SEX DETERMINATION 259 



If one were inclined to be facetious, he might say that in all 

 these lower animals, duplicity is synonomous with f emaleness, 

 simplicity with maleness! 



It should be noted in passing that among plants as well as 

 among animals, an unfertilized gamete may undergo multi- 

 plication and growth while in the simplex, reduced condition. 

 The ordinary fern plant is a zygote with a duplex chromo- 

 some number. But it produces reproductive cells (spores) 

 containing the reduced (simplex) chromosome number, and 

 these after growing into a small inconspicuous little plant, 

 known as a prothallus, produce the functional gametes (egg 

 and sperm-cells) without further reduction. Union of these, 

 egg with sperm, produces duplex zygotes again, which 

 develop into the ordinary fern plant. 



In many animals in which males and females alike arise 

 from fertilized eggs, there occurs nevertheless a difference in 

 chromosome number between males and females, the female 

 always containing the higher number, as in the partheno- 

 genetic plant lice. One of the best-known cases is that of 

 the common squash bug, Anasa tristis, first worked out by 

 E. B. Wilson, but since fully confirmed by the observations 

 of others. In this animal the body-cells of the female contain 

 twenty-two chromosomes, those of the male twenty-one. 

 Historically this is a famous case, the first one in which the 

 mechanism of sex determination was definitely ascertained. 

 The egg, according to Wilson, always undergoes reduction to 

 the simplex chromosome number, eleven. But reduction in 

 the male is less sunple because the male contains an odd 

 number of chromosomes, viz., twenty-one. All the sperm 

 cells cannot receive the same number of chromosomes at the 

 reduction division, unless the odd chromosome splits, but this 

 it refuses to do. The division occurs into cells with eleven 

 chromosomes, and those with ten. Both metamorphose into 

 spenn cells. The 10-chromosome spenn cells, if they ferti- 

 lize an egg, cause It to develop into a male, since Egg 11 -f- 

 Spemi 10 = 21, the number characteristic of the male. But 

 the 11-chromosome spenn fertilizing an egg causes it to 



