THE ULTIMATE SEX-ELEMENTS. 



89 



the level of the simplest animals or Protozoa, which (with the 

 exception of very loose colonies) remain always unicellular. 

 The simplest organisms leave off where the higher plants and 

 animals begin, i.e., as unit masses of living matter. They corre- 

 spond, in fact, to the reproductive cells of higher animals, and 

 may be called, according to their predominant character, protova 

 and protosperms. A fertilised ovum, as we have seen, pro- 

 ceeds by division to form a "body;" the Protozoon remains, 

 with few exceptions, a single cell, in which there is obviously 

 no distinction between reproductive elements and entire 

 organism. 



Reference will have to be made to the Protozoa in three 

 connections, which may be here simply noted : — 



{a) In their chief groups, and in the stages of their life- 



Ophrydium, a colonial infusorian.— From Sa\ ille Kent. 



histories, they express phases in the same cell cycle which recurs 

 in higher forms in the component elements of the body, and 

 in the reproductive cells. The contrast, in other words, between 

 an infusorian and an amoeba, between the ciliated and amceboid 

 stage in the life-history of many forms, is a forecast of the 

 contrast between a ciliated cell and a white blood corpuscle, 

 between a mobile spermatozoon and a young ovum. That is 

 to say, a predominance of the same protoplasmic processes is the 

 common explanation of such similarities of form (see p. 121). 



{b) It is among the Protozoa that we must presently look, if 

 we hope to understand the origin and import either of "male 

 and female," or of fertilisation (see pp. 119, 128). 



{c) Among the loose colonies which some Protozoa form. 



