360 STRUCTURE AND LIFE HISTORIES 



sperms one-half possess it (x) and one half do not (no x) . 

 In other cases (first recorded for hemipterous insects by 

 Wilson) the ^-chromosome in the cells of the male is 

 accompanied by a companion chromosome of a different 

 type, called by Wilson the y-chromosome. In the reduction 

 division one half the sperms receive the ^-chromosomes, 

 the other half the ^-chromosomes. In some cases the 

 x- and ^-elements are not single chromosomes but groups 

 of small chromatin bodies. The ^-element is always 

 associated with f emaleness, the no x (or y) with maleness. 

 If an egg (x) is fertilized by a sperm possessing the x- 

 chromosome a female zygote is determined (formula xx) ; 

 the union of an egg with a no x or y sperm results in 

 a male zygote (formula xo or xy) , thus ; 



Egg x + sperm no x = zygote x (male) . 



Egg x -f- sperm y = zygote xy (male) . 



Egg x + sperm x = zygote xx (female) . 

 This condition is illustrated diagrammatically in Fig. 265. 

 In sea-urchins and some other animals the condition may 

 be reversed, the sperms being all alike and the eggs unlike. 

 Careful investigations have, so far, failed to reveal any- 

 thing corresponding to the x- chromosome in plants, 

 except in two species of the dioecious liverwort, Sphcero- 

 carpos. In 1 9 1 9 Allen 1 (for S. Donnellii) and Miss Schacke 2 

 (for S. texanus) demonstrated the presence in the cell- 

 nuclei , of the female plants (gametophytes) of one x- 

 chromosome, clearly distinguished from the seven other 

 chromosomes by its much greater size. Likewise they 



1 Allen, C. E. The basis of sex inheritance in Spharocarpos. Proc. 

 Amer. Phil. Soc., 58: 289-316. 1919. 



2 Schacke, Martha A. A chromosome difference between the sexes of 

 Sphcerocarpos texanus. Science, N. S., 49: 218-219. Feb. 28, 1919. 



