MENDELISM AND SEX 141 



Stevens has observed a similar condition of 

 degeneration of those sperm-cells which are devoid 

 of the accessory chromosome in another genus 

 of plant-lice, Aphis. And in bees the degener- 

 ation of one half of the sperm-cells has also been 

 observed by Meves. 



In this way our knowledge of the behaviour 

 of the accessory chromosome, while rendering it 

 certain that sex is a gametic differentiation, that 

 it is a quality not depending upon environmental 

 influences, also enables us to explain the phenomenon 

 that in bees, plant-lice, and water-fleas, fertilised 

 eggs develop only into females. For femaleness, 

 as has been shown by Professor E. B. Wilson in the 

 insect Protenor, and by Professor Morgan in certain 

 species of Phylloxera, is characterised by the presence 

 in the body cells of the full complement of chromo- 

 somes, while maleness is characterised by the absence 

 of one or two of these. 



It may therefore be accepted as a demonstrated 

 fact that in certain insects and in certain phyUoxerans 

 and aphids the body cells of the females possess 

 one — or in some cases two — chromosomes more 

 than those of the male. It is also a fact that in some 

 insects, phyUoxerans, and aphids there are two kinds 

 of spermatozoa, one kind, which is smaller than 

 the other, containing one or two chromosomes less 

 than the larger kind. The smaller ones, which are 

 the carriers of maleness, degenerate, and leave, 

 therefore, only the larger ones, which are carriers of 



