36 H. J. VAN CLEAVE. 



chemical bodies found in the cell. However on the basis of 

 facts pointed out earlier in this paper it seems certain that since 

 there is no possibility of environmental factors acting as the 

 controlling element in mitosis the ultimate cause of the process 

 must be sought within the cell. In this connection invaluable 

 support is found in the field of experimental embryology. 

 Morgan ('95, '01, and '03) and Driesch ('98 and 'oo) have both 

 shown that in the embryos of echinoderms developed from 

 isolated blastomeres of the two cell stage the number of cells 

 present at any point in the development is approximately half 

 of the number present in a normal embryo. Similarly from one 

 of the blastomeres of the eight cell stage the gastrula is composed 

 of only one eighth the number of cells found in the normal 

 gastrula. Loeb ('06: 59) interprets these results as supporting 

 the hypothesis of Sachs ('93 and '95) which regards the factors 

 determining cleavage controlled by the ability of each nucleus 

 to gather around itself and control a definite amount of proto- 

 plasm. Yet what determines the amount of protoplasm present 

 in the developing individual? The cytoplasm is constantly 

 being replaced through the processes of anabolism which experi- 

 ments with enucleated cells have shown to be under the control 

 of the nucleus. Consequently it seems that the view just stated 

 comes not much short of being an argument in a circle. Does 

 the amount of cytoplasm determine the number of nuclei that 

 are to be formed or is the numerical relation of nuclei to cyto- 

 plasm a mutual one brought about not through the influence of 

 either cytoplasm or of nucleus but through some fundamental 

 factor which determines the number of nuclei and at the same 

 time indirectly the amount of cytoplasm that is to be formed? 

 The writer interprets the data of Morgan and of Driesch in an 

 entirely different manner. If by the first division of the egg 

 there are set apart two units, each of which has the possibility 

 of developing into a given number of cells by the process of 

 mitosis and this tendency is retained, even though the two units 

 become separated, it seems logical to conclude that within the 

 fertilized egg there are resident potencies which through the 

 process of mitosis become divided between the two daughter 

 nuclei of the first and then of each succeeding generation of cells. 



