260 Experimental Transformation of Species 



some. In moths and butterflies a similar mechanism seems 

 to operate. 



The XX-XY (or XX-XO) form of sex determination, 

 in which there are two kinds of sperm cells, is found also 

 among many plants. In some animals there is no distinct 

 sex chromosome, but a portion of one of the other chromo- 

 somes (called autosomes) acts like an X. Among bees, 





Corn-position of (yy) 

 Parents v y 



9 



XY 



cf 



(wz) 



zz 



V 



d 



Gametes Produced (^ @ [J] 

 Offspring 





XY 



(f 







o^"'^ 



+ 



ZZ 



d' 



Fig. 72. Two Types of Sex Determination 



Each individual, whether male (large square) or female (large circle), 

 arises from the fusion of two gametes, an egg (small circle) and a sperm 

 (small square). The inheritance of sex cannot, therefore, mean a direct 

 transfer of the sex of one of the parents to the offspring. Among certain 

 plants and animals each egg (small circles, X, X) may develop into either 

 a male or a female, depending upon the kind of sperm cell with which it 

 has combined (small squares X and Y). When two X chromosomes get 

 into a fertilized egg the individual develops into a female; when only one 

 X chromosome enters the combination, a male develops. In such cases sex 

 is " determined by the sperm," whether the offspring is male (XY) or 

 female (XX). Among birds and butterflies there are apparently two kinds 

 of egg cells produced by the female (small circles W and Z), but only one 

 kind of sperm cells by the male (small squares, Z, Z). When two Z chro- 

 mosomes get into a fertilized egg, the resulting individual develops into a 

 male; when only one Z chromosome comes into the combination the result 

 is a female. In such cases the sex is " determined by the egg," whether 

 the offspring is a male (ZZ) or a female (WZ). 



wasps and ants, the males seem to consist of individuals that 

 develop from unfertilized t^^s^ and therefore have one-half 

 the number of chromosomes normally present in the females. 

 The formation of sperm cells in this order of insects is ex- 

 ceptional, since there is no true reduction-division — the 

 sperms contain the same number of chromosomes as the body 

 cells of the male. 



