CHROMOSOME STUDIES IN TETTIGID.E. 2 19 



have accepted the thought that this reduction in number takes 

 place by the union of chromosomes in pairs we have essentially 

 also accepted the theory of the individuality of chromosomes. 

 Then how do these individuals come together and finally separate 

 again? 



i . Synapsis and Heredity. 



One can scarcely follow out the discussion of Janssen on 

 Batracoseps attennatus, and more especially when close attention 

 is given to his drawings, without coming to the conclusion that 

 with this form, at least, the chromosomes unite by parasynapsis 

 and that it is perfectly possible in the twisting of certain chroma- 

 tin elements to have an exchange of parts of homologous chromo- 

 somes, thus forming a convenient mechanism for the "crossing 

 over" of hereditary characteristics if we accept the chromosome 

 hypothesis of heredity. The "chiasmatypie" theory of Janssen 

 ('09) furnishes a very convenient means for explaining some 

 ratios in Mendelian inheritance. 



An application of this chiasmatype theory, which should not 

 pass without mention, is the work on Drosophila, particularly 

 the work of Morgan and his students ('15). From the behavior 

 of the hereditary characteristics a chromosome map has been 

 constructed in which not only are the determiners of the char- 

 acteristics located on definite chromosomes, but also the relative 

 distances that these genes are from each other, is given. The 

 position of the genes on the chromosomes is calculated from the 

 percentages of cross-overs. In addition to the vast amount of 

 genetic evidence for this condition in Drosophila there is also 

 some cytological evidence. 



Metz ('14) in dealing with five different types of chromosome 

 groups of Drosophila shows that the chromosomes exhibit a 

 close association in pairs at nearly all times and that before 

 each cell division the pairs become so intimately associated that 

 they may be said actually to conjugate. He further states that 

 the union of the chromosomes is "unquestionably a side-by- 

 side, or parasynapsis one." In a later paper (Metz '16) the 

 same author in considering chromosome pairing in about eighty 

 species of Diptera, many of which belong to the genus Drosophila, 

 states that the pairing "certainly involves the essential features 

 of a synaptic (parasynaptic) union." 



