ASSOCIATION OF CHROMOSOMES IN DIPTERA 215 



cytological study of these insects have caused them to be gener- 

 ally avoided by cytologists. These difficulties, however, may 

 very largely be overcome by care and persistence. Although 

 certain principles must be observed in making preparations, the 

 task is mainly one of securing and preparing enough speci- 

 mens to get material in the proper stages and in sufficient quan- 

 tity for study. No more difficulty is experienced in studying 

 the nuclear phenomena, when the proper material is secured, 

 than is the case in other insects; indeed the chromatic elements 

 in the ffies, when well prepared, appear with a brilliancy that 

 is surpassed by very few objects.^ 



The observations included here are concerned chiefly with 

 chromosomal behavior in somatic cells and in germ-cells outside 

 the sphere of maturation. These cells I shall briefly term 'dip- 

 loid' cells, in distinction to oocytes and spermatocytes. Since 

 all of the 'diploid' cells agree in respect to the phenomena dealt 

 with, no confusion should arise from such a terminology. Phe- 

 nomena associated with the maturation processes are considered 

 only in so far as they bear directly upon those in'diploid' cells. 

 Likewise the relationships between the chromosomes in dif- 

 ferent species of flies are only briefly considered. I hope to re- 

 turn to both of these questions in subsequent papers. 



In order to facilitate the treatment of the subject matter 

 I will outline at once the main points considered in the paper, 

 and will indicate in advance some of the conclusions attained. 

 This may best be accomplished by taking account of certain 

 genetic hypotheses which intimately involve the chromosomes 

 and which have furnished the occasion for this investigation. 



These hypotheses are all contained in one comprehensive 

 theory which has recently been brought into prominence by 

 the rapid development of Mendelism. According to this theory 

 the chromosomes are complex, accurately differentiated bodies 

 whose organization and behavior are directly correlated with the 

 genetic factors located in them. In any biparental organism, 

 the diploid chromosome group is composed of two equivalent, 



1 Except in maturation stages, which are often very unfavorable for study. 



