378 INTRODUCTION TO CYTOLOGY 



A. lumbricoides has in the male an X-complex made up of five elements 

 (Fig. 212). All five pass to one pole in /, and two classes of spermatozoa 

 are formed, with 19 and 24 chromosomes respectively. This is some- 

 times referred to as the " XnO type."^ 



The "XF type" is well exemplified by Drosophila. In D. melano- 

 gaster the somatic complement comprises four chromosome pairs, includ- 

 ing a pair of sex-chromosomes (Figs. 196, 212). In the female the two 

 members of this pair are alike {XX), while in the male they differ slightly 

 in size and greatly in function (XF). Owing to disjunction in meiosis, 

 half of the sperms carry X and half carry Y, while every egg has X. 

 Fertilization of an egg by an X-sperm results in a female with XX, while 

 that by a F-sperm results in a male with XYJ In some genera the X 

 and F divide equationally in I and disjoin in II. It will be shown later 

 in the chapter that sex is correlated with the ratio of X-chromosomes to 

 autosome pairs, rather than with the simple presence of X' or F or the 

 number of X's. 



The XF type is the most prevalent form of sex-chromosome differen- 

 tiation in animals. Like the XO type, it is best known in insects and 

 mammals,^ including man. In a number of cases the genetic evidence 

 indicates the presence of an XF pair,^ although it has not been possible 

 to identify it with the microscope. Apparently the sex-chromosomes 

 are often not visibly differentiated from the other members of the comple- 

 ment. A variant of the XF type is seen in Prionidus, in which the X 

 is represented by three small elements (Fig. 212). In the female there 

 are six of these X-elements, while in the male there are three X-elements 

 and a F. In spermatogenesis the X-elements disjoin from the Fin 77. i" 



^ Works on sex-chromosomes in Ascaris: Boveri (19096), Boring (1909), Edwards 

 (1910, 1911), Frolowa (1912), Walton (1918, 1921, 1924). Walton concludes from 

 his extensive researches on nematodes that each autosome in A. megalocephala is equiv- 

 alent to 22 somatic chromosomes, and that the X is equivalent to 8. Sex-chromo- 

 somes attached to autosomes are also reported for Choaborus plumicornis (Frolowa, 

 19296). 



'Stevens (1907, 1908a), Morgan (1911), Metz (1914). In Drosophila Willistoni 

 it has been shown by Lancefield and Metz (1921, 1922) that the sex-chromosomes are 

 represented by one of the large bent pairs rather than the shorter rod-shaped pair; and 

 there is evidence in the linkage of genetic factors that the rod-shaped chromosome of 

 melanogaster corresponds to one arm of the large bent sex-chromosome in Willistoni 

 and obscura. 



8 Works on mammals: Painter (1921a, 19226) on opposum, (1922a, 19236) on mon- 

 keys, (1926) on rabbit; Agar (1923), Greenwood (1923) and Saez (1930c) on mar- 

 supials; Cox (1926) and Minouchi (1928e) on the mouse; Minouchi (1928c) and 

 Pincus (1928) on the rat; Starks (1928) on the seal; Minouchi (1928a) on the cat. 

 Winiwarter and Sainmont (1909) reported XO in the cat. 



'E.g., in the fish genera Lebistes (J. Schmidt, 1920; Winge, 1922, 1923a, 19276, 

 19306; Vaupel, 1929; Iriki, 1932) and Aplocheilus (Aida, 1921, 1930; Iriki, 1932). 



^° Payne (1909). Several other cases of this XnY type of differentiation are known. 

 In the Mantidse the X-complex consists of two elements, the male having 24 -|- Xa YXb 

 and the female 24 + XaXaXbXb (King, 1931; Oguma, 1921). 



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