THE MECHANISM OF SEX-DETERMINATION 51 



seen is the reverse of the actual conditions. Stevens 

 (1905) made out the relations of the XY pair of chro- 

 mosomes to sex and Wilson in the same year (1905) 

 the correct relation of the accessory chromosome to sex. 

 The results described above for the insects are for the 

 most part from Wilson's studies on the chromosomes ; 

 those for ascaris from the recent work of Sophia 

 Frolowa, which confirms in the main the work of Boveri, 

 Gulick, Boring, and Edwards. 



In the fruit fly, Drosophila ampelophila, it appears 

 from the recent work of Metz, that, in the male, there 

 is an XY pair of sex chromosomes, instead of only an 

 X, as Stevens supposed. The female has, of course, 

 two X's. An analysis of certain experimental evi- 

 dence has led H. J. Muller to the conclusion that prob- 

 ably the Y chromosome carries no factors that influence 

 development. If this proves true, we can better under- 

 stand how it might be completely lost in certain types. 



The whole history of the sex chromosomes of ancyro- 

 canthus, a nematode worm, is strikingly shown in a 

 recent paper by Carl Mulsow (Fig. 29 and 29a, A). 

 This is a typical case in which the male has one less 

 chromosome than the female, as in protenor. The 

 case is striking because the chromosomes can be seen 

 and counted in the living spermatozoa. Some sperm 

 have six, some have five chromosomes. The sperm- 

 nucleus can be identified in the egg after fertilization 

 because it lies nearer the pole opposite to the polar 

 bodies. The entering sperm nuclei show in half of 

 the fertilized eggs six chromosomes and in the other 

 half five chromosomes. 



An interesting confirmation of these conclusions in 



