THE MECHANISM OF SEX-DETERMINATION 51 



seen is the reverse of the actual conditions. Stevens 

 (1905) made out the relations of the XF 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 ; 

 these for ascaris from the recent work of Sophia 

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

 Gulick, Boring, and Edwards. 



A case similar to ascaris has been described by Stevens 

 for the mosquito, in which there is an X and a F in the 

 male, each attached to another chromosome. In the 

 guinea pig also, there seems to be in the male an X and 

 a F, attached to another pair of chromosomes. Find- 

 ing these cases so widely distributed, it seems not un- 

 likely that in other cases, where an unpaired X or an 

 X and a F have not been detected, they are parts of 

 other chromosomes. 



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 



