THE FACTORS WHICH DETERMINE SEX 673 



Payne 1 has recently shown that in Galgulus oculatus there are 

 two sorts of spermatozoa possessing respectively sixteen and nineteen 

 chromosomes, whereas the eggs are uniform in containing nineteen 

 chromosomes. Eurthermore, the females are believed to have three 

 more chromosomes than the males (i.e. thirty-eight as compared with 

 thirty-five). 



In another insect, Zygwus hicrucis, the male differs from the, 

 female, not in having fewer chromosomes, but in the possession of a 

 pair of different sized chromosomes. The body-cells in the male 

 have twelve ordinary chromosomes and two sex chromosomes which 

 behave differently from the others. One of the sex chromosomes is 

 larger than the other, and has been called the X chromosome, the 

 second or smaller one being termed the ^ chromosome. 



After synapsis, or the pairing of the chromosomes preparatory 

 to spermatogenesis, there are six double chromosomes, and the two 

 sex chromosomes, X and Y, which remain separated. At the first 

 maturation division all the chromosomes divide, the two resulting 

 cells each having eight chromosomes, incldding the X and Y. At the 

 second division, however, although all the other chromosomes divide, 

 the X and Y chromosomes do not divide, but each passes into a 

 separate product of division, that is, into a separate spermatozoon. 

 Each spermatozoon, therefore, contains^six ordinary chromosomes and 

 either an X or a Y chromosome. In the female body-cells there are 

 twelve ordinary chromosomes and two sex chromosomes which are 

 similar, X and X. Preparatory to oogenesis there are seven double 

 chromosomes, the X's uniting together just as the other chromosomes 

 unite in pairs. The polar bodies are formed, and reduction occurs in 

 the usual way, each egg containing six ordinary chromosomes and 

 one X chromosome. The ova, therefore, are all similar, but those 

 fertilised by spermatozoa with X chromosomes become females, and 

 those fertilised by Y-bearing spermatozoa become males.^ 



The mode of sex inheritance displayed- by Lygceus is believed to 

 be characteristic of the very large class of organisms in which the 

 males are heterozygous as regards sex, and the females homozygous.^ 



tion with smear preparations instead of sections, find no evidence of a persisting 

 accessory chromosome in xinasa tristis, and believe that the body described as 

 such by Wilson is a plasmosome and not a chromosome. 



1 Payne, " On the Sexual Differences in the Chromosome Groups in Galgxilus 

 omdatus," Biol. Bull, vol. xiv., 1908. Con-ens also has shown that in some plants 

 there are two classes of male germ-cells, and that these are produced in equal 

 numbers {Die Bestim/mimg und Vererbung des Oeschlechtes nach neuen Versuclien 

 mit Iioheren Pflanzen, Berlin, 1907). 



2 Morgan, Heredity and Sex, New York, 1913. 



•' The term heterozygote has been given by Bateson to offspring lesulting 

 from the union of dissimilar gametes. Such organisms, according to the 

 Mendelian theory, produce more than one sort of gamete (see p. 20*). 

 Hom'ozygotes are formed by the union of similar gametes, and produse gametes 



