SEXUAL DIFFERENTIATION 85 



cytoplasm and is therefore more heavily weighted than the male 

 gamete. This cytoplasm, moreover, is in many cases densely 

 charged with food material, which constitutes the capital with 

 which the zygote, formed by the union of the two gametes, has 

 to begin its new life cycle. 



It is quite clear that the primary distinction between the sexes 

 is a simple case of division of labour accompanied by a 

 corresponding structural differentiation. Two ends have to be 

 secured by the gametes. They must come together in order that 

 they may conjugate, and therefore one or both must be capable 

 of active locomotion. They must also contain between them 

 sufficient material, either in the form of actual protoplasm or of 

 some substance that can easily be worked up into protoplasm, to 

 give the new individual which results from their union a fair 

 start in life. 



A cell body heavily weighted with food material is, however, 

 clearly incompatible with great activity, so one of the two gametes 

 remains unencumbered and becomes specialized as the active 

 partner, charged with the duty of seeking out its mate 

 and bringing about their union, while the other, more or less 

 burdened with the necessary supplies, passively awaits the event. 

 The conjugation of such differentiated gametes leads to a more 

 satisfactory result than can be attained in cases of isogamy 

 like that of Copromonas, for the new individual will have a better 

 chance in life owing to the greater amount of capital with which it 

 commences. Such sexual differentiation of the gametes finds its 

 most complete expression in the formation of female ova and 

 male spermatozoa, which are especially characteristic of the 

 higher animals (compare Fig. 69), though they also occur in 

 many plants and even in some unicellular forms. The process 

 of conjugation in such a case is often spoken of as the fertiliza- 

 tion of the ovum by the spermatozoon. 



These considerations enable us to understand at once the great 

 difference in size which usually distinguishes the male from the 

 female gamete, whence the general terms microgametes and mega- 

 gametes so often applied to them. The microgamete is as small 

 as possible in order that its activity may not be impaired ; the 

 megagamete is swollen out with nutrient material. 



We may illustrate these general principles by a brief description 

 of a few more cases of conjugation amongst unicellular organisms. 



Bodo, or Heteromita (Fig. 38), is a very minute flagellate monad 



