The Visible Basis of Heredity t^ 79 



a fellow preparatory to the reduction division 

 at the time the sperm are formed. It goes 

 entire to one or the other of the sperm so that 

 these are of two sorts, one with the odd chromo- 

 some, one without it. It was Henking who 

 made this discovery in 1899 in a locust or 

 grasshopper, Phyrocorrus; it was soon con- 

 firmed by other investigators working on other 

 animals. McClung, the same year, suggested 

 that this odd chromosome is a sex-determiner. 

 If an egg is fertiUzed by a sperm that contains 

 the odd chromosome, the fertilized egg develops 

 into a female, but if fertilized by a sperm of the 

 other t)^e it develops into a male. Thus in the 

 white man there are forty-seven chromosomes 

 in the male, but forty-eight in the body cells of 

 the female. Evidently the chromosome which 

 is an odd one in the male finds a mate in the 

 cells of the female since the chromosome number 

 is even. The female may be said to have a 

 double dose of the sex chromosome, the male a 

 single dose. Representing the haploid number 

 of chromosomes by n the formula for the cells 

 of a female is usually given as 2n-\-2, for the 



