818 



SCIENCE 



[N. S. Vol. XXXVII. No. 961 



sential facts concerning the chromosomes, 

 leaving aside most of the complicated tech- 

 nical details. In each species of plant or 

 animal the chromosomes are of constant, or 

 nearly constant, number. They divide as 

 the cell divides, and are thus transmitted 

 from cell to cell. In the fertilization of the 

 egg two similar groups of chromosomes are 

 brought together, one contributed by the 

 egg, one by the sperm-cell ; and as the egg 

 step by step divides to build up the body 



against it must give way before the fact 

 that in certain hybrids — in particular, cer- 

 tain fish-hybrids observed by Moenkhaus — ■ 

 the chromosomes of maternal ancestry can 

 actually be distinguished by the eye from 

 those of paternal. Finally, when new 

 germ-cells are produced for the formation 

 of the following generation the double 

 chromosome-groups are again reduced to 

 single ones in preparation for the succeed- 

 ing process of fertilization. 



of the embryo the chromosomes also divide 

 at each cell-division. Every cell-nucleus 

 thus receives a double group of chromo- 

 somes, consisting of two single groups de- 

 scended respectively from the two groups 

 that originally came together in the fer- 

 tilized egg. The two single groups of each 

 nucleus are thus of maternal and of pater- 

 nal ancestry, respectively. This all-impor- 

 tant conclusion has been obstinately con- 

 tested and is still denied by a few writers, 

 I think, however, that all arguments 



These are not theories but observed facts. 

 It is impossible to overlook the very precise 

 parallel which they show to what Gregor 

 Mendel and his successors have proved to 

 be true also of the unit-characters, as will 

 be made elear by the accompanying dia- 

 gram (Fig. 2). When two similar or 

 nearly similar individuals unite in fertili- 

 zation they contribute to the germ two cor- 

 responding groups of unit-characters, which 

 are designated in the diagram by two series 

 of letters A-D and a-d, respectively. The 



