84 INTRODUCTION. 



presence of such organs in the embryo is indispensable as a 

 stimulus to the development of the permanent structures of the 

 adult. Should this theory prove to be well founded, it will 

 afford a ready and welcome explanation of many perplexing facts 

 in the development of animals. 



The Origin of Sex. 



The simplest mode of reproduction is a mere act of fission or 

 cell division, as seen in an Amoeba or in an ordinary epithelial 

 cell. Such a form of reproduction is characteristic of the simpler 

 Protozoa, and of the component tissues of Metazoa. It may 

 concern one individual alone, or may be preceded by the con- 

 jugation or fusion of two or more originally separate individuals 

 or cells. 



The higher Protozoa, or Infusoria, show considerable advance 

 on this simple method. In Parainecium, or Stylonychia, reproduc- 

 tion is effected, as before, by fission, i.e. by division of the single 

 animal into two separate animals ; and under favourable circum- 

 stances this process may be repeated again and again with great 

 rapidity. Sooner or later the rate slackens, and ultimately the 

 process stops altogether ; and it does not recommence until 

 conjugation, usually temporary, has occurred between two indi- 

 viduals, which on the completion of the process begin to divide 

 actively once more. Maupas' researches have shown that this 

 conjugation is absolutely necessary, and that it must not take 

 place between two closely allied individuals, but between ones 

 of different broods. 



In Vorticella there is further complication, for the conjuga- 

 ting individuals are in this case unlike ; one being an ordinary 

 large, stalked Vorticella ; the other a small free-swimming indi- 

 vidual, of which a number, usually eight, are formed by simul- 

 taneous division of a large Vorticella. The conjugation is in this 

 case a permanent one, the small Vorticella fusing completely 

 with the large one ; and the whole process corresponds singularly 

 closely with the sexual reproduction of Metazoa, the small 

 free-swimming Vorticella playing the part of the spermatozoon, 

 while the large fixed one behaves as the ovum. This may be 

 taken as the first definite establishment amongst animals of sexual 

 differentiation, and the two Vorticellre may not inappropriately 

 be spoken of as male and female respectively. 



