Manchester Memoirs, Vol. Ixii. (191 7), No. 2 7 



less by placing the animals in beef extract, which had previously 

 been shown to act as a deterrent to male production. 



A very interesting result of the experiments was the demon- 

 stration that there are definite male and female-producing females, 

 and that the same female does not give rise to both male and 

 female-producing daughters. Whether a female is to bef a male- 

 producer or a female-producer is decided during the growth 

 period of the parthenogenetic egg from whicihl it is hatched]. 

 Thus " sex is determined a generation in advance." It appears 

 evident that both external and internal agents are at work in the 

 change from parthenogenesis to sexual reproduction. These 

 internal agents are probably capable of causing the appearance 

 of males without any outside aid; indeed the evidence goes to 

 prove that when external agents, with few exceptions, operate 

 male production is prevented or diminished. 



It is possible that some external stimulus may modify the 

 normal course of events in the internal economy of the egg. The 

 discovery that oxygen increases male-production indicates that 

 the cause may be attributed to an increased speed of reaction in 

 the protoplasm. There is evidence also that the chromosome 

 number is different in the two types of egg, and that the method 

 of maturation is also 'different. It is possible therefore that the 

 external agents may so act on the ,egg as to cause the 'appearance 

 of one or other mode of maturation, (also, as we shall see later, 

 there is evidence that in the Aphjids the external and internal 

 factors act together in the production of males and sexual 

 females. 



Cytological Aspect. 



As is well known, during the maturation divisions of the germ 

 calls of sexual female and male animals, the somatic number of 

 p chromosomes is reduced to one half; and thus when fertilisation 

 occurs the somatic number is again obtained. 



From the researches of numerous eytologists during recent 

 years it has been demonstrated, that in some species the males 

 produce two kinds of spermatozoa, which differ one from the other 

 in the number of chromosomes, one type possessing one or more 

 than the other. The eggs are all alike in having an equal 

 number. These chromosomes are also in many cases charac- 

 terised by differing in shape 'and size from the others, and are 

 known as X-chromosomes or sex chromosomes. Thus if 2n 

 represents the somatic chromosome number, omitting the sex 

 ones, the female will possess 2n -f~ 2X5, and the, male 2n -j- X. 

 After maturation divisions all the ova will have n-j-X chromo- 

 somes, while the sperms will be, of two. types : 



(1) n-|-X chromosomes; 



(2) n chromosomes. 



