ASSOCIATION OF CHROMOSOMES IN DIPTERA 233 



evitable under the circumstances, are the two members of a 

 pair in any other relation than this during metaphase. 



Musca domestica (figs. 68-72). In Musca the chromosomes 

 are very similar in form and behavior to those of Calliphora, 

 except in respect to the sex-chromosome pair, which is almost 

 as large as the autosomes. Haploid groups of Musca have 

 already been published by Stevens ('08, fig. 3). The accompany 

 ing figures are taking solely from diploid groups to illustrate the 

 pairing phenomena. They are all from ovarian tissue far in 

 advance of maturation stages and may be said to represent 

 the characteristic features of prophase and metaphase in early 

 ovarian and somatic cells. Figures 68 and .69 are metaphases 

 showing the six pairs of chromosomes in the equatorial plate. 

 It will be noticed that in each figure the members of one pair of 

 chromosomes are displaced and are not closely associated. These 

 are in all probability the sex-chromosomes (XX). Prophases 

 showing the early appearance and the disjunction of the chromo- 

 somes are represented in figures 70, 71 and 72. The former is 

 from an entire, or nearly entire nucleus, the latter two are from 

 cut nuclei, but each includes almost all of the chromatin. 



Phormia regina (figs. 73-80). Haploid groups of this spe- 

 cies are shown in figures 73 to 75 (second spermatocytes) and 

 figure 76 (first spermatocyte). As shown in figures 73 and 

 75 in contrast to 74, the sex-chromosomes (smallest in each 

 case), are very unequal. In figure 76 they may be seen sepa- 

 rating from one another in the reduction division. Figure 77 

 is taken from a spermatogonial cell in early anaphase (or late 

 metaphase), and shows the six pairs of chromosomes, correspond- 

 ing to the six single chromosomes of the haploid group; each of 

 these is split lengthwise in the process of division. In the cen- 

 ter may be seen the unequal X-Y pair splitting in the same 

 manner as are the autosomes. A comparison of this figure (77) 

 with that of a similar stage in the reduction division (76) clearly 

 brings out the relation between the two groups (haploid and 

 diploid). Prophases from early ovarian tissue showing the 

 origin and behavior of the pairs in preparation for division in 



