenting any evidence whatever 

 of further division of the somatic nuclei after the adult body form 

 had been attained. These observations were upon over two 

 hundred individuals collected during a period of four years at 

 various localities and at various seasons of the year. 



In conclusion, if the determination of the mitotic process were 

 to find its explanation in terms of environmental factors, or to 

 a continuous production of combinations of environmental 

 factors during the process of development, then in every instance 

 the number of mitotic divisions a fertilized egg would undergo 

 would be a direct resultant of the complex of environmental 

 factors operative during its process of development. At least 

 in those forms with a high degree of cell constancy it seems 

 obvious that purely environmental factors have but one relation- 

 ship to the process of mitosis and that consists in the modifiability 

 of the rate, either as an acceleration or as a retardation. In this 

 respect purely environmental factors have the same general 

 effect upon the process of mitosis as they exert upon purely 

 physical and chemical reactions. 



2. INTERNAL FACTORS. 



Our incomplete knowledge of the finer structure of the cell r 

 and of the nucleus in particular, make it impossible to associate 

 the control of the mitotic process directly with any structure or 



