CH. xiv] NUCLEUS AND HEREDITY 207 



the belief, which is daily gaining ground, that the chromo- 

 somes have a preponderant share in the transmission of 

 hereditary characters. 



The arguments for ascribing this leading part to the 

 chromosomes in hereditary transmission, like those for the 

 individuality of the chromosomes themselves, are of several 

 distinct kinds, some direct and some purely inferential. 

 They may be classed at the outset into two groups, firstly, 

 those which indicate that the nucleus 'rather than the cyto- 

 plasm is the more important factor, and secondly those 

 which point to the chromosomes rather than any other 

 nuclear constituent as being the "bearers of hereditary 

 characters." The main lines of argument will be dealt with 

 in this order. 



One obvious reason for supposing that the nucleus plays 

 the chief part in the transmission of inherited characters 

 is that the spermatozoon consists almost entirely of nucleus, 

 middle-piece, and tail, and of these the tail in some forms 

 is thrown off as the spermatozoon enters the egg, and the 

 middle-piece seems to give rise to little beside the centro- 

 somes. Further, the egg and spermatozoon are equivalent 

 as regards the transmission of most characters, and the 

 nuclei are the only parts of them which are alike ; if the cyto- 

 plasm took any preponderant share, it is hardly possible to 

 believe that the maternal characters would not be more 

 strongly developed in the offspring than the paternal. There 

 are also several kinds of experimental evidence pointing to 

 the same conclusion. It is known that non-nucleated frag- 

 ments of Sea-urchin eggs can be fertilised by a spermatozoon 

 and give rise to larvae which show the normal characters 

 of the species. BOVERI (1895, 1903 b] attempted to use this 

 fact to test the hypothesis that the nucleus bears the deter- 

 miners for hereditary characters by fertilising non-nucleated 



