382 PERMANENT HYBRIDS 



ordinary chiasma-pairing by pairing as found in the sex chromosomes 

 of other Hemiptera, which seems to depend on the property of 

 terminal affinity, and, secondly, the development through genetic 

 change of a special spindle mechanism. The progressive change in 

 this series is represented in the diagram (Fig. 120). It is described 

 here as differentiation, meaning the accumulation of structural 

 differences. It has been described by the Schraders as 

 " degeneration " — a term which should be avoided in speaking of 

 chromosomes here and elsewhere, because it implies an analogy with 

 evolutionary changes in gross structures that is somewhat misleading. 

 But there is a fourth genus, Gossyparia, whose behaviour seems 

 to conflict with this or any other evolutionary hypothesis, although 

 the Schraders have represented it as the last step in the process of 

 differentiation. Here again meiosis is normal in the female : 

 14 bivalents are formed. In the male half the chromosomes are 

 precocious and do not associate with the other half, but they do 

 pair at metaphase amongst themselves. It is believed that this is 

 not true pairing and does not lead to segregation, all 28 chromosomes 

 dividing equationally at the first division. The precocious 

 chromosomes separate from the rest at the second division, which 

 is otherwise abortive. Thus four nuclei are formed with the haploid 

 number of chromosomes, and if it were possible to consider these as 

 of two alternative sex types, the precocious chromosomes repre- 

 senting those characteristic of the male, the " Y " set, and those 

 that are homozygous in the female, the " X " set, it would be easy 

 to see in Gossyparia an exaggeration of the differentiation of 

 Pseudococcus and Protortonia. But Schrader (1929) finds that half 

 the sperm die, and considers these to be of one type, the '' Y " set. 

 If this is so, obviously the chromosomes that are lost in the 

 degenerating sperm of one male are precisely those that were not 

 lost in its own male parent. Moreover, since the half that survive 

 must contain both male-determining and female-determining 

 chromosomes, these must have been derived in part from opposite 

 parents. This upsets the notion of the constancy of behaviour 

 being due to any properties inherent in the chromosomes concerned. 

 The view that the sperm which die contain what they describe as 

 the " degenerate " set of chromosomes is held by the Schraders 



