60 MENDELISM AND 



Pyrrhocoris and Protenor (Hemiptera) Brachystola 

 and many other Acrididae, Anasa, Euthoetha^ 

 Narnia, Anax. In a second class of cases the sex- 

 chromosome is double, consisting of two components 

 which pass together to one pole. Examples of this 

 are Syromaster, Phylloxera, Agalena, In a third 

 class the sex-chromosome is accompanied by a fellow 

 which is usually smaller, and the two separate at the 

 differential division. The sizes of the two differ in 

 different degrees, from cases as in many Coleoptera 

 and Diptera in which the smaller chromosome is very 

 minute, to those (Benacus, Mineus) in which it is 

 almost as large as its fellow, and others {Nezara, 

 Oncopeltus) in which the two are equal in size. 

 Again, there are cases in which one sex-chromosome, 

 say X, is double, triple, or even quadruple, while the 

 other, say Y, is single. In all these cases there are 

 two X chromosomes in the oocytes (and somatic 

 cells) of the female, and after reduction the female 

 gametes or unfertilised ova are all alike, having a 

 single X chromosome or group. On fertilisation 

 half the zygotes have XX and half XY, whether Y 

 is absence of a sex-chromosome, or one of the other 

 Y forms above mentioned. The sex is thus deter- 

 mined by the male gamete, the X chromosome 

 united with that of the female gamete producing 

 female individuals, while the Y united with X pro- 

 duces male individuals. 



Professor T. H. Morgan has made numerous 

 observations and experiments on a single culture 

 of the fruit-fly, Drosophila ampelophila, bred in 

 bottles in the laboratory for five or six years. He 

 has not only studied the chromosomes in the gametes 

 of this flv, and made Mendelian crosses with it, 



