IV.] FOUNDATION OF A THEORY OF HEREDITY. 253 



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 fertihzing sperm-nucleus 

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

 nucleus and sperm-nucleus are identical would be furnished. 

 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 compensated for 

 by the fact, ascertained by Berthold, that in certain Algae 

 [Ecfocarpits and Scytosiphon) there is not only a female, but 

 also a male parthenogenesis ; for he shows that in these species 

 the male germ-cells may sometimes develope into plants, which 

 however are very weakly^. Furthermore the process of con- 

 jugation may be considered as a proof that this view as to the 

 secondary importance of sexual differentiation is the true one. 

 At the present time there can hardly be any hesitation in ac- 

 cepting the view that conjugation is the sexual reproduction of 

 unicellular organisms. In these the two conjugating cells are 

 alm.ost always identical in appearance, and there is no evidence 

 in favour of the assumption that they are not also identical in 

 molecular structure, at least so far as one individual of the same 

 species may be identical with another. But there are also forms 

 in which the conjugating cells are distinctly differentiated into 

 male and female, and these are connected with the former by 

 a gradual transition : thus in Pandorina^ a genus of Volvocineae, 

 we are unable to make out any differences between the con- 

 jugating cells, while large egg-cells and minute sperm-cells 

 exist in the closely allied Volvox. If we must suppose that 

 the conjugation of two entirely identical Infusoria has the same 

 physiological effect as the union of two sexual cells in higher 

 animals and plants, we cannot escape the conclusion that the 

 process is essentially the same throughout : and that therefore 



^ I quote from Falkenberg, in Schenk's ' Handbuch der Botanik.' Bd. 

 II. p. 219. He further states that these are the only instances hitherto 

 known in which undoubted male cells have proved to be capable of 

 further development when they have been unable to exercise their 

 powers of fertilization. It must be added that the two kinds of germ- 

 cells do not dilTcr in appearance, but only in behaviour ; the female 

 germ-cells becoming fixed, and withdrawing one of their two flagella, 

 while the male cells continue to swarm. But even this slight degree of 

 differentiation requires the supposition of internal molecular differenti- 

 ation. 



