174 CYTOLOGY chap. 



homologous chromosomes in the last chapter, and it is a plausible 

 hypothesis that these differences are due to their different factorial 

 composition. 



(4) The Interchange of Hereditary Factors between Homologous 



Chromosomes 



The simple proposition that the characteristics of organisms are 

 represented by factors in the chromosomes and that their distribution, 

 according to the classical Mendelian scheme, depends upon the movements 

 of these chromosomes in meiosis, requires some elaboration, however. 

 For if each independently inheritable character resided in a separate 

 chromosome, it is clear that there could only be as many such characters 

 as there are chromosomes in the gamete. By separately inheritable 

 characters we mean those which can be separated from each other and 

 made to enter into fresh combinations by crossing. To put it in another 

 way, all the characteristics of an organism should be capable of correla- 

 tion into the same number of groups as there are chromosomes in the 

 gamete, each group of characters behaving in heredity as a unit. 



Now organisms present far more characters which are separately 

 inheritable than they have chromosomes. This has been shown definitely 

 for some organisms, and there can be little doubt that it is the general 

 rule. Thus in the fruit-fly Drosophila ampelophila, which has been the 

 subject of such exhaustive study in America, more than a hundred 

 separately inheritable characters are known, though the number of 

 chromosomes (haploid) is only four. 



The hypothesis that hereditary factors are located in the chromosomes 

 has therefore to be supplemented by the supposition that there are 

 many factors in each chromosome, each located in a definite part of the 

 chromosome {i.e. represented by a definite unit in the structure of the 

 chromosome) and that exchange of corresponding factors may take 

 place between homologous chromosomes. It is natural to. suggest the 

 chromomeres as the seat of the separate factors, and syndesis as the 

 moment at which exchange takes place. By chromomeres, in this case, 

 must be understood the numerous small bead-like bodies often observable 

 in syndesis, not the few, much larger swellings on the chromosomes, 

 sometimes found in late prophase. ^ According to this view, a chromo- 

 some is to be considered as containing a linear series of factors ABCDE 

 . . . and two chromosomes in syndesis can be represented thus : 



ABCDE . . . 



a b c d e ... 



1 Although even in syndesis the chromomeres are probably often already compounded 

 of several smaller units. 



