22 THE GERM-PLASM 



physiological processes, — investigation cannot proceed from 

 the simple to the more complex without taking into considera- 

 tion the objects and processes for which it was first undertaken. 

 It must, on the contrary, avoid the densely overgrown path 

 and skirt the hedge which surrounds the enchanted castle of 

 the secret of Nature, in order to see if there be not somewhere 

 a gap through which it is possible to enter and obtain a firm 

 foothold. 



Such a gap in the hedge which encloses the secret of heredity 

 may be found in the processes of fertilisation, if we connect them 

 with the facts of heredity as observed in the organisms which 

 have adopted sexual reproduction. 



As long as we were under the erroneous impression that 

 the fertilisation of the ovum by the spermatozoon depended 

 on an aura se//i2Jtalis which incited the egg to undergo devel- 

 opment, we could only partially explain the fact that the father 

 as well as the mother is able to transmit characters to the 

 children by assuming the existence of a spiritus rector, con- 

 tained in the aura seminah's which was transferred to the ovum 

 and united with that of the latter, and thus with it directed the 

 development. The discovery that development is effected by 

 material particles of the substance of the sperm, the sperm-cells, 

 entering the ovum, opened the way to a more correct interpre- 

 tation of this process. We now know that fertilisation is noth- 

 ing more than the partial or complete fusion of two cells, the 

 sperm-cell and the egg-cell, and that normally only one of the 

 former unites with one of the latter. Fertilisation thus depends 

 on the union of two protoplasmic substances. Moreover, although 

 the male germ-cell is always very much smaller relatively than 

 the female germ-cell, we know that the father's capacity for 

 transmission is as great as the mother's. The important con- 

 clusion is therefore arrived at that only a small portion of the 

 substance of the ovum can be the actual hereditary substance. 

 Pfliiger and Nageli were the first to follow out this idea to 

 its logical conclusion, and the latter observer stated definitely 

 that it is impossible to avoid the assumption that no more 

 hereditary substance is contained in the egg-cell than in the 

 male germ-cell, and that consequently the amount of the sub- 

 stance must be infinitesimal, for the sperm-cell is. in most cases, 

 many hundred times smaller than the ovum. 



The numerous and important results of the investigations of 



