REPRODUCTION AND SEX 521 



good intentions — ^must be set ciside at once because of obviously 

 fatal defects in their scientific procedure. Some lay stress on unveri- 

 fiahle factors, such as the desire of the parents or parent to have a 

 male child. Others allege the operation of factors which are physio- 

 logically absurd. Others base a generalisation on an outrageously 

 small number of cases. 



THE PROBLEM STATED.- The general problem is: What 

 determines whether a fertilised egg-cell will develop into a male or 

 a female organism? But let us look at particular forms of the 

 problem. What are called "true twins" in the human race arise 

 from the division of a single ovum into two independently-developing 

 ova, and they are said to be always of the same sex — identical in 

 this as in their other features. But ordinary twins, which arise from 

 two distinct ova developing simultaneously, are often of different 

 sexes. Why is there this difference ? 



In one household the family consists of boys and girls, in a 

 second of boys only, in a third of girls only. What determines this? 



The unfertilised eggs of a queen-bee develop into drones, while 

 the unfertilised eggs of Aphides (produced all through the summer 

 months) develop into parthenogenetic females, until the end of the 

 season, in autumn, when males are produced. What does this 

 mean ? 



A step would be gained if we could narrow the issue by answering 

 the question. When is the sex of the offspring finally determined? 

 How long may a germ-cell remain with the potentiality of either 

 sex? Is there sex-determination before fertilisation or during 

 fertilisation, or not until after fertilisation? 



Prof. V. Haecker has proposed a useful terminology. Sex-dif- 

 ferentiation implies that one of the two sex-primordia in the germ- 

 cell is activated, while the other remains latent ; or, to put it more 

 cautiously, one of two lines of development (towards maleness or 

 towards femaleness) is in some way determined. 



{a) This may occur before iexi\\\s2±\0Y\.— progamic sex-differentia- 

 tion — as in the large and small ova of Dinophilus, Rotifers, and 

 Phylloxera. 



(6) Or it may occur at the moment of fertilisation — syngamic 

 sex-differentiation — as in the case of the hive-bee, where the 

 fertilised ova become females (queens and workers) and the unferti- 

 lised ova males or drones. 



(c) Or it may (theoretically) occur after fertiHsation at some 

 stage in development — epigamic sex-differentiation. Of the last, 

 however, no convincing case is at present certain. 



DIFFERENT WAYS OF ATTACKING THE PROBLEM —The 



problem of the determination of sex has been attacked scientifically 



