Transmifision of Hereditary/ Character s. 469 



theredity are present in relatively far greater abundance in 

 the spermatozoon than in the ovum, and that by far the 

 greatest mass of the ovum consists of nutritive plasm. Now 

 the newer doctrine maintains that those matters which are 

 active in heredity are situated in the nucleus alone, and the 

 demonstration of such a transmitter of hereditary characters, 

 the existence of which is not merely inferred, but which is 

 actually visible, has usually been regarded as an important 

 advance. In my opinion such a display of precision should 

 rather be termed a retrocession or deviation : for Naii:eli'3 

 idea was simply a delicate logical construction which was 

 thoroughly consistent with the facts, and its justice has sub- 

 sequently been proved most umistakably by tlic experiment of 

 Boveri, to which reference shall shortly be made. For the 

 more modern view, on the contrary, not only were the actual 

 starting-points too slight, but many facts made themselves 

 felt against it even at the time ; that it is untenable as a 

 theory is moreover demonstrated by the above-mentioned 

 communications from Fol. 



I shall now adduce the chief arguments which are brought 

 forward in favour of the doctrine of the seat of the processes 

 of heredity in the nucleus and give an analysis of each. 



In the first place it is alleged as the chief reason that the 

 heads of the spermatozoa consist solely or almost solely of 

 nuclear substance, and that this is the only portion of the 

 spermatic elements which is active in the process of fertiliza- 

 tion. Kolliker goes furthest in this respect, since he declares 

 the spermatozoa of certain animals to be simply nuclei. Tiie 

 majority of investigators do not follow him in this, doubtless 

 partly because we otherwise know nothing of an existence 

 and activity of naked nuclei, and because the greater number 

 of those who have watched the process of spermatogenesis 

 have maintained the cellular nature of the spermatic filaments. 

 Moreover, most of the adherents of the theory which is iiere 

 to be criticized assume the presence of an extremely thin 

 protoplasmic envelope round th6 nucleus of the spermatozoon, 

 even when such an envelope is hardly to be detected or is 

 absolutely invisible, thus allowing the tail of the spermatic 

 filament to rank as protoplasm ; and at any rate in the pro- 

 cess of fertilization the layer surrounding the nucleus must 

 penetrate with it into the ovum, as has indeed positively been 

 proved to be the case in certain instances. But, in addition 

 to this, the following also must be conceded : the spermatozoa 

 arise from cells through repeated indirect nuclear division 

 (and finally cell-division). Now recent investigations have 

 shown that in indirect nuclear division the so-called polar 

 bodies or " centrosomata " {Centrosomen) are of quite general 



