48 



CHARLES W. METZ 



Diagrams 1 to 5 Diagrams 1 a, 2 a, 3 a, 4 a, 5 a represent metaphase plates of 

 Types I, II, III, IV and V respectively. Diagrams 1 b, 2 b, 3 b, 4 b, 5 b represent 

 anaphases of the same. In these anaphases, for the sake of simplicity, only one 

 member of each chromosome pair is figured; m., michrochromosomes; h., sex- or 

 heterochromosomes ; I., large autosomes; s., small autosomes. 



Diagrams 6 to 11 The same types of chromosome groups as above, but arranged 

 in a dichotomous series according to their relationship to one another, and their 

 divergence from Type I. 6, Type I; 7, Type IV; 8, Type V; 9 and 11, Type III; 

 10, Type II. As the diagrams indicate, Type III may be derived from either Type 

 V or Type II. 



1. The 711 or inichrochromosomes.^ One pair; small, spherical; 

 usually occupying the center of the plate in metaphase; going to 

 the poles as short, straight rods in anaphase (m, diagrams 1 a and 

 lb). 



2. The sex-chromosomes or heterochromosomes. One pair; 

 unequal in the male,^ equal in the female; elongate, rod-like, 



' A name first applied by Wilson to a similar small pair seen in many coreid 

 Hemiptera; (E. B. Wilson, Studies on chromosonjes. II. 1905. Jour. Exp. Zool. 

 vol. 2, pp. 508-545). Whether the pair in Drosophila is strictly comparable with 

 these in the Hemiptera is not certain, and I have used the term here merely for 

 convenience. It remains to be seen how the pair behaves in the maturation stages. 



■* This inequality in the male was first pointed out by Miss Stevens ('08) in the 

 case of D. ampelophila. In so far as the diploid chromosome groups of this species 



