1 86 The Study of Animal Life PART m 



tion of special reproductive cells ; (b) the production of two 

 different kinds of reproductive cells (spermatozoa and ova), 

 which are dependent on one another, for in most cases an 

 ovum comes to nothing unless it be united with a male-cell 

 or spermatozoon, and in all cases the spermatozoon comes 

 to nothing unless it be united with an ovum ; (c) the pro- 

 duction of spermatozoa and ova by different (male and 

 female) organs or individuals. 



(a) It is easy to think of simple many-celled animals 

 being multiplied by liberated reproductive cells, which 

 differed but little from those of the body. But as more 

 and more division of labour was established in the bodies 

 of animals, the distinctness of the reproductive cells from 

 the other units of the body became greater. Finally, the 

 prevalent state was reached, in which the only cells able 

 to begin a new life when liberated are the reproductive 

 cells. They owe this power to the fact that they have not 

 shared in making the body, but have preserved intact the 

 characters of the fertilised ovum from which the parent 

 itself arose. 



(b) But, in the second place, it is easy to conceive of a 

 simple multicellular animal whose liberated reproductive 

 cells were each and all alike able to grow into new 

 organisms. In such a case, we might speak of sexual 

 reproduction in one sense, for the process would be different 

 from the asexual method of liberating more or less large 

 parts. But yet there would be no fertilisation and no sex, 

 for fertilisation means the union of mutually dependent 

 reproductive cells, and sex means the existence of two 

 physiologically different kinds of individuals, or at least 

 of organs producing different kinds of reproductive cells. 

 We can infer from the Protozoa how fertilisation or the union 

 of the two kinds of reproductive cells may have had a 

 gradual origin. For in some of the simplest Protozoa, e.g. 

 Protomyxa,) a large number of similar cells sometimes flow 

 together ; in a few cases three or more combine ; in most a 

 couple of apparently similar units unite ; while in a few 

 instances, e.g. Vorticella^ a small cell fuses with a large one, 

 just as a spermatozoon unites with an ovum. 



