ASSOCIATION OF CHROMOSOMES IN DIPTERA 257 



Whatever may be the fundamental cause of this phenomenon 

 it seems certain from the evidence that its manifestations dif- 

 fer markedly in different organisms. There seem to be various 

 intermediate conditions between that of intimate pairing (Dip- 

 tera) and that of very slight pairing. It may be true there- 

 fore, that the tendency to associate in pairs is inherent in the 

 chromosomes of multicellular organisms, being manifest in 

 all the cells of some, but only in the maturing germ-cells of 

 others. At all events it seems certain that the Diptera are not 

 sharply differentiated from other animals by reason of any 

 primary distinction in organization responsible for the pairing 

 of their chromosomes. 



Many of the questions suggested by this study are intimately 

 involved with those of maturation, and can only be satisfactorily 

 treated when the phenomena of maturation in the Diptera 

 are better known. For this reason a full discussion of them will 

 be reserved for a subsequent paper in which I hope to consider 

 the maturation processes in detail, but in conclusion a word may 

 be said as to the bearing of the pairing phenomena upon the 

 theoretical question of synapsis. It is a significant fact that 

 in the diploid cells of the flies a process may actually be followed 

 which agrees in all essential respects with parasynapsis. In 

 metaphase corresponding chromosomes, although arranged in 

 pairs, are usually not closely applied; but in anaphase the mem- 

 bers of these pairs often become associated side by side as they 

 pass toward the poles, until the approximation becomes very 

 intimate. ^^ In these cases there can be no doubt about the 

 reality of a process which, whether or not it actually corre- 

 sponds to that of synapsis, certainly involves the essential fea- 

 tures of a synaptic (parasynaptic) union, and removes the a 

 priori objections urged against the conception of synapsis. 



1* With regard to the synapsis during maturation it may be noted that during 

 the final spermatogonia! anaphase (and probably oogonial also) the chromosomes 

 behave in this same manner, and hence are brought into a closely paired arrange- 

 ment before maturation. 



