198 MANUAL OF ELEMENTARY ZOOLOGY 



of sex exists only to bring it about. Sex is primarily the 

 differentiation of the individuals of a species into two kinds 

 adapted to the production of two kinds of gametes, though 

 the differentiation may extend to other functions, such as 

 the part played by the parents in bringing about the union 

 of the gametes or the nourishment and protection of the 

 young. It is an example of the physiological division of 

 labour between individuals. The basis upon which sex is 

 established is the unlik eness or ' ' dimorphism " of the gametes. 

 In Polytoma the two gametes are identical so far as observa- 

 tion shows. There is here conjugation without difference 

 either between the gametes or between their parents. In 

 Paramecium there is dimorphism of the gametes. The 

 migratory pronucleus, which is said to be also somewhat 

 the smaller, represents a male gamete; the stationary 

 pronucleus represents a female gamete. The female 

 gamete possesses the cytoplasm ; the male is specialised 

 for that part of the process which involves locomotion. 

 These gametes, however, are produced by the same in- 

 dividual. Some kinds of Hydra are in this condition, 

 though in them the gametes of one kind usually ripen 

 before those of the other, so that the parent has a kind of 

 temporary sex. In some (if not in all) species of Monocystis 

 there is a dimorphism of the gametes together with a 

 restriction of the production of each kind of them to 

 certain individuals, which, however, show no further 

 difference. We have here the simplest case of sex. The 

 same thing is seen in some kinds of Hydra and in medusae, 

 where the sexes are alike in all respects both of structure 

 and of behaviour, save that one produces only eggs and the 

 other only sperm. In the frog the parents differ, not only 

 in respect of the gametes which they form, but also in 

 certain points of structure, such as the generative ducts 

 and the pad upon the hand of the male, and conspicuously 

 in their behaviour. One, the female, plays in coition the 

 same passive part which the germs she forms play in 

 conjugation. The other, the male, plays the active part 

 in coition and forms germs which play the active part in 

 conjugation. Throughout the animal kingdom it is the 

 case that, so far as there is a difference between the sexes, 

 the male is the active organism, the female relatively 



