350 THE CELL 



of nuclear division is the division of the mother-granule ; all the 

 other processes only serve to convey one of the daughter-grannies, 

 which have been derived by division from the same mother- 

 granule, into the centre of each daughter-cell." If we replace the 

 term " mother-granule " by the expression " idioblast," we have 

 established a connection between the process of nuclear segmenta- 

 tion and the theory of heredity. 



This conception of the nuclear substance as an hereditary mass 

 is important, since it offers some explanation of the facts that the 

 nuclear substance takes less part in the coarser processes of 

 metabolism, than the protoplasm does, and that, for its better 

 protection, it is enclosed in a vesicle provided with a special 

 membrane. 



3. The Prevention of the Summation of the Hereditary Mass. I 

 consider the third point, viz. the prevention of the summation of 

 the hereditary mass, during sexual reproduction, to be a most 

 important point in the argument. In consequence of the nature 

 of the process of nuclear division, each cell receives the same 

 quantity of nuclear substance as the fertilised egg-cell, A. Now 

 when two of its descendants unite, as sexual cells, the product of 

 generation, B, ought to contain twice as much nuclear substance 

 as the cell A originally did. Then when members of the third 

 generation conjugate, the product G ought to contain twice as 

 much nuclear substance as B, or four times as much as A, and 

 thus with each new act of fertilisation the nuclear mass would 

 increase by geometrical progression. Such a summation, however, 

 must be prevented by nature in some way or other. 



This would also be true of the idioplasm, if the full quantity of 

 it were transmitted to each cell, and if it were doubled each time 

 by the act of fertilisation. By this means, its nature, per s*, 

 would not be changed. For instead of twice, each individual 

 elemental germ would be represented four, eight, or even more 

 times. Thus, although the quantity would be increased, the 

 quality would always remain the same. But it is self-evident 

 that the mass cannot thus increase to an unlimited extent. Nageli, 

 and especially Weismann. have laid stress upon this difficulty, and 

 have tried to solve it. 



Nageli remarks : " If during each act of reproduction by means 

 of fertilisation, the volume of the idioplasm of whatever constitu- 

 tion it may be, were to become doubled, after a few generations 

 the idioplasmic bodies would have increased so much, that there 



