214 CHARLES W. METZ 



(Stevens '08, p. 372). In a later paper on "The chromosomes 

 in the germ-cells of Culex"- (Stevens '10, p. 215), correspond- 

 ing phenomena called forth a similar statement to the effect 

 that "perhaps the most interesting point in the history of the 

 germ-cells of Culex is the fact that, as in the Muscidae, pairing 

 or synapsis, occurs in connection with each spermatogonial 

 and oogonial mitosis as well as in anticipation for maturation." 

 Although only able to study somatic mitoses to a very hmited 

 extent, Stevens surmised that, "it may therefore be true that 

 pairing of homologous chromosomes occurs in connection with 

 each mitosis throughout the hfe history of these insects" (p. 

 215). Now this would be a very important point to establish, 

 as Stevens realized, and she doubtless would have followed it 

 up had it not been for her untimely death in 1912. Most un- 

 fortunately, however, her work on the Diptera was stopped at 

 its very beginning and many promising questions suggested 

 by it have remained uninvestigated. 



Nothing further appeared on chromosomes of the Diptera 

 until 1914 when three papers were published, one by the author 

 on Drosophila chromosomes; the others on the chromosomes 

 on Culex pipiens, one by Miss Taylor, and one by Lomen. 

 Both of the latter took exception to Stevens' conclusions that 

 the chromosomes are paired in Culex and other Diptera, on the 

 ground that the chromosome pairs which she described were 

 really only precociously split univalent chromosomes. Their 

 evidence on this point, however, is very inadequate, and their 

 conclusions are surely erroneous (see pp. 244 and 245). 



The purpose of the present paper is to describe in some 

 detail the phenomena involved in 'chromosome pairing' in the 

 Diptera, and to consider their bearing on current theories re- 

 specting the nature of the chromosomes and their role in hered- 

 ity. Because of their remarkably definite paired association 

 the chromosomes of the Diptera are especially suitable for 

 studies on the relationships between individual chromosomes 

 and on the qualitative characteristics of chromosomes as indi- 

 cated by their behavior, but as I have mentioned in a previous 

 paper (Metz '14) the technical difficulties involved in an extensive 



