CHROMOSOMES 239 



in the zygote. In this way every somatic nucleus of 

 the latter contains a double set of chromosomes, half 

 of them being descended from the chromosomes intro- 

 duced by one parent, whilst the other half came from 

 the second parent. 



There is reason to believe that the set of chromo- 

 somes derived from one parent is complete in itself, 

 containing everything necessary for the development 

 of a normal individual. Indeed, in some cases of 

 parthenogenesis (development of the unfertilized egg), 

 egg cells have been known to develop which contained 

 only a single set of chromosomes. Boveri proved 

 very prettily that the paternal set of chromosomes is 

 equally adequate for complete development. By dint 

 of violent shaking Boveri contrived to remove the 

 nucleus from the egg-cells of a sea-urchin, and he 

 afterwards allowed a sperm-nucleus to enter the 

 enucleated egg, which presently developed into a 

 complete embryo. Thus it was shown that the 

 paternal as well as the maternal set of chromosomes 

 is sufficient by itself to determine the proper production 

 of all the organs of the embryo. But Boveri also 

 showed that if any chromosome ot the paternal (or 

 maternal) set were wanting in such a case, normal 

 development of the embryo could no longer take place. 

 Let it once more be emphasized that the somatic cells 

 of an ordinary organism contain a double complement 

 of essential nuclear material. 



Since the gametes contain only half as many chromo- 

 somes as the somatic cells, and since the number of 

 chromosomes present in the latter is constant for each 



