52 HEREDITY AND SEX 



regard to the relation between sex and the sex chromo- 

 somes was found in another direction. It has long been 

 known that the fertiUzed eggs of aphids or plant lice 

 produce only females. The same thing happens in 

 near relatives of the plant lice, the phylloxerans. 





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Fig. 29. — 1 and 2 are spermatogonia ; 3, growth period ; 4-7, prophases ; 

 8, equatorial plate of first division, 9-10 ; 11, spermatocytes of second order ; 

 12-13, division of same; 14-16, the four cells or spermatids that come 

 from the same original cell, two with 5, two with 6 chromosomes; 17, 

 spermatids; 18, mature sperm; 19, living sperm. (After Mulsow.) 



In these insects a study of the chromosomes shows 

 that the male has one less chromosome than the female. 

 At the first maturation division in the male (Fig. 30), 

 all the chromosomes divide except one, the X chromo- 

 some, and this passes to one cell only. This cell is 

 also larger than the sister cell. The small cell lacking 

 the X degenerates, and does not produce spermato- 



