276 THE CELL 



ally in the replacement of the male elements, which have been 

 expelled from the egg, by an equal number of similar elements, 

 which are introduced by the spermatozoon. 



More careful investigation shows that these theories are not 

 tenable. For the empirical foundation, upon which they were 

 based, is destroyed by the fact which was proved on p. 237, namely 

 that the polar cells are morphologically nothing but egg-cells, 

 which have become rudimentary. This follows from a comparison 

 of the development of egg and sperm-cells in Nematodes. Hence 

 the nuclear segments, expelled from the egg in the polar cells, 

 cannot be the discharged male constituents of the germinal vesicle, 

 as is stated in the restitution theory. 



Apart from this, we are unable, with the methods of investiga- 

 tion at our command, to discover the least difference between the 

 nuclear substances of the male and female cells. Nuclein and 

 centrosomic substance are identical, both as regards quantity and 

 composition. There is no specific male or female fertilising 

 material. The nuclear substances, which come into contact with 

 one another during the process of fertilisation, differ only in this, 

 that they are derived from two different individuals. 



Now, if, in consequence of this, it can no longer be allowed that 

 the egg and sperm-nuclei are sexually opposed in the way under- 

 stood by the supporters of the restitution theory, what meaning 

 must be attached to the terms male and female sexual cells or 

 male and female nuclei ? 



These terms do not really touch the essential part of fertilisa- 

 tion, and do not express an opposition based upon fundamental 

 processes of reproduction; they refer rather to secondary differ- 

 ences of minor importance which have developed between the 

 conjugating individuals, between the sexual cells and their nuclei, 

 and which must be classed as secondary characteristics. Hence 

 we will state at once that the formation of two separate sexes is 

 not the cause of sexual generation, as might be concluded from a 

 .superficial investigation, but that the reverse is really true. All 

 .sexual differences, if we trace them back to their sources, have 

 arisen because the union of two individuals of one species, which 

 originally were similar, and hence sexless, is advantageous to the 

 maintenance of the vital processes ; without exception, these 

 differences only serve one purpose, namely to facilitate the com- 

 bination of two cells. On this account solely have the cells de- 

 veloped the differences which are termed male and female. 



