364 PERMANENT HYBRIDS 



prophase behaviour. The attached X chromosome condenses in 

 advance of the autosomes, and is therefore distinguishable from 

 them until metaphase. Some species are uniform in having the 

 attachment of the X {e.g., in Leptynia and Mermiria) others have 

 races with the free X and races with the attached X {e.g., in 

 M enexenus 3.nd Hespeyotettix, cf. Fig, 46). 



Such a fusion has probably taken place in Ascaris megalocephala 

 (Edwards, 1910 ; Geinitz, 1915) for male individuals occur with a 

 single X chromosome separate instead of included in the ordinary 

 compound chromosome. In the male Macropus (Agar, 1923) the 

 special behaviour of the two elements which condense precociously 

 at meiosis shows them to be X and Y chromosomes. But the X 

 chromosome is permanently fused with an autosome in both sexes, 

 A configuration consisting of three unequal chromosomes is formed 

 at meiosis and segregates like that in Tenodera and Rumex. 



The apparent fusion is probably as elsewhere an interchange, 

 in this case an interchange of the active part of an autosome for 

 the inert part of the X chromosome ; one product of interchange 

 being entirely inert can then be lost. When it is not inert it is not 

 lost and a chain of four, as in Humulus Lupulus, will arise, instead 

 of a chain of three. 



That changes of this type have been effective in Drosophila is 

 indicated by a comparison of linkage maps and chromosomes in 

 D. melanogaster and D. Willistoni (Lancefield and Metz, 1922). 

 The sex chromosomes of the latter species are V-shaped, while one 

 pair of autosomes are rod-shaped. The reverse is the case in 

 D. melanogaster (Fig. 121). One arm of the X chromosome of D. 

 Willistoni corresponds with the X chromosome of D. melanogaster 

 in the linked factors located in it {cf. KoUer, 1932 bon D. " obscura," 

 and Dobzhansky, 1934), 



8. Hagerup's observation (1927) of two XY pairs in the tetraploid 

 Empetrum hermaphroditum (corresponding with those in the diploid 

 relative) is improbable on genetical grounds, while XX and YY 

 pairs as found in trisomic Rumex {v. infra), are more probable on 

 cytological grounds on account of differential affinity. Differentia- 

 tion of sex chromosomes has not therefore been proved in any 

 polyploid species but Fragaria elatior. Where it arises it will not 



