xi] APHIDS 163 



somes but differing in the existence of only the haploid 

 number of autosomes in the male instead of the diploid 

 number that is present in almost all other groups. 



The phenomena seen in the parthenogenetic Aphids and 

 Phylloxerans are more closely comparable with those of the 

 sexually reproducing species, although some points of great 

 interest have not yet been completely worked out. Our 

 knowledge of the main facts depends chiefly on the work of 

 Miss STEVENS (1909, 1910 a), T. H. MORGAN (1915) and VON 

 BAEHR (1909) ; MORGAN'S account of Phyllaphis coweni may 

 be taken as typical (Pis. XIX, XIII). In this species the 

 nuclei of the "stem-mother" (female produced from a 

 fertilised egg) contain six chromosomes, two of which are 

 ^-chromosomes. She produces two kinds of eggs, (i) eggs 

 that will develop parthenogenetically into females, and 

 (2) eggs that develop parthenogenetically into males. The 

 females produced from these eggs are either sexual females or 

 again reproduce parthenogenetically like the stem-mother. 

 There are thus three kinds of eggs in all, (i) and (2) which de- 

 velop parthenogenetically and produce respectively females 

 and males, and (3) sexual eggs which require fertilisation and 

 produce females (stem-mothers). The oocytes of class (3) 

 undergo a synapsis (synizesis) stage and then have a double 

 polar division, reducing the chromosomes to three two 

 autosomes and one ^-chromosome. The oocytes that will 

 develop parthenogenetically have no synapsis, and these 

 eggs undergo only one maturation division. In those which 

 will produce females all the chromosomes divide equation- 

 ally, so that the female offspring, like the mother, have six 

 chromosomes. In those that produce males, however, the 

 two ^-chromosomes pair together before the single polar 

 division, and while the autosomes are dividing equationally 

 the X-chromosomes separate from each other, one going 



II 2 



