164 SEX-DETERMINATION [CH. 



into the polar body and the other remaining in the egg. As 

 a result, the male has five chromosomes four autosomes 

 and one Jf-chromosome as compared with the six of the 

 female. As in many other Hemiptera, therefore, the male 

 has one Jf-chromosome while the female has two, and this 

 condition is reached by the Jf-chromosomes undergoing a 

 reduction division at the time when the autosomes are 

 dividing equationally in the single polar mitosis of the male- 

 producing parthenogenetic egg. 



In the Aphids, however, as in other parthenogenetic 

 forms, only females arise from fertilised eggs, and since the 

 male has only one X-chromosome one would expect that 

 two kinds of spermatozoa would be formed, and therefore 

 the production of nothing but females from fertilised eggs 

 requires explanation. The explanation of this apparent 

 anomaly is that the first spermatocyte division is an unequal 

 one. Just as in other forms the autosomes come together 

 in pairs in the preceding growth-phase, giving rise to three 

 chromosomes in all, two bivalent autosomes and the single 

 X-chromosome. In the first spermatocyte division, as in 

 other Hemiptera, the bivalent autosomes divide into 

 their separate components and the ^-chromosome splits 

 longitudinally, but in the anaphase of the mitosis both 

 halves of the Jf-chromosome travel together into one 

 daughter-cell; they lag behind the autosomes but in the 

 telophase both halves are included in one nucleus, which 

 thus contains two single autosomes and a divided X- 

 chromosome, while the other nucleus contains only two 

 autosomes (cf. PI. XIII, 9-11). When the cell divides the 

 division is unequal; the cell containing the three chromo- 

 somes is much larger than that which has only two. The 

 first spermatocyte division thus results in a large secondary 

 spermatocyte containing two autosomes and an JJf-chromo- 



