364 THE GERM-PLASM 



The greater the extent to which sexual dimorphism occurs, 

 and the larger the parts which it affects, the larger the two 

 groups of determinants must be, and the earlier in ontogeny 

 will one of them remain inactive in a cell and cease to undergo 

 further division, while the other gives rise to further cell- 

 divisions. 



Sexual dimorphism largely consists in the arrest of develop- 

 1)1 ent in an organ in one sex. In many female butterflies, for 

 instance, wings are wanting. This must be due to the group of 

 determinants for the wings, which existed in a double condition, 



— i.e., were male and female in the earlier phyletic stages, — be- 

 coming arrested as regards their male portion. Such females 

 commonly possess rudiments of the wings, and in this case the 

 two groups of wing-determinants must pass through ontogeny 

 together up to the stage in the caterpillar in which the forma- 

 tion of the imaginal disc of the wing arises from a cell of the 

 hypodermis. If the animal is a female, the arrested group, 

 and if a male, the perfect group of wing-determinants will 

 then become active. It is, however, also conceivable that the 

 development of the group of determinants in the female might 

 continue to be arrested until they disappeared completely, as in 

 the case of female Fsychida, in which the wings are altogether 

 wanting. 



But the highest degree of sexual dimorphism is not reached 

 even when certain parts disappear completely. In various 

 groups of the animal kingdom species exist in w/iick the inales 

 differ from the females in nearly all their characters. In many 

 Rotifers the males are very much smaller than the females, and 

 exhibit an entirely different structure ; the alimentary canal, 

 moreover, is entirely wanting. In Bonellia viridis, a marine 

 gephyrean worm, the male differs so much from the female that 

 one might be tempted to class it with an entirely different group 



— the Turbellaria. The difference in the sizes also of the two 

 sexes is still more marked in this instance, the length of the male 

 being 1-2 mm., and that of the female 150 mm.; the former, 

 moreover, lives as a parasite within the latter. In this case the 

 eggs which give rise to males cannot be distinguished from 

 those which develop into females, even in size ; and the rela- 

 tively enormous bulk of the female simply results from subse- 

 quent growth. Even the young male and female larvae cannot 

 be distinguished from one another, and their development only 



