OTHER METHODS OF SEX-DETERMINATION 229 



X-chromosomes come together (there are four X's pres- 

 ent). Two of them pass out of the egg into the single 

 polar body that is given off (Fig. 134). The autosomes 

 at this time divide, and half of each is eliminated. The 



a 



d e f ' g 



Fig. 133. 

 First maturation division of the bearberry aphid. At the first divi- 

 sion, a-c, the large X-chromosome passes into one cell. At the 

 second division, e, f, g, this cell divides again producing two func- 

 tional female-determining sperms. The rudimentary cell does not 

 divide again. 



egg is left with a diploid set of autosomes and half of 

 the X-chromosomes. It develops, by parthenogenesis, into 

 a male. 



In the Aphids a similar process takes place. The actual 

 extrusion of one of the X's from the egg (there are only 

 two X's present) has not been observed, but since, after 

 the single polar body is given off, there is one less chro- 

 mosome present in the egg, there can be no doubt but that 

 one is lost, as in the Phylloxerans. 



In these two groups the male sex is determined by a 



