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THE AMERICAN NATURALIST [Vol. XLIV 



rapid division of the body cells this ratio decreases. In 

 fact the chief difference between the yolk-free body cells 

 and the (yolk-free) germ cells is the slow rate of division 

 of the latter. Finally, the eggs will not divide at all un- 

 less specially stimulated, by fertilization. The question 

 now arises : what causes the cell to divide. Sacks found 

 that plant cells divide when they have reached a certain 

 size. This rule has been extended to animals, and the 

 final size of the cell found to be determined by the ratio 

 of nucleus to cytoplasm. This rule may apply to the 

 germ cells, since it appears that after the egg cell, pri- 

 mary oocyte, readies a certain size any additional food 

 absorbed does not cause growth of the protoplasm, but 

 is precipitated as yolk. 



If the egg is properly stimulated, rapid growth of 

 protoplasm and cell divisions follow. From the study 

 of artificial parthenogenesis it appears probable that 

 stimuli which lead to development of the egg increases 

 the permeability of its plasma membrane. If this be 

 true we may say that the germ cells are distinguished 

 by the fact that their jil;i>ina membranes are poorly 

 permeable and retard those reactions between the cell 

 contents and environment which lead to growth and cell 

 division. In other words, the optimum intensity of stim- 

 ulation toward growth and division is higher for the 

 germ cells than for the body cells. 



This difference is probably due to a difference in the 

 colloids of the cell, which in animals could be explained 

 as the result of an unequal mitosis. This explanation 

 may be modified so as to apply to plants. Klebs has 

 shown that those conditions which are adverse to vege- 

 tative growth of plants (too strong stimuli?) call forth 

 flowers. Perhaps there are slight differences in the 

 sensitiveness of plant cells to stimuli, and as the stimuli 

 increase, those initially least sensitive cells acquire 

 further immunity to the stimulus, whereas those initially 

 more sensitive cells are overstimulated and weakened. 

 Thus the difference between germ and body cells is grad- 



