PROBLEMS OF MODERN SCIENCE 



cycle of the organism in which, in the matura- 

 tion of the germ-cells, the chromosomes, instead of 

 dividing, arrange themselves in pairs, which then 

 separate, and in this way, with complications which 

 need not concern us, the number of chromosomes 

 in the germ-cells is reduced to half that in the body- 

 cells. In fertilisation the nuclei of two germ-cells 

 unite, but their chromosomes maintain their 

 separate identity, and so the double number is 

 restored. In recent years it has been shown in 

 various plants and animals that the paired arrange- 

 ment of the chromosomes frequently takes place 

 immediately after fertilisation and persists through- 

 out the development of the individual ; and, 

 whether the pairing is early or late, each pair con- 

 sists of one chromosome of maternal and a cor- 

 responding one of paternal origin. When the 

 chromosomes differ from each other in size or shape, 

 as they are now known to do in many organisms, 

 it can then be shown that there are two of each 

 kind. Every nucleus in our bodies thus contains 

 corresponding elements derived from both parents. 

 So fine are the meshes of the warp and woof out 

 of which we are woven. 



Now we have seen that the number of chromo- 

 somes is a constant for each species ; e.g., in the lily 

 family, the genus Lilium has twelve pairs of long 

 chromosomes, while Aloe has seven pairs of different 

 lengths. In the evening primroses (CEnothera) the 

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