EVOLUTIOX OF SCALE-INSECTS 



381 



pair, but yet are segregated to opposite sexes they will vary 

 independently in so far as they vary at all and therefore become less 

 and less like one another as evolution proceeds. 



In another Coccid, Llaveia bouvari, a condition is found more 

 nearly approaching normal meiosis (Hughes-Schrader, 1931). The 

 chromosomes as a rule pair normally, even forming chiasmata 



Fig. 120. — Diagram of chromosome behaviour at meiosis in the male 

 in a series of forms showing a gradation from normal pairing to a 

 complete failure. Circles represent chromosomes, white ones 

 being those confined to the male sex (the " Y " set) ; the X 

 chromosome is contributed to the female progeny. In Llaveia 

 there is 95 per cent, of normal pairing, 5 per cent, of failure of one 

 pair. There is probably no differentiation between the pairing 

 chromosomes. In Protortonia only two chromosomes pair and 

 these are perhaps differentiated. In Gossyparia there is no 

 pairing and half the sperm degenerate. I eery a is haploid in the 

 male tissue. (From F. and S. H. Schrader, 1931.) 



apparently, though each pair remains in a separate vesicle. 

 Occasionally, however, only one pair associates at metaphase, as in 

 Protortonia. 



These species are most easily considered as representing steps in 

 the successive differentiation of the two sets of chromosomes in the 

 male, so that the whole of a set develops the character of X or Y 

 and the " Y " set is carried exclusively in the male line. Such a 

 differentiation might be conditioned by, first, the replacement of 



