282 THE EVOLUTION OF THE METAZOA 



this has taken place cannot be discussed in the present study. 

 This bisexuality has proved to be of great advantage to 

 organisms in their further development and so it was quite 

 generally accepted. Later it was even combined with propaga- 

 tion and thus with the preservation of the continuity, a function 

 which it did not originally possess. The sexual phase of the 

 life-cycle had been developed by Protozoa along different lines 

 and in very different ways. Very primitive conditions can be 

 found preserved even now in various species of Protista. In 

 these the sexual phase is represented by a special sexual 

 generation which however does not differ morphologically 

 from the usual, the so-called vegetative generation (Dogiel 

 prefers to call it a "neutral" generation). This most primitive 

 type of syngamy has been called, as is well known, a hologa- 

 mous isogamy. The sexualized generation differs from the 

 vegetative generation in its chemical properties only— sexuaHty 

 itself was originally a biochemical phenomenon. We are hardly 

 justified to speak of gametes during this primitive stage of the 

 sexuality. The two units are naturally attracted because of the 

 biochemical differentiation into two opposite directions which 

 cover each other in the 0-point (there is, according to the well- 

 founded theory by Hartmann, no absolutely -f and no absolu- 

 tely — developed sexuality). This finally leads to a union of the 

 two. This was the basis from which special conditions had 

 evolved parallel to the various lines of evolution of Protozoa 

 (we will omit here the Protophyta). The development went 

 mainly into two directions. On one hand it led to a strong 

 differentation of the two partners in the sexual generation. 

 Two gametes had evolved that differ from each other, in 

 agreement with the function they have: the slender, actively 

 swimming androgametes have also been called microgametes 

 {usually in the form of spermatozoa, resembling flagellates; 

 secondarily, however, they can become atypical and they can 

 lose their free mobility), and the passive gynogametes (macro- 

 gametes) which are usually equipped with a reserve food sub- 

 stance. The gametes no longer feed independently; they have 



