200 WM. REES B. ROBERTSON 



(figs. 67-77; Tables IX, X) in having autosomes with a great 

 range of sizes between the smallest and largest, an exceedingly 

 large gap between 4's and 5's, a large difference between 5's 

 and 6's, and a sex chromosome (Ix) in length near the smallest 

 autosome. 



The differences from granulatus are greater than from incur- 

 vatus. The two largest pairs in obscurus differ more in length 

 than in any other species of the genus. This is most evident 

 in first spermatocytes, the lengths being respectively 20.67 and 

 29.60. This difference cannot be far from correct, since in 

 spermatogonia the lengths are 22.1 to 29.4. The ratio of these 

 chromosomes in the first spermatocytes of incurvatus is 22.5 

 to 27.15, and in granulatus, 21.84 to 25.15. The last number 

 (25.15), however, is not normal, as comparison with the sper- 

 matogonial chromosomes (22.48 and 28.14) will show. Compar- 

 ing measurements of the spermatocytes in incurvatus with those 

 of the spermatogonia in granulatus and obscurus, it appears that 

 incurvatus is more nearly like granulatus than is obscurus. In 

 the last species the 6's are relatively the longest (29.4) and the 

 gap between the 5's and 6's the largest yet found (figs. 69-77). 



The intermediate pairs (3's and 4's) are almost indistinguish- 

 able from each other, especially in spermatogonia, though meas- 

 urement shows them to be slightly different. In this respect 

 they are similar to those of the other species of the genus. How- 

 ever, the difference in size between the no. 3's and the no. 2's 

 is greater in obscurus than in granulatus or incurvatus (Tables I, 

 VII, IX). 



In the spermatogonia of obscurus the 2's are slightly larger 

 than the I's and the sex chromosome. But in the spermatocyte 

 the sex chromosome appears larger than either I's or 2's. This 

 is due to the fact that in the spermatocyte the diameter of the 

 sex chromosome is less than that of the other chromosomes 

 and its length therefore greater than it should be. The more 

 correct measurement is probably that of the spermatogonia, 

 where the sex chromosome is the smallest of the series (figs. 

 67-70). For this reason it has been numbered Ix. It is 'woolly' 

 in appearance (figs. 67-68). Here, again, is clear evidence of 

 the 'differential chromosome' in spermatogonia. 



