THE NUCLEUS IN HEREDITY 31 



with regard to the cytoplasm. The bulk of the sperm con- 

 sists of nucleus in most cases, and there is very little cyto- 

 plasm. Under normal conditions the centrosomes in the 

 fertilised ovum are derived from the sperm. Besides the 

 centrosomes a small amount of cytoplasm is brought into 

 the ovum by the sperm. Several observers have declared 

 that the cytoplasm thus brought in degenerates shortly 

 after fertilisation has taken place. It would appear that 

 the centrosomes cannot be the probable bearers of hereditary 

 characters generally, as otherwise the offspring would inherit 

 the characters of the male parent only, and not those of the 

 female. These observations taken alone suggest strongly 

 that the nucleus is the only part of the sperm that continues 

 to exist in the fertilised ovum as a probable bearer of here- 

 ditary characters. There is, however, a great deal more 

 evidence that suggests that the hereditary characters are 

 determined by the nucleus. Oscar and Richard Hertwig 

 broke the unfertilised eggs of a sea-urchin by shaking them. 

 They took some portions of the egg which did not contain 

 any part of the nucleus and fertilised them with sperms. 

 These portions of the egg proceeded to segment, and de- 

 veloped up to a certain point. In these cases the cells 

 formed contained nuclei derived entirely from the sperm, 

 while the bulk of the cytoplasm was derived from the ovum. 1 

 Morgan subsequently repeated this experiment, and 

 showed that in the cells of such an embryo only half the 

 number of chromosomes appeared. 2 



Boveri carried these experiments still further. He showed 

 that such enucleated portions of the ovum as were used by 

 the Hertwigs in their experiments and fertilised by a sperm, 

 developed into larvae perfect in every way except in size. 

 An even more interesting and suggestive experiment was 

 subsequently carried out by Boveri. He fertilised the enu- 

 cleated portions of the ovum of one species of sea-urchin 



1 Hertwig, 0. and R., Uber den Befnichtungs- und Teilungsvorgang des 

 tierischen Eies unter dem Einfluss aiisserer Ageiitien, Jena, 1887. 



2 Morgan, T. H., op. cit., p. 30. 



