120 



CYTOLOGY CHAP. 



stitution of their grandmother or even earlier ancestor. Morgan (1909) 

 has, however, suggested a way in which even this determination may 

 be preceded and hence " caused " by a differential partition of the 

 chromosomes. 



(c) Some other Cases. — An entirely different method of sex regulation 

 is found in the gall-fly Neuroterus lenticularis (Doncaster, 1910, 191 1). 

 This species produces two generations in a year. The generation 

 which hatches in the spring consists of females only, which reproduce 

 parthenogenetically. Their eggs hatch out into males or sexual females. 

 All offspring produced by one individual parthenogenetic female are 

 of the same sex ; i.e. her eggs are all either male-producing or female- 

 producing. It is also found that the parthenogenetic eggs fall into 

 two types : (i) in which there are the usual two meiotic divisions 

 resulting in the formation of two polar bodies ; (2) in which there are 

 no meiotic divisions or maturation processes. Embryos developing from 

 type (i) have haploid chromosome groups, and from (2) have diploid. 

 Moreover, all eggs laid by one individual are alike in this respect, being 

 either all of type i or all of type 2. Finally, the female of the sexual 

 generation has the usual diploid group in its pre-meiotic nuclei, but in 

 the male this is haploid. 



Combining these observations, therefore, it becomes clear that the 

 parthenogenetic eggs with two polar bodies produce haploid embryos 

 which are males, while those with no polar body produce diploid 

 embryos which are females. The maleness of the haploid individuals is 

 in accordance with the general rule for facultative parthenogenesis 

 in the Hymenoptera (Chapter III.). 



In the case of some groups of animals with alternation (though in most 

 cases irregular alternation) of sexual and parthenogenetic generations, 

 nothing is yet known of any accompanying changes in the chromosome 

 complex. In the Cladocera the common individual is the parthenogenetic 

 female, which produces females Uke itself for a variable number of genera- 

 tions. After a time, however, males and sexual females are produced. 

 The sexual eggs fertiHzed by the spermatozoa of these males invariably, 

 so far as is known, develop into females. 



One great difficulty in the cytological investigation of this group is 

 that males and females are produced by the same parthenogenetic 

 female and it is not possible to determine into which sex an egg will 

 develop. So far as has been observed, all the parthenogenetic eggs produce 

 only one polar body, but whether this applies to the comparatively rare 

 parthenogenetic eggs which will develop into males, as well as to the 

 majority which will develop into females, is not known, though it is 

 probable from the fact that the male is diploid. The other problem, 



