IV THE SEX CHROMOSOMES 113 



with the distinction between the X and Y pair in the males of many 

 other animals. 



In Abraxas grosstilariata Doncaster (1914 h) found that whereas 56 

 is the typical somatic number both for males and females, yet females 

 of certain strains have only 55 chromosomes and produce two classes of 

 eggs with 27 and 28 chromosomes respectively. It is not an unreasonable 

 hypothesis, therefore, that the females with 56 chromosomes have an X 

 chromosome paired by an inert Y, the latter having been lost in the 

 strain with 55 chromosomes. 



(7) Some special Life Histories 



The apparently simple and obvious relation between the presence 

 or absence of the X chromosome in one of the gametes and the sex 

 of the resulting zygote in the examples already dealt with raises at 

 once some interesting questions regarding certain cases of reproduction 

 of a different type. What is the condition of the sex chromosomes, for 

 instance, in a case of alternation of bisexual and hermaphrodite genera- 

 tions, such as is found in Ascaris nigrovenosus ; or where a female 

 produces parthenogenetically both males and females, the sex therefore 

 being determined by something other than the spermatozoon (Cladocera, 

 Aphids, etc.) ; or again where all fertilized eggs develop into females 

 (Aphids, Apidae, etc.), males only developing from unfertihzed eggs ? 



The chromosome cycle in a number of these life histories has been 

 worked out. 



(a) Ascaris nigrovenosus (Fig. 52). — The life history of this species 

 exhibits an alternation of hermaphrodite and bisexual generations, the 

 former being parasitic in the lung of the frog, while the latter is free- 

 living. The chromosome cycle in this species was worked out independ- 

 ently by Boveri (1911) and Schleip (1912), the two accounts agreeing 

 in all important points. 



In the free -living bisexual generation the male has eleven chromo- 

 somes and the female twelve ; the male produces two kinds of spermatozoa, 

 one with five and one with six (5 + X) chromosomes, while all the eggs 

 have six (5 + X) . Now all the animals developing from the zygotes 

 formed by the union of these gametes are hermaphrodites, which are of 

 the female form of body and have twelve chromosomes (10+ XX). It 

 is therefore to be supposed (though this matter was not actually deter- 

 mined by observation) that the spermatozoa without the X chromosomes 

 do not take part in fertilizing the eggs (cf. Aphis and Phylloxera, 

 below). 



The hermaphrodites produce eggs and spermatozoa in the same 

 reproductive organ, and up to the onset of the meiotic phase the cells 



I 



