360 STRUCTURE AND LITE 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 x-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 a:-chromosomes, 

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

 X- and j-elements are not single chromosomes but groups 

 of small chromatin bodies. The x-element is always 

 associated with femaleness, 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 3; sperm results in 

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



Egg X -f- sperm no — x = zygote x (male). 



Egg X + 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 1919 Allen^ (for S. Donnellii) and Miss Schacke^ 

 (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 Sphcerocarpos. Proc. 

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



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

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



