XII.] CONJUGATION AND SEXUAL REPRODUCTION. 113 



which unite during fertilization seem to be necessary to determine 

 the commencement of development. But this refers only to the 

 restoration of a certain amount of nuclear substance, rendering its 

 quantity sufficient for development, and parthenogenesis shows 

 us that when the second polar body is absent this quantity can be 

 supplied by a single sexual cell. In the precise meaning of the 

 word, as it is ordinarily used, there is no such thing as a 

 fertilizing substance, and the progress in thought from the old 

 to the new doctrine of fertilization can only take place when the 

 idea of such a substance in the old sense is completely aban- 

 doned, and when it is recognized that fertilization has no signifi- 

 cance except the union in the single offspring of the hereditary 

 substance from two individuals. 



The advance which has occurred is due to Strasburger's 

 writings as well as my own : the former agreed with O. Hertwig 

 and me as to the essential similarity, as regards their chief 

 constituents, of the two sexual cells, and as to the secondary 

 nature of their differences : Strasburger in fact went so far as to 

 say that all differentiations of sex were simply the means 

 adapted to bring together the two cell-nuclei which were 

 necessary for the sexual act. With this view I not only entirely 

 agreed, but totally rejected the pre-existing dynamic theory 

 of fertilization, in as much as I could not recognize the object 

 of fertilization as the ' vitalization of the germ' or the 'reju- 

 venescence of vital processes,' but regarded it as simply the union 

 of the different hereditary tendencies of two individuals. This union, 

 which has hitherto been regarded, to some extent, as merely 

 a necessary consequence, has become the important feature, while 

 the ' vitalization of the germ ' by the interaction of two opposed 

 sexual cells, — formerly looked upon as the essential part of the 

 process, — has declined from this high position and is regarded 

 as only the means by which the process is effected. 



I was, at that time, so completely convinced that the facts 

 warranted no other explanation, that I maintained that the 

 nucleus of an ovum might be fertilized as fully by the nucleus 

 of another ovum,— i. e. might be rendered equally capable of 

 development, — as by the nucleus of a spermatozoon. The passage 

 in which I advocated this view runs as follows : — ' If it were 

 possible to introduce the female pronucleus of an o^gg into 

 another Q:gg of the same species, immediately after the transfor- 



VOL. II. r 



