18 THE BIOLOGICAL PROBLEM OF TO-DA Y 



grows and the cells for the maintenance of the 



o 



species. He has enunciated the proposition that, 

 when the sexual cells have been separated from the 

 cells of the young embryo, the material of the germ 

 has been divided into shares for the individual and 

 shares for the species ; that the sexual cells take 

 no part in the formation of the body, and that 

 body-cells never give rise to ova or spermatozoa. 



Weismann differs from Nussbaum in one im- 

 portant point. He lays no stress on the direct 

 origin of the sexual cells, as cells, from the egg at 

 the beginning of its development. He found, for 

 instance, that, in the case of hydroids, the sexual 

 cells did not arise in such a fashion. He considers, 

 therefore, that the chain of events is as follows : 

 The whole of the protoplasm of an egg- cell is not 

 required to build up the new being, aud the super- 

 fluous part remains unaltered to form the sexual 

 cells of the new generation. Unlike Nussbaum, 

 then, he asserts a continuity, not for the sexual 

 cells, but for the germinal protoplasm which he 

 believes to pass along definite cell- tracks until it 

 forms the sexual cells. From this germinal proto- 

 plasm, which makes the germ-cells, he distinguishes 

 the somatic protoplasm which makes the mortal, 

 somatic cells. 



The germplasm theory entered a new phase in 

 the year 1885, after the independent appearance in 

 1884 of essays by Strasburger and by me, in which 

 we gave reason for thinking that the cell nucleus 

 was, as I expressed it, the bearer of the characters 

 which were transmitted by parents to their off- 



