ASSOCIATION OF CHROMOSOMES IN DIPTERA 227 



staining capacity, so that very little can be determined about 

 the behavior of individual chromosomes. It is significant, 

 however, that these chromosomes normally enter the telophase 

 in a closely paired condition (figs. 31, 95, 169, 171) and it seems 

 highly probable that they retain this relationship during the 

 transformations in the resting nucleus. Such a conclusion is 

 rendered almost certain by their subsequent behavior in com- 

 ing out of the resting stage. The earliest prophase or spireme 

 stages in which the chromatic threads may be distinguished 

 with any degree of clearness show these threads to be intimately 

 associated in pairs (figs. 11, 14, 34, 58, 65, 70, 71, 78, 80, 91, 92, 

 100, 123, 130, 131, 155, 165); and from this time on they may 

 be seen to retain this association during their condensation and 

 contraction from early prophase up to the time at which definite 

 chromosomes are formed ready to go on the spindle. Some 

 of the earliest prophases in which the chromatic threads were 

 well defined are shown in figures 58 to 63 (Calliphora) . Each 

 of the double threads in these figures represents a pair of chromo- 

 somes. In figure 62 all six pairs are shown (the smallest being 

 very faint), but in the others only parts of the nucleus are repre- 

 sented. Fig-ure 65 is a later stage showing the chromosomes 

 more condensed and contracted, but still closely apposed in 

 pairs. Figure 66 is a still later stage, in which the chromosomes 

 are assuming their definite shape preparatory to disjoining and 

 going on the spindle. It is followed by the late prophase and 

 metaphase stages represented in figures 53, 54, 55, 56 and 57, 

 These are succeeded in turn by the late metaphase and anaphase 

 in which each of the twelve chromosomes divides equationally 

 as described above. Other early prophases are shown in figures 

 100 to 102 (Homalomya). The chromosome group here is 

 indistinguishable from that of Calliphora (five large and one 

 small pairs). In figure 100 the long, delicate but double threads 

 are clearly distinguishable. It is impossible to determine 

 precisely how many double threads are present, for some 

 are broken, but the number is clearly about five or six, cer- 

 tainly not ten or twelve. Part of a similar nucleus is shown 

 in figure 101. One of the most interesting features about 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 21, NO. 2 



