XII.] CONJUGATION AND SEXUAL REPRODUCTION. 19I 



I selected a Moneron as defined by Hackel, viz. an organism 

 without a nucleus. I purposely abstained from considering 

 those unicellular beings which possess nuclei, because I was 

 then only concerned with bringing forward the general concep- 

 tion that sexual reproduction exists in order to ensure indivi- 

 dual variability. I was, however, well aware that in the nu- 

 cleated Protozoa, and especially in those Infusoria, which 

 although unicellular are extremely highly differentiated, such 

 a simple transmission of acquired peculiarities was hardly con- 

 ceivable. Now that we possess accurate knowledge of the most 

 essential points in the process of conjugation, it is possible to 

 approach this problem somewhat more closely. 



The fact, as we now know it, that conjugation in Infusoria is 

 a mingling of the nuclear substances of two individuals, permits 

 the conclusion that, in these animals, the whole individuality of 

 the cell, and thus of the cell-body, is contained in the nuclear 

 material as predispositions or hereditary tendencies, just in the 

 same manner as has been proved in the case of the germ-cells 

 of the Metazoa. Nussbaum's experiments upon the artificial 

 fission of Infusoria, and those which Gruber undertook at 

 my suggestion in the Zoological Institute at Freiburg, prove that 

 the nucleus determines the regeneration of the mutilated animal, 

 and that it contains, in some way, the essence of the whole or- 

 ganism in all its details. Hence we must believe that all those 

 variations which appear in Infusoria, in consequence of external 

 influences, can only pass on to the products of fission when they 

 are accompanied by corresponding changes of the nuclear substance ; 

 or, in other words, we come to the conclusion that the heredi- 

 tary transmission of somatogenic changes does not, as a rule, 

 take place, or only does so when they are accompanied by 

 corresponding blastogenic changes. The use of both these ex- 

 pressions only implies a correspondence, and not a similarity of 

 application, the ' soma ' of Metazoa corresponding to the cell- 

 body of Infusoria, the ' germ ' to the nuclear substance. The 

 broken bristles of an Infusorian are renewed in the products of 

 fission because the predisposition to form them is contained in 

 the nuclear substance. Mutilation is no more hereditary here 

 than in the Metazoa. Furthermore, all changes in the cell-body 

 of an Infusorian are not accompanied by corresponding changes 

 in the nuclear substance, and all cannot therefore be inherited ; 



