68 THE REPOET OF THE Xo. 36 



Lutz found that when a normal Crioceris asparagi with three yellow spots on 

 blue-black elytra was crossed with a variety where; the upper spot was united with 

 the middle one, the normal form was dominant. Intermediate conditions occur, 

 but these may be heterozygous forms. 



I 1n-Beeedixg. 



Darwin's memorable work on cross-fertilization with morning-glories, pansies, 

 cabbage, lettuce, buckwheat and beets showed conclusively the value of cross- 

 fertilization over self-fertilization in the maintenance of the vitality of these plants. 

 But it has also been shown that many self -fertilizing plants, .like tobacco, wheat 

 and barley, are vigorous and have retained their vigor for thousands of years. Dr. 

 East says very properly that Darwin's phrase " Xature abhors perpetual self- 

 fertilization '' should read : " Nature resists any sudden change in long established 

 conditions." 



Eegarding the effects of in-breeding among animals differences of opinion 

 exist, but Mendelism has cleared away some of the difficulties. It shows that in- 

 breeding in itself is not necessarily injurious, but great care must be exercised to 

 prevent injury. The duplex dose of determiners develops prepotency in all char- 

 acters, good and bad alike. 



Castle in-bred Drosophila for about sixty generations without any appreciable 

 loss of vitality. 



The Sex Cpieomosome. 



On account of the fact that approximately the same number of . males and 

 females of the human species are born, students of heredity have suspected that 

 sex is a character dependent upon factors which act in a Mendelian manner. The 

 explanation of this equality of sexes was forthcoming by comparatively recent re- 

 searches in which insects played an important part. Henking, in 1890, dis- 

 covered two kinds of sperm cells in the firefly, Pyrrliocoris. McClung, in 1902, 

 found two kinds of chromosomes in the sperm cells of the grasshopper. Stevens, 

 in 1905-1908, found accessory chromosomes in certain species of aphis and Diptera. 

 Wilson, in 1905-1907, discovered diffeTent kinds of chromosomes in loertain 

 Hemiptera; and Morgan, in 1908, described two kinds of sperm cells in Phylloxera. 

 Later researches by Doncaster, Eaynor, Morgan and others have extended the list. 



This accessory chromosome has been called the sex or X chromosome inas- 

 much as its presence in the germ cell is believed to determine the sex of the off- 

 spring. In most animals, including man, all the female germ cells contain an 

 X chromosome, while it is present in only half of the male germ cells. On the 

 other hand, in ( hickens, ducks, canaries, and in the English currant moth 

 (Abraxas) all the male germ cells, but only half the female germ cells, contain 

 the X chromosome. 



In the first case investigations have revealed the fact that when fusion occurs 

 between an Qgg and a sperm, both containing an X chromosome, the result is a 

 female, but when an Qg^ and a sperm without an X chromosome fuse the fertilized 

 egg has only one X chromosome and the result is a male. In fertilization, there- 

 fore,- the chances are that approximately an equal number of males and females 

 are produced, as shown by diagram: 



