No. 524] CHROMOSOMES AM) II KUKDIIY 



489 



sible ways of explaining the same facts, and the ease 

 might be turned to ridicule on that score, but the expla- 

 nations are only variations of the same hypothesis. 

 Whatever the final decision mav be it is interotin- and 

 important to find that the inheritance of sex ran be 

 treated by the same methods used for other alternate 

 characters and gives consistent results. Let us be as 

 sceptical as we will, yet the facts will impress them- 



Such are the experimental results. Looked at not too 

 critically they show that by the time that reduction of the 

 chromosomes occurs, or after that event, there seems to 

 exist a distinction between the cells, so that half of the 

 cells are destined to become males and half females. 

 But, as has been said, unless we assume this process to 

 take place in one sex onlv the results can not be ex- 

 plained. 



Let us turn then to the evidence which the study of the 

 germ cells has revealed, which shows that in certain 

 forms exactly such a process occurs in one sex and not 

 in the other. I may say at once that the evidence re- 

 lates to the chromosomes of germ-cells. 



Only a few years ago it was generally held that the' 

 number of chromosomes in each species of animals is 

 constant for all individuals of the species. Every cell 

 in the bodv contained the same number. 



We now know, however, that in seme species of ani- 

 mals, the female contains one more chromosome than 

 does the male, and we have a complete account of the 

 mechanism by means of which this difference arises. 



In Anasa tristis and in Protenor, and in a number of 

 other insects, as shown by Wilson, one chromosome in 

 the male has no mate. At one division it passes to one 

 pole of the spindle, so that one of the two resulting cells 

 has one more chromosome than the other. This chro- 

 mosome is the accessory, or the odd, or the sex chromo- 

 some, or, as Wilson has called it, the X-element. At the 

 other division it, like the other chromosomes, divides 

 into two parts, so that both of the derived cells from this 



