364 PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 



the number found in the body cells. The union of sperm and 

 egg, each with its half number of chromosomes, restores the full 

 number in the somatic cells and primordial germ cells of the 

 new organism. The parallel of these chromosome relations with 

 the relations of unit characters could not long escape attention; 

 and today the view is very general that the chromosomes are 

 the bearers of such Mendelian hereditary units.* 



Twenty-five years ago biologists stated unhesitatingly that 

 the chromosome number in the somatic cells was always an 

 even number; but, in 1891, Henking published the revolutionary 

 discovery of an odd chromosome number in the male of an 

 insect. Here the spermatozoa are necessarily of two classes, the 

 spermatozoa of one class having one more chromosome than 

 those of the other class. Eleven years later, largely because of 

 the numerical equality of the two classes, McClung suggested 

 the theory that this "accessory chromosome" or "odd chromo- 

 some" or "x-chromosome", as it has been variously named, may 

 be correlated with the determination of sex. This view, although 

 in modified form, has now been almost certainly demonstrated 

 for a variety of animals by the careful investigation of E. B. 

 Wilson and other workers. 



We may confine our attention to this simplest and most 

 typical case of sex determination, and disregard the complications 

 introduced by the presence of a y-chromosome, paired with the 

 x-chromosome and sometimes visibly indistinguishable from it, 

 or by the substitution of a chromosome group for the single x- 

 or y-chromosome. 



In the female the somatic chromosome number is even, in- 

 cluding two x-chromosomes, and the eggs are all alike, each 

 containing an x-chromosome. The fertilization of an x-contain- 

 ing egg by an x-containing spermatozoon produces a female ani- 

 mal; the fertilization of an x-containing egg by an x-lacking 

 spermatozoon produces a male. In other words, the female is 



* The existence of any particulate "bearers" of hereditary "units" is 

 totally denied by certain able investigators, who, nevertheless, recognize 

 the correlation in behavior of chromosomes and hereditary characters. 



