214 



THE EVOLUTION THEORY 



In the Coccidia, which are extremely simple unicellular organisms, 

 equipped, however, with a nucleus, the adaptations relating to amphi- 

 mixis are more extensive and more complex than in the Rhizopods. 

 For while in the latter the two conjugating cells are absolutely alike 

 in external appearance, in the former the male cell is distinct from 

 the female, and indeed the differences are as marked as those that 

 usually occur in multicellular animals. 



We owe our present knowledge of these processes especially to 





Fig. 121. Life-cycle of CoccicUwn Utiiohii, a cell-parasite of the centipede 

 Lithohius ; after Schaudinn. i, a ' sporozoite ' ; 2, the same penetrating into an 

 intestinal epithelial cell ; 3, the same groAving into a ' schizont ' capable of 

 division ; 4, the same dividing, and 5, breaking wp into numerous pieces "which 

 separate from the 'residual body' in the centre, and either, as in i, migrate 

 into epithelial cells and lepeat the history, or pass on to the phase of sexual 

 rei^roduction. In the latter case, after eliminating a portion of the nucleus 

 (reduction) in 6 and 6 a, they form the ' macrogamete * (the ovum) ; or within 

 the mother-cell they produce microgametes (or sperm-cells), 7 and 7 a. The 

 penetration of a sperm-cell into an egg-cell (amphimixis) is shown in 8, the 

 fertilized egg-cell (9) becomes the so-called oocyst or permanent spore, from 

 which by repeated division (10 and 11), ncAv sporozoites, as in i, arise, and 

 begin the cycle afresh. 



Schuberg, Schaudinn. and Siedlecki, and, because of their theoretical 

 importance, I should like to summarize the essential points. 



One of these Coccidia lives in the intestinal cells of a small 

 centipede, Lithohius', in Fig. lai the parasite is shown as a so-called 

 ' Sporozoite,' that is, as a minute sickle-shaped cell, which at first 

 moves freely about the intestine of the host (i), but then soon 

 penetrates into an epithelial cell (2). There it grows to a spherical 



