34^ THE GERM-PLASM 



single idant of the other form would result in a much smaller 

 percentage. I have been able to prove the existence of 24-26 

 idants in the case of Arteinia salina^ a crustacean which multi- 

 plies parthenogenetically. If even several of these belonging to 

 one of the two ancestral varieties were present in a minority, 

 it is questionable whether they would ever become accumulated 

 in one and the same germ-plasm in the course of generations 

 and the corresponding reducing divisions, so as to form a pre- 

 ponderating majority : such an accumulation is, however, con- 

 ceivable. This would partly depend on chance, for the majority 

 of the ova produced by an individual always perish, and rare 

 combinations which would produce reversions are therefore also 

 usually lost. 



We may thus account for the fact that reversion only occurred 

 rarely in my experiments and only in certain individuals of a 

 colony at the same time, as well as for its appearing either 

 suddenly, or else after the production of intermediate forms. 

 The latter case is to be explained theoretically by supposing 

 that a balance of the two kinds of idants was first produced, and 

 that this then in the offspring partially led to the preponderance 

 of the idants on which reversion depended. 



The possibility of reversion in parthenogenesis therefore de- 

 pends upon two factors : firstly, upon the composition of the 

 germ-plasm out of different kinds of ids and idants, — i.e., upon 

 the occurrence of sexual reproduction in a previous generation ; 

 and, secondly, upon the 'reducing division' which always takes 

 place at the formation of germ-cells. 



7. Proof that the Determinants become Disintegrated into 



Biophors 



I will conclude this chapter with some remarks which might 

 have been more suitably inserted in the chapter on the control 

 of the cell by the idioplasm., but in that place they would have 

 been unintelligible, as they depend upon a knowledge of the 

 phenomena of maturation of the ova and spermatozoa. 



The fact that actual germ -plasm is removed from the animal- 

 ovum by means of the cell-divisions occurring before maturation, 

 — that is, in the process of the separation of the two polar 

 bodies, — was not by any means easily determined, and was only 

 proved after at least ten years of prolonged and difficult investi- 

 gation and reasoning. I have not thought it advisable to give 



