252 RECENT CYTOLOGY 



cells, precisely that chance distribution of allelomorphs 

 which is required on the Mendelian theory would thus 

 be arrived at. 



De Vries' explanation throws light on one pheno- 

 menon which is not accounted for on the supposition 

 that each chromosome represents a separate allelo- 

 morph. In the diagrams previously given of the 

 behaviour of Mendelian characters within the cells we 

 have given no indication of a conjugation in pairs 

 previous to the reducing division. Such a process of 

 fusion is, however, one of the most marked phenomena 

 in the behaviour of the chromosomes at the parallel 

 stage of their existence. On the chromosome-allelo- 

 morph view, the phenomenon of mitosis as bringing 

 about an equal division of hereditary particles between 

 the cells, and the process of reduction in the number 

 of the chromosomes, are both accounted for, but there 

 is no explanation of the fusion between the pairs of 

 chromosomes. On de Vries' view, however, this pro- 

 cess is necessary in order to bring about the necessary 

 redistribution of allelomorphs between the chromo- 

 somes, and so between the germ-cells into which the 

 latter pass. 



In cases where the phenomenon of correlation or 

 coupling has been observed we must suppose that there 

 is some mechanism which causes the representative 

 particles of the respective characters concerned to 

 remain in company during the process by which the 

 other allelomorphs are being reasserted between the 

 chromosomes. Of this process of coupling the cytolo- 

 gists have not yet been able to observe any visible 



