XI.] PROBLEMS OF THE DAY. 91 



These tendencies are exclusively contained in the nuclear 

 loops ; the cell-bodies of the spermatozoon and ovum are in 

 this respect indifferent, and serve only as the nutritive mate- 

 rial which is formed and transformed in a definite w^ay by the 

 dominating idioplasm of the nucleus, as clay is moulded by the 

 hand of a sculptor. That the ^%^^ and the spermatozoon differ 

 so greatly in appearance and function, and that they mutually 

 attract each other, depend on secondary adaptations, which 

 ensure that they shall find each other, that their idioplasm or 

 nuclear substance shall come into contact, while, at the same 

 time, a certain amount of nutriment shall be provided for the 

 embryogeny, &c. &c. And just as the differentiation of cells 

 into male and female reproductive elements is secondar}'', so 

 is that of male and female individuals : all the numerous dif- 

 ferences in form and function which characterize sex among 

 the higher animals, all the so-called 'secondary sexual charac- 

 ters,' affecting even the highest mental qualities of mankind, 

 are nothing but adaptations to bring about the union of the 

 hereditary tendencies of two individuals. 



These are briefly the ideas on fertilization which I indicated 

 in the year 1873, and which I published in a detailed and 

 definite form in 1885, after the discoveries of Van Beneden on 

 the morphological processes which take place during the 

 fertilization of the Q.%g of Ascaris'^. Towards the end of the 

 essay I used these words, ' If it were possible to introduce the 

 female pronucleus of an ^gg into another &g^^ of the same 

 species, immediately after the transformation of the nucleus of 

 the latter into the female pronucleus, it is very probable that 

 the two nuclei would conjugate just as if a fertilizing sperm- 

 nucleus had penetrated. If this were so, the direct proof that 

 egg-nucleus and sperm-nucleus are identical would be fur- 

 nished. Unfortunately the practical difficulties are so great 

 that it is hardly possible that the experiment can ever be 

 made ; but such want of experimental proof is partially com- 

 pensated for by the fact, ascertained by Berthold, that in 

 certain Algae {Ectocarpus and Scyiosiphon) there is not only 

 a female, but also a male parthenogenesis ; for he shows that 

 in these species the male germ-cells may sometimes develop 

 into plants, which however are very weakly ^' 



^ See Vol. I, Essay iv, p. 163. ^ See Vol. I, pp. 352, 253. 



