ASSOCIATION OF CHROMOSOMES IN DIPTERA 251 



sex-chromosomes, which shows that members of pairs, whether 

 associated together or not, are separate and distinct chromo- 

 somes instead of daughter halves of single chromosomes. 



The actual behavior of chromosomes in the Diptera shows 

 with the greatest clearness that neither the criticisms of jMeves 

 nor of Dehorne can be vahd in this group. The evidence leaves 

 no doubt that the chromosomes are arranged in pairs and are 

 paired in accordance with their size and form. In DasylHs 

 thoracica (figs. 155-158) for instance the five pairs include four 

 sizes, of which the smallest, next smallest and largest are in- 

 dividually distinct. Similar relations are seen to exist in various 

 other species, such as Asilus lecythus (figs. 146-148), Asilus 

 notatus (figs. 149, 150), Deromyia winthemi (figs. 159-164), 

 Neuroctena analis (figs. 115, 116), Volucella obesa (figs. 124- 

 126), Mesogramma marginata (figs. 127, 128), Chaetopsis 

 fulvifrons (figs. 117-119), Anthrax sinuosa (figs. 134-137), 

 Spogostylum simson (fig. 141), Asilus sericeus (see p. 241) and 

 certain species of Drosophilidae (see especially figs. 21-26). 

 When the haploid and diploid groups of any of these species are 

 compared they are seen to contain the same series of sizes, 

 the former having one and the latter two representatives of 

 each size. There can be little doubt, therefore, that each pair 

 in the diploid group is composed of one paternal and one mater- 

 nal member; indeed it only remains to establish the continuity 

 of the chromosomes to make this a demonstrated fact. 



Another question upon which the Diptera present definite 

 evidence is that of gonomery. In contrast to the more or less 

 continued spatial separation of the two parental chromosome 

 groups found (Haecker, Van Beneden, Riickert, Conklin ('02), 

 Blackman, Ferguson, etc.) in some organisms, the parental 

 groups in the flies intermingle, and the corresponding chromo- 

 somes become arranged in pairs at an early stage in the cleavage 

 of the egg, — perhaps during fertilization and before the first 

 cleavage, although this has not been observed. The earliest 

 stages which I have been able to study with accuracy are those 

 immediately following the migration of the cleavage nuclei 

 to the surface of the egg; and these show the chromosomes 



