THE PHYSICAL BASIS OF MENDELISM 361 



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. 60. 

 Here it is possible to recognize in the female two large pairs of curved 

 chromosomes very similar in size and shape. There is also a very smaU 

 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 



FEHllE HILC 



Fig. 60. — Diagram showing the characteristic pairing, size relations, and 

 shapes of the chromosomes of Drosophila melanogaster. In the male an X and a 

 Y chromosome correspond to the X pair of the lemale. On the basis of X ioq 

 the length of each long autosome 159, of each small autosome 12, and of Y 112, of 

 the long arm of F 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 



