IV.] FOUNDATION OF A THEORY OF HEREDITY. 25 1 



half of the nucleus necessary for development. But such eggs 

 (e. g. that of the oyster) do not develope, but always die if they 

 remain unfertilized. 



This argument can only be met by a new hypothesis, the 

 construction of which I must leave to the defenders of the 

 above-mentioned theory. But the observation in question 

 seems to me to furnish at the same time a proof of the co- 

 existence of two different nucleoplasms in the egg-nucleus. If 

 the nucleoplasm of the polar bodies was also germ-plasm, we 

 could not understand why such eggs are unable to develope 

 parthenogenetically, for at least as much germ-plasm is con- 

 tained in the unfertilized &gg as would have been present after 

 fertilization. 



The only objection which can be raised against this con- 

 clusion depends upon the supposition that the nucleoplasm of 

 the sperm-cell is qualitatively different from that of the egg-cell. 

 I have already dealt with this view, but I should wish to refer 

 to it again rather more in detail. Some years ago I expressed 

 the opinion^ that the physiological values of the sperm-cell and 

 of the egg-cell must be identical ; that they stand in the ratio 

 of I : I. But Valaoritis '^ has brought forward the objection that 

 if we consider the function of a cell as the measure of its physio- 

 logical value, it is only necessary to point to the respective 

 functions of ovum and spermatozoon in order to show that 

 their physiological values must be different. ' The egg-cell 

 alone, by passing more or less completely through the phyletic 

 stages of the female parent, developes into a similar organism ; 

 and although the presence of the spermatozoon is in most cases 

 required in order to render possible such a result, the cases of 

 parthenogenesis prove nevertheless that the ^g'g can do without 

 this stimulus.' This objection appeared to be fully justified as 

 long as fertilization was looked upon as the 'vitalization of the 

 germ,' and so long as the sperm-cell was considered as merely 

 ' the spark that kindles the gunpowder,' and further so long as 

 the germ-substance was believed to be contained in the cell- 

 body. But now we can hardly give to the body of the egg-cell 

 a higher significance than that of the common nutritive soil of 



^ ' Zeitschrift fur wissenschaftliche Zoologie,' Bd. XXXIII. p. 107. 

 1873. 



^ Valaoritis, 1. c, p. 6. 



