2oo Basis of Sex Determination 



with and one without an accessory or as it is now called 

 an X chromosome, all the cells of the female have one 

 chromosome more than the cells of the male. From 

 this he concludes correctly that in such species a female 

 is produced when the egg is fertilized by a spermato- 

 zoon containing an X chromosome, while a male is 

 produced when a spermatozoon without an X chromo- 

 some enters the egg. 



Such a form is Protenor,one of the Hemiptera. Wilson 

 made sure that all the eggs are alike in the number of 

 chromosomes, each egg containing an X chromosome in 

 addition to the six chromosomes characteristic of the 

 species Protenor. There are two types of spermatozoa 

 in equal numbers in this species, each with six chromo- 

 somes, but one with, the other without, an X chromo- 

 some. The two possible chromosome combinations 

 between egg and spermatozoa are therefore as follows 

 (see the diagrammatic Fig. 39) : 



Egg Spermatozoon Result 



(i) 6 + X +6 = 12 + X = Male 



(2)6-j-X +6 + X = 12 + 2 X = Female 



The egg which receives a spermatozoon without an 

 X chromosome has after fertilization 12 + X chromo- 

 somes and develops into a male; while the egg into 

 which a spermatozoon with an X chromosome enters 

 gives rise to a female. Since all the body cells arise 

 from the fertilized egg by nuclear division and the 



