ASSOCIATION OF CHROMOSOMES IN DIPTERA 229 



division (figs. 8, 28, 40, 77, 99, etc.), and by the time the equatorial 

 arrangement is completed they may all exhibit a longitudinal 

 split. It is this stage that demonstrates unquestionably the 

 presence of a diploid instead of a haploid group. 



As shown by the figures, especially numbers 7, 28, 29, 68, 

 it occasionally happens, as mentioned above, that the members 

 of a pair appear in metaphase on opposite sides of the spindle, 

 or separated from one another by other chromosomes. This 

 disarrangement apparently takes place in late prophase while 

 the chromosomes are becoming equatorially oriented. Several 

 cases have been observed in which the members of a pair were 

 partially separated by other chromosomes, and it seemed a 

 question as to whether they would be forced completely apart, 

 or would succeed in taking their places together. The frequent 

 appearance of the condition in which the two members are on 

 opposite sides of the plate appears to be due to their having ap- 

 proached the equator of the spindle vertically instead of hori- 

 zontally, i.e., from one pole instead of from the side — and thus 

 having been pulled diametrically apart, with their points of 

 attachment near together but their extremities pointing in 

 opposite directions. At first sight it would appear that chromo- 

 somes once separated in this manner would have difficulty in 

 associating again, and that after many divisions all the pairs 

 would be disarranged. An examination of chromosome ar- 

 rangement in late anaphase indicates one reason at least why 

 such a confusion does not occur. During this stage the chromo- 

 somes are drawn out in a slender cone with their apices brought 

 close together at the pole. As a result all of the chromosomes 

 are rather closely approximated throughout their lengths, and 

 an ample opportunity is afforded for the reunion of separated 

 members of a pair, even if they previously lay on opposite sides 

 of the spindle. 



In my paper on Drosophila chromosomes (Metz '14, p. 56), 

 I mention the apparent occurrence of a 'second conjugation' 

 of chromosomes in early metaphase, after the separation which 

 normally occurs in prophase. The details of this phenomenon 

 were obscure at the time, and were left for further study. It 



