494 HEREDITY AND SEX 



has been recently afforded by the work of T. H. Morgan on 

 Phylloxera and of von Baehr on Aphis saliceti. In these forms 

 half of the spermatocytes degenerate (as Meves pointed out in 

 the bee), namely those without the accessory chromosome ; 

 therefore all the spermatozoa are female-producers, and every 

 one 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 eagerly 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 chromo- 

 some, but it seems at present somewhat discrepant and difficult. 

 There are many other cases now known. 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. In a variety of 

 types, e.g. house-fly and pig, it has been shown that the presence 

 or absence of an accessory chromosome in the spermatozoa is 

 associated with size-dimorphism. 



Accessory chromosomes have been demonstrated in numer- 

 ous vertebrate types, e.g. the Amphibian Necturus, guinea 

 fowl, common fowl, armadillo, opossum, bat, rat, white 

 mouse, guinea-pig, sheep, horse, pig, bull, cat, dog, and man 

 himself. 



