ASSOCIATION OF CHROMOSOMES IN DIPTERA 255 



Y-chromosome, or in other words the two sex-chromosomes 

 would be expected to associate in approximately the same 

 manner as do corresponding autosomes. In reality the as- 

 sociation of the sex-chromosomes differs slightly from that of 

 the autosomes, in that the former appear to condense earlier 

 in prophase and become separated more frequently in metaphase 

 than do the latter, but in essential features pairing is the same 

 in both. 



As suggested above the phenomena of chromosome pairing 

 in somatic and primordial germ-cells appear to be closely cor- 

 related with those of maturation in spermatocytes and oocj^tes. 

 The latter phenomena are obviously much more complicated 

 than the former, and the association of the chromatic elements 

 during synapsis is perhaps much more intimate than during the 

 resting stage or prophase of somatic cells; but the similarity 

 between the figures in the somatic cells of flies and those in 

 germ-cells of many animals (including flies) makes it seem very 

 probable that essentially the same cause is operative in both 

 cases. If this be true it would seem that in the development 

 of a fly each cell division is preceded by an attempt at synapsis. 

 Or, in other words, the tendency to undergo synapsis is so marked 

 as to bring about a close approximation of homologous chromo- 

 somes during each cell generation. 



No positive answer can be given to the question as to why 

 pairing outside the sphere of maturation should be exhibited 

 by some organisms and not by others. Among animals a defi- 

 nite pairing of all the chromosomes in somatic as well as germ- 

 cells is kno"^Ti to occur only in the Diptera. Among plants it 

 has been reported in several orders. But whether the phenomena 

 are really the same in the two kingdoms is not clear, for the 

 details of the process in plants are still obscure. In the Diptera 

 one of the most characteristic features of pairing is the close 

 apposition of the early prophase threads, upon which subse- 

 quent behavior seems to depend. Whether a similar apposition 

 is found in the prophases of plant cells, or whether a pairing 

 takes place just preceding metaphase, is not certain. The ob- 

 servations of Stomps on Spinacia, of Overton on Thalictrum 



