FACTORS CONCERNED IN THE PRODUCTION OF MITOSIS. 37 



As to the nature of this partition with each mitotic division two 

 explanations present themselves. According to the simplest of 

 these mitosis may be the result of the direct chemical activity of 

 certain substances, present in the fertilized egg, which become 

 used up in the mitotic process so that each cell of the two cell 

 stage receives an equal amount of the substance present in the 

 fertilized egg after the amount necessary for the first mitotic 

 division has been eliminated. On this hypothesis with each 

 succeeding mitotic division the materials resident in the de- 

 veloping embryo become partitioned and dissipated in the process 

 of development. Thus if x equals the entire amount of the 

 material for the execution of the mitotic process present in the 

 fertilized egg, and a the amount required for the realization of 

 the first mitotic division then each cell of the first cleavage would 

 receive (x -- a)/2. In this manner each succeeding division 

 would reduce the amount of the material present until the 

 amount apportioned to each cell would be less than the amount 

 required for the execution of the mitotic process, thus bringing 

 about an automatic check upon the course of the series of nuclear 

 divisions. The following objection to such a hypothesis shows 

 its weak point. Upon this basis each blastomere of the early 

 stages would be required to produce the same number of cells, a 

 supposition which the facts of cell lineage do not support. 



The second explanation of the method of control over the 

 number of divisions of the nucleus seems more natural and does 

 not convey so much of the idea of predestination, in that it 

 requires less emphasis upon the inherent qualities of the fertilized 

 egg. According to this hypothesis mitosis is just as markedly 

 a result of chemical processes going on within the egg as indicated 

 above but the individual cells may in varying degrees retain the 

 power of synthesizing the materials necessary to initiate and 

 carry out the mitotic division of the nucleus. In the application 

 of this explanation the check to the course of the nuclear divisions 

 may come as the result of an accumulation of materials within 

 the cell, probably as metabolic by products, which serve to retard 

 and finally to prohibit the chemical activity incident to nuclear 

 division. Thus at the end of any definite period of physio- 

 logical activity the organism will be composed of a definite 



