660 LECTURE XXIII. 



and an antherozoid, do not differ in nature, and the various 

 external sexual differences he regards as being simply means 

 to ensure the coalescence of the appropriate gametes. 



With regard to heredity, Strasburger is of opinion that 

 the nucleus of the reproductive cell is the means by which 

 the hereditary characters are transmitted, and he adduces 

 evidence to prove that the idioplasmic filament of the 

 nucleus consists of a number of segments derived from 

 previous generations. Such a view clearly puts a limit to 

 the time within which reversion may manifest itself, for, 

 in the course of a few generations the amount of nucleo- 

 idioplasm in the spore which can be traced back to any 

 particular ancestor must be very small. Strasburger himself 

 points out that in the twentieth generation it would be 

 scarcely one millionth part. The fact that all the ancestral 

 characters may not manifest themselves in any given in- 

 dividual is explained by Strasburger, in agreement with 

 Naegeli, on this wise, that inherited characters may remain 

 latent; that is, the inherited properties of the nucleo-idio- 

 plasm of the spore may not all influence the cytoplasm 

 simultaneously. Strasburger also agrees with Naegeli in 

 assuming inherent variability, and in denying the inherit- 

 ance of acquired characters. 



Finally, there is Weismann's Theory of a special repro- 

 ductive substance (Keimplasma). He assumes that each gene- 

 ration passes on to its offspring a certain amount of this 

 reproductive substance ; in the individual the amount of the 

 reproductive substance is increased, the individual in fact 

 produces from it its own reproductive cells, but the increase 

 in quantity is not accompanied by any change in kind. 

 There is then a "continuity of the reproductive substance" 

 fr6m one generation to another. As to the exact seat and 

 nature of the reproductive substance, Weismann considers, 

 in view of the importance of the two nuclei in the sexual 

 process, that it is contained in the nuclei of the reproductive 

 cells; his reproductive substance appears in fact to be nucleo- 

 idioplasm. 



The account which he gives of the extrusion of the polar 



