358 PERMANENT HYBRIDS 



history X or Y in the Bryophyta. The significance of this Hmitation 

 we shall see later. 



The sex heterozygote in the higher organisms is usually the 

 male. Only in the Lepidoptera, Trichoptera, the birds and some 

 fishes, amongst animals, and in Fragaria among plants is the female 

 known to be the heterozygote. 



The types of differences between X and Y chromosomes, as seen 

 at mitosis, may be classified as follows : — 



1. Organisms in which the X and Y chromosomes are not 

 structurally distinguishable (e.g., Oncopeltus, Wilson, 1912 ; 

 Drosophila Willistoni, Metz, 1926) or scarcely so {e.g., Nezara, 



a b e d e f g h i 



Fig. no. — The X chromosome (below) and the Y (above) paired end- 

 to-end at meiosis (first or second metaphase) in the heterozygous 

 sex in various insects (after Wilson and Stevens) showing the 

 transition from equality in size to the disappearance of the Y. 

 a, Oncopeltus fasciatus. b, Nezara hilaris. c, LygcBtis bicrucis. 

 d, Euschistus fisilis. e, Thyanta custator. /, Lygcsus turcicus. 

 g, Nezara viridula. h, Trirhabda. i, Proienor belfragei. (From 

 Witschi, 1929.) 



Wilson, 1905, 1911). In Drosophila funebris two similar chromo- 

 somes are diagnosed as sex chromosomes by Heitz (1933) merely 

 by one chromosome having a larger heterochromatic segment than 

 its partner in the male. 



2. Organisms in which the X and Y chromosomes differ more or 

 less considerably. Every gradation is found between the slight 

 difference in Drosophila melanogaster (Fig. 121) and the considerable 

 difference in Sphcerocarpus Donellii (Lorbeer, 1927, 1930), and 

 in man (Evans and Swezy, 1929). The X chromosome may be 

 the largest member of the complement (as in Sphcerocarpus 

 Donellii ; Leptophyes, Mohr, 191 5 ; Phragmatohia fuliginosa, Gryllus 

 campestris, Ohmachi, 1929 a, and Humulus japonicus) or the smallest 

 (as in Asilus notatus, Metz and Nonidez, 1923, and Macropus, Agar, 



