494 HEREDITY AND SEX 



knows that all the fertilised ova produce females. An interesting 

 accessory discovery is that in Phylloxera and Aphides the males 

 have in their bodies one chromosome fewer than the females have. 

 "The male-producing egg," Wilson notes, "must therefore 

 eliminate one chromosome, and this, we cannot doubt, is the 

 X-element." 



These cytological studies are so very striking that one in- 

 quires anxiously as to the distribution of the phenomena in the 

 animal kingdom. There have been some noteworthy recent 

 extensions. 



An accessory chromosome is reported by Boveri and Gulick in 

 Heterakis, a Nematode of the pheasant. The ovum has five 

 chromosomes ; the sperms are of two types, one with four, the 

 other with five — a condition similar to that described by Wilson 

 for Protenor, one of the Hemiptera. In the common Ascaris 

 megalocephala there is also evidence of an accessory chromosome, 

 but it seems at present somewhat discrepant and difficult. As 

 one would expect from the difficulty of the inquiry, there is still 

 considerable discrepancy of description in regard to many cases 

 in which an accessory chromosome has been affirmed. It is very 

 interesting to inquire whether there is any hint of an accessory 

 chromosome in Vertebrates. In a recent paper, Prof. M. F. 

 Guyer brings forward evidence to show that in man half of the 

 spermatids (or immature spermatozoa) have ten, and half twelve 

 chromosomes, which would correspond to one of Wilson's cases, 

 Syromastes, where half of the spermatids were found to possess 

 two more chromosomes than the others. Guyer has found 

 evidence, still unpublished, which leads him to think that, as 

 regards accessory chromosomes, conditions obtain among Verte- 

 brates (fowl, guinea-pig, rat, and man) similar to those found 

 in numerous Tracheates, and he ventures to express the expecta- 

 tion that the somatic cells of man will be found to contain 

 twenty-two chromosomes, and those of woman twenty-four 

 chromosomes. 





