402 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
however, are qualitatively very different. Similarly within the species 
the chromosomes are not all alike; on the contrary, especially in 
certain forms, they exhibit very marked differences in size and shape. 
This is peculiarly well illustrated in Drosophila as shown in Fig. 70. 
Here it is possible to recognize in the female two large pairs of curved 
chromosomes very similar in sizeand shape. There isalsoa very small 
pair of chromosomes, and finally there is a pair of straight ones about 
two-thirds as long as the large curved chromosomes. In the male the 
same relations hold except that instead of the pair of straight chromo- 
somes there is a pair consisting of one straight and one somewhat 
larger hooked chromosome. The significance of this difference in 
chromosome content in the sexes will be pointed out in a consideration 
FEMALE MALE 
Jt 4 
Fic. 70.—Diagram showing the characteristic pairing, size relations, and 
shapes of the chromosomes of Drosophila ampelophila. In the male an X and a 
Y chromosome correspond to the X pair of the female. On the basis of X 100 
the length of each long autosome 159, of each small autosome 12, and of Y 112, of 
the long arm of Y 71, and of the short arm of Y 41. (From Babcock and Clausen, 
after Bridges.) 
of the inheritance of sex. The pair of straight chromosomes we call 
the sex or X-chromosomes, the unequal mate of the X-chromosome in 
the male of this species is called the Y-chromosome. The other 
chromosomes are called autosomes when it is desired to distinguish 
them as a class from the sex chromosomes. Drosophila is not unique 
in possessing chromosomes of such characteristic shapes and sizes; but 
more and more as cytology advances it is becoming possible to dis- 
tinguish chromosomes, and to recognize them at every cell division. 
Moreover, the characteristic paired relations which exist among 
the chromosomes of Drosophila are of general significance. When 
mature germ cells are formed in an individual, reduction divisions 
occur by means of which the chromosome number is reduced in the 
germ cells to one-half that characteristic of the body cells. Thus the 
