CHROMOSOMES OF MOSQUITO 559 



is absolute in the male but partial in the female, and that the 

 coupled factors bear a linear relation to each other. 



Both genetic and cytological studies show that in the Lepi- 

 doptera the female is digametic, the male monogametic, while 

 the reverse condition obtains in the Diptera. This, however, 

 seems to have nothing to do with partial coupling which occurs 

 in both sexes of the rat. 



Thus far results from Primula and sweet peas (Bridges '14) 

 and extensive evidence from Drosophila show that where partial 

 coupling of three pairs of factors occurs, the factors stand in 

 linear relationship to each other and that they are scattered, as 

 if by chance, along a line, not collected together into groups. 

 Moreover the factors which are very closely coupled are still 

 in linear relationship, so that it would seem necessary, as Morgan 

 and his students have done, to suppose that there is a transverse 

 break in a pair of chromosomes and a re-fusion in such a manner 

 that the broken end of each of the original members is united 

 to the broken end ol the other. 



This theory, known as the chiasmatype theory, was first 

 advanced by Janssens ('09) who suggested that it might explain 

 genetic results. He supposed that the early tetrads twisted 

 spirally and that homologous chromatids touched each other, 

 broke at the point of junction, which was called the chiasma, 

 and fused at this point with each other. Thus in text figure 4, 

 B and C would undergo a break and re-fusion, while A and D 

 would remain intact. 



Janssen's investigations were done upon the amphibian Batra- 

 choseps. As Wilson ('12, p. 442) says, his theory "gives the 

 only simple mechanical explanation thus far offered as to how 

 such an orderly exchange of materials may be effected." He is 

 inclined to give credence to Janssen's results; but some doubt 

 about the matter has arisen through more recent work. It is 

 most important to investigate the conditions in some form in 

 which the mechanism of heredity in regard to partial coupling 

 is known, or to investigate the mechanism of heredity in forms 

 in which the synaptic conditions are better understood. 



