ASSOCIATION OF CHROMOSOMES IN DIPTERA 223 



features. Likewise in figure 3 the side by side arrangement is 

 obvious. Figures 4 and 5, 17, 19 and 20, from species possess- 

 ing another type of chromosome group, bring out the same 

 relations. In each case the two members of a pair he side by 

 side, not one above the other — with the exception of one mis- 

 placed chromosome in figure 19. Similarly in figures 21-24, 

 representing another type of group, the side by side pairing is 

 very distinct. Other examples are given in figures 27, 28, 37, 

 39-46, etc. These figures are not selected from among many 

 in which pairing is less evident, but are perfectly typical and 

 represent the normal condition in their respective species. 



The manner in which division takes place during late meta- 

 phase or early anaphase is shown by figures 7, 8, 9, 16, 28, 32, 

 40, etc. Figures 7, 8, and 9 represent the same type of chromo- 

 some group as do figures 4, 5, 17, 19, 20, namely, a group com- 

 posed of two long U-shaped pairs, one straight pair, and one 

 small spherical pair. In all of these figures each chromosome 

 (save the smallest in 8 and 9) may be seen dividing equationally, 

 in the ordinary manner. In figure 16 the mode of division in 

 a similar group is seen at a somewhat later stage. The dark 

 chromosomes are seen at a high focus, the hght ones at a lower 

 focus. It is evident that each member of the diploid group has 

 divided and sent a daughter half toward either pole. The 

 smallest pair cannot be seen in this figure. In figures 28, 32 

 and 40 the same process is indicated in the case of two other 

 species. Earher stages in the same species are represented in 

 figures 27 and 39 respectively. The features indicated by fig- 

 ure 28 are brought out even more clearly by figure 32 (a side 

 view at the same stage). In figure 32 each of the short chromo- 

 somes has divided, while the two long ones have split in prepara- 

 tion for division. 



Passing now to the later anaphases it may be seen that during 

 this period a diploid, not a haploid, group goes to each pole, 

 and in many cases the two members of a pair of chromosomes 

 are so clearly separated from one another that they cannot be 

 considered the result of a precocious split as suggested by Tay- 

 lor and by Lomen. This fact is demonstrated conclusively 



