Metz and IMcises : Chrc^mosomes 



201 



^ 





^ 



1 



JooC 





j)ooi. ^ 





10 





11 



goo- 



12 



13 



Sol 



15 



» 

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16 



POSSIBLE RELATIONSHIP OF THE VARIOUS CHROMOSOME GROUPS 



Figure 4. This chart is intended only to show how the various groups may be arranged 

 into a graded series, with the assumption of a few comparatively simple changes. These are 

 fusion of two of the rod-like chromosomes to form a V, or vice versa, the loss of the m- 

 pair, and changes in the size of other pairs. In order to determine whether this or any other 

 arrangement represents the true condition, it would be necessary to study these relationships 

 genetically. Note the similarity between the most common types shown here at 6 and 9. If 

 the V's of number 9 were broken into rods the two types would be alike. Solid black 

 chromosomes are known to be, or are believed on good evidence to be, the sex-chromosomes ; 

 the heavily barred ones are considered to be the sex-chromosomes in making up the series, 

 but cytological evidence with regard to this is lacking. The stippled chromosomes indicate 

 the types in which genetical studies are now under way that are expected to throw light on 

 the homology of the chromosomes in the various groups. 



placing the types as shown. Some 

 types may be put in different places 

 in the series according to whether one 

 or another pair is assumed to be the 

 sex-chromosome pair. This is only 

 done in one case here, however (dia- 

 grams 4 and 15). Diagram 1 in this 

 figure represents the most aberrant type 

 thus far observed and is included 

 merely for the sake of completeness. 

 It has been found in only one species. 

 The other types show more resem- 

 blances to one another and may be 

 arranged in such a wav as to make suc- 



cessive differences relatively slight. In 

 the arrangement given here, for ex- 

 ample, all of the differences might be 

 accounted for on the assinnption of a 

 few comparatively simple changes. 

 These would involve mainly the fusion 

 of rod-like chromosomes in two's, end 

 to end, to form V's or, vice versa, the 

 breaking in two of V's to form rods, 

 together with alterations in the size of 

 individual pairs (particularly the m- 

 pair) and in the form of certain pairs. 

 The series included in diagrams 3, 6, 

 5, 8, 9, 10, 11 and 12 (duplicate of 3) 



