300 THE STUDY OF ANIMAL LIFE CHAP. 



(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 ; and (c) the produc- 

 tion 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 reproduc- 

 tive cells or germ-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 reproduc- 

 tive cells. We can infer from the Protozoa how fertili- 

 sation 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 simi- 

 lar 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. 



