230 AN INTRODUCTION TO MODERN GENETICS 



before the extrusion of the polar bodies, and a differential maturation 

 occurs; if the fertilizing sperm contains the X chromosome, the egg 

 pushes out its X into the polar body, so that the fertilized egg becomes 

 XYy and similarly if the sperm contains F, it is the Y which gets 

 eliminated from the egg. Thus fertilized eggs normally contain the 

 whole F complex, shared between the X and the Y. Occasionally the 

 differential maturation mechanism breaks down and diploid XX or F F 

 individuals result; they are exceptional diploid males. 



6. Progamic Sex Determination^ 



In a few animals, it has been suggested that sex determination is 

 dependent not on genetic factors but on the size or physiological con- 

 dition of the eggs. This is often spoken of as progamic sex determina- 

 tion. Thus in Dinophilus, a worm, there are two classes of eggs, large 

 and small, and the large ones develop into females and the small into 

 males. The interpretation of these facts in terms of a determination 

 dependent on a balance between genetic factors, is still obscure. In 

 frogs, where a genetic basis is certainly present, it has been shown by 

 Hertwig and Witschi that the rates of development of the male and 

 female substances can be affected by the physiological state of the egg 

 when it is fertilized;^ overripe eggs develop as males even if they have a 

 female genotype. Again, Uttle is known of the chromosome cycles of 

 organisms with progamic sex determination. In Dinophilus, there are 

 reported to be ten chromosomes in both eggs and sperm, but it is 

 possible that there is an XF pair, whose members are not distinguish- 

 able cytologically, and that the dimegaly of the eggs is correlated with a 

 differential maturation, so that in the large eggs the F is passed into the 

 polar body while in the small eggs it is the X which is extruded. 



D. THE THEORY OF SEXUALITY^ 



I. The Two Sexes 



Differentiation into two sexes is an extremely common property of 

 living organisms. Several theories have been put forward to account for 

 this. One of these theories can now be dismissed. BiitschU suggested 

 that sexual union is necessary to rejuvenate living substance, basing the 

 idea on the alleged fact that cultures of infusorians cannot be kept 



1 Rev. Whiting 1935^. - Cf. Witschi 1934. 



' General references: Darlington 1932a, 1938, Hartmann 193 1, Mainx 1933, 

 Muller 1932a. 



I 



