Chromosome Pairs 109 



the carriers of the same hereditary unit were represented in consider- 

 able quantity ; they became gradually differentiated to an extent 

 commensurate with newly acquired characters. It was also neces- 

 sary that, in proportion as this happened, the mechanism of nuclear 

 division must be refined. At first processes resembling a simple con- 

 striction would suffice to provide for the distribution of all hereditary 

 units to each of the products of division, but eventually in both 

 organic kingdoms nuclear division, which alone insured the quali- 

 tative identity of the products of division, became a more marked 

 feature in the course of cell-multiplication. 



Where direct nuclear division occurs by constriction in the 

 higher organisms, it does not result in the halving of hereditary 

 units. So far as my observations go, direct nuclear division occurs 

 in the more highly organised plants only in cells which have lost 

 their specific functions. Such cells are no longer capable of specific 

 reproduction. An interesting case in this connection is afforded by 

 the internodal cells of the Characeae, which possess only vegetative 

 functions. These cells grow vigorously and their cytoplasm increases, 

 their growth being accompanied by a correspondingly direct multipli- 

 cation of the nuclei. They serve chiefly to nourish the plant, but, 

 unlike the other cells, they are iYicapable of producing any offspring. 

 This is a very instructive case, because it clearly shows that the 

 nuclei are not only carriers of hereditary characters, but that they 

 also play a definite part in the metabolism of the protoplasts. 



Attention was drawn to the fact that during the reducing 

 division of nuclei which contain chromosomes of unequal size, 

 gemini are constantly produced by the pairing of chromosomes of 

 the same size. This led to the conclusion that the pairing chromo- 

 somes are homologous, and that one comes from the father, the other 

 from the mother 1 . This evidently applies also to the pairing of 

 chromosomes in those reduction-divisions in which differences in 

 size do not enable us to distinguish the individual chromosomes. In 

 this case also each pair would be formed by two homologous chro- 

 mosomes, the one of paternal, the other of maternal origin. When 

 the separation of these chromosomes and their distribution to both 

 daughter-nuclei occur a chromosome of each kind is provided for each 

 of these nuclei. It would seem that the components of each pair 

 might pass to either pole of the nuclear spindle, so that the paternal 

 and maternal chromosomes would be distributed in varying pro- 

 portion between the daughter-nuclei; and it is not impossible that 

 one daughter-nucleus might occasionally contain paternal chromo- 

 somes only and its sister-nucleus exclusively maternal chromosomes. 



1 First stated by T. H. Montgomery in 1901 and by W. S. Sutton in 1902. 



