Inheriting Sex 259 



a female; if fertilized by one of the latter a male individual 

 develops. 



This type of sex determination, in which there are two 

 kinds of sperm cells (X and Y, or X and O) and one kind of 

 Qgg cell (X) , has been reported for most of the insects, out- 

 side of moths and butterflies, for the sea urchins, for many 

 round worms, in the horse, and in the opossum. The indica- 



^mm\ 



JliUHii;il»MM«('CCiliH 



Fig. 71. Chromosomes in Man 



The forty-eight chromosomes are shown distinct but crowded together in the cell that 

 is to give rise to sperm-cells, at A. The smallest chromosome is shown at Y. The twenty- 

 four pairs are spread out, B, showing the two sex chromosomes, X and Y, diflferent in 

 size. After Painter, from Conklin, Heredity and Environment in the Development of 

 Man, published by Princeton University Press. 



tions point to the guinea pig and most of the bony fishes as 

 having also this type of sex-determining mechanism. 



Among birds the relation. of chromosomes to sex de- 

 termination is somewhat different. In these animals the 

 study of the chromosomes in the females shows one pair to 

 consist of a large and a small unit, whereas the cells of the 

 males have two equal chromosomes for the corresponding 

 pair (Fig. 72). The hen therefore produces two kinds of 

 eggs, one containing the large chromosome of this particular 

 pair, called the Z, and the other containing the small mem- 

 ber, called W. The male, on the other hand, produces only 

 one kind of germ cells, each sperm containing one Z chromo- 



