200 



The Journal of Heredity 



there appears to be a very small V- 

 or U-shaped chromosome pair, more 

 or less intermediate between the rod- 

 like and the large V-shaped kinds. 

 Only one representative of this type 

 is known (D. mclanica), but we have 

 numerous good figures of both male 

 and female groups, including haploid 

 (male) grt)ups, and are able to dis- 

 tinguish the small U-shaped pair in 

 nearly every case. 



In type K two pairs are found which 

 are intermediate between the V's and 

 the rods in length. One has a sub- 

 terminal spindle fibre attachment giving 

 it an S or a hook shape. The other 

 has a median or sub-median attach- 

 ment and is much like the small V- 

 or U-shaped pair just described in 

 D. mclanica. In some figures both of 

 these pairs are somewhat S-shaped, and 

 were so indicated in previous papers. 

 But we have recently put up new mate- 

 rial and have examined a series of 

 figures which have convinced us that 

 one pair is really difi^erent from the 

 other. Some of these figures are rep- 

 resented in the accompanying camera 

 lucida drawings (Figure 3, Nos. ;5-10) 

 taken from spermatogonia and second 

 spermatocytes. The letters indicate re- 

 spectively the X and Y (sex) chromo- 

 somes, the small rods (r), the small 

 U- or V- shaped chromosomes (U) 

 and the J-shaped chromosomes (J). 

 Numbers 3, 4 and 5 are diploid groups* 

 from spermatogonia. Numbers (5, 7 

 and H are the X-containing haploid 

 groups of the second spermatocytes, 

 and 9 and 10 are the corresponding 

 Y-containing groups. In Numl^er 9, 

 there appear to be two m-chromosomes 

 l)resent, but one of these may be a 

 granule accidently carried into the mid- 

 dle of the field. 



Cases of what appear to be simply 

 modifications of the V's, rods or M's 

 are seen in types B, G, and L. In 



* In these, X and Y appear to be correspondingly segmented, as indicated by the clear 

 band across each. This segmentation is distinct from the constriction at the point of spindle 

 fibre attachment and its significance is not known. It seldom, if ever, appears in the second 

 spermatocyte figures, and is presumably evident in the spermatogonia only in smear prepara- 

 tions. 



t Perhaps this should be classed with the U-chromosomes of Types E and K, but it 

 seems to be nearer the large V's in size. 



Tyi^e B one pair of V's is unusually 

 long ; in Type G one pair of rods is 

 unusually long and the other four are 

 unusually short, while the M's are ex- 

 ceptionally small (compare with Type 

 F) ; and in Type L one pair of V's 

 appears to be smaller than usual.! 

 Otherwise the types are distinguished 

 merely by possessing different combi- 

 nations of the three main sorts of 

 chromosomes, except where distinctions 

 are based on sex-chromosome dififer- 

 ences, as discussed above. 



Possible Relationships Between Differ- 

 ent Types of Chromosome Groups 



One of the most striking facts about 

 the series of types is that many of 

 them show such resemblances to one 

 another that in some cases graded 

 series may be arranged between widely 

 divergent types. This suggests that 

 the relationships are not purelv for- 

 tuitous. It remains for genetical stu- 

 dies to establish these relationships, if 

 they actually exist, but some of the 

 possibilities suggested by the cytolog- 

 ical findings may be outlined here. 



These are represented schematically 

 in Figure 4. In presenting this series 

 of diagrams we wish to emphasize 

 the fact that the arrangement is based 

 purely on the superficial appearance of 

 types and is not intended to indicate 

 relationship between the particular 

 species involved. Furthermore. it 

 represents only one of various schemes 

 that could be employed. It is given 

 rather to present the problem than to 

 indicate its solution. 



In the diagrams the lowermost pair 

 of chromosomes in each group is in- 

 tended to represent the sex-chromo- 

 somes. Those given in solid black are 

 known to be, or are believed on good 

 evidence to be, the sex-chromosomes. 

 The heavily banded ones are merely 

 assumed to be the sex-chromosomes in 



