140 THE MENDEL JOURNAL 



this result we must first consider another fact in con- 

 nexion with the chromosomes of germ -cells. We have 

 already seen that in certain insects there are two kinds 

 of sperm- cells or spermatozoa ; one of these carries an 

 accessory chromosome and its union with the egg-cell 

 produces a female, while the other kind is devoid of 

 the accessory chromosome and the egg fertilised by it 

 developes into a male. Now, if we suppose that some- 

 thing happened by which all the latter kind of sperm- 

 cells became destroyed, it is obvious that male offspring 

 would never be produced. The product of fertilisa- 

 tion would be females always, and this we know 

 is the case, not only with the fertilised eggs of the 

 bee, but also of the green-fly [Aphis), of a water flea 

 (Daphnia), and of a certain genus of plant-lice 

 (Phylloxera). Is there then any evidence that this 

 seemingly improbable selective destruction of a 

 particular type of sperm- cell does actually occur ? 

 Such a hypothesis appears to be exceedingly 

 fanciful. None the less, it expresses an actual fact 

 which has been ascertained by careful observation. 

 For Professor Morgan has observed that in a Phyl- 

 loxera one half of the sperm-cells are small in 

 size and do not contain the accessory chromosome, 

 and ultimately degenerate ; they therefore can 

 take no part in fertilisation. But these are the 

 cells which determine maleness in the offspring 

 which arise from the eggs fertilised by them. There- 

 fore, fertilisation being effected only by the other 

 kind of cell, females alone are produced. Miss 



