THE VITAL PROPERTIES OF THE CELL 275 



tion substances. All other substances (protoplasm, yolk, nuclear 

 sap, etc.) are not concerned in fertilisation as such. 



This proposition is supported by two important facts : 



Firstly, the complicated processes of preparation and matura- 

 tion which the sexual cells must undergo. As follows from the 

 statements given on pp. 235-239, the chief result of these processes 

 is not that the nuclear substances are increased through fertilisa- 

 tion, but that they remain constant in amount for the species of 

 plant or animal in question. 



Secondly, the phenomena of fertilisation seen in Infusoria. Here, 

 as Maupas and Richard Hertwig both assert, similar individuals 

 remain in contact for a sufficient period in order to exchange 

 halves of equal nuclei. When this exchange of migratory nuclei 

 has been effected, the process of fertilisation is completed, and 

 the two animals separate. Hence it is evident, that the ultimate 

 result of the complicated processes consists in this, that after the 

 fusion of the migratory and stationary nuclei the nucleus in each 

 fertilised individual is composed of nuclear substance derived 

 from two different sources. 



It' the important substance of fertilisation is contained in the 

 nucleus, the question arises whether the nuclear substance of the 

 spermatozoon differs from that of the egg-cell. This question has 

 been answered in very different ways. Formerly it was generally 

 considered, as Sachs expressed it, that the male element intro- 

 duced into the ovum a substance which it did not contain before. 

 One view especially has obtained many adherents; it may be de- 

 scribed as the doctrine of the hermaphroditism of nuclei and the 

 theory of restitution. 



Many investigators consider that the cells possess hermaphro- 

 dite nuclei, that is to say, nuclei with both male and female 

 properties. For instance, according to van Beneden's hypothesis, 

 which has been the most clearly worked out, immature egg and 

 sperm-cells are hermaphrodite ; they only gain their sexual 

 character after the egg-cell has lost its male, and the sperm-cell 

 its female constituents of their normal hermaphrodite nuclear 

 apparatus. The male nuclear constituents are expelled from the 

 egg in the nuclear segments of the polar cells. The reverse pro- 

 cess occurs in a similar manner with sperm-cells. Thus the egg 

 and sperm-nuclei, being halved, become pronuclei, and possess 

 opposite sexual characteristics. 



Regarded from this point of view, fertilisation consists essenti- 



