256 RECENT CYTOLOGY 
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 of 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 
