214 



THE EVOLUTION THEORY 



In the Coccidia, which are oxtreiuely simple unicellular organisms, 

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

 mixis are more extensive and nune 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 ditlerences ai'e as marked as those tliat 

 iisually occur in multicellular animals. 



We owe our present knowledge of tliese processes especially to 



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Fig. 121. Life-cycle of CoLXuHiim litliobil, 11 coll-iianisite of tho centipede 

 LithoUus; after Schiradinn. i, a ' sporozoite ' ; a, the same penetrating into nn 

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

 division ; 4, the same dividing, and 5, liroaking up into numerous pieces whicli 

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

 into epithelial cells and repeat tho liistory, or jiass on to the phase of sexual 

 reproduction. In the latter case, after olimiuating a portion of the nucleus 

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

 the mother-cell they produce mioroganieto.s (or sperm-cells), 7 and 7 a. Tlio 

 penetration of a sperm-cell into an ogg-ooll (amphimixis) is shown in 8, tho 

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

 Avhich by repeated division (10 and 11), new sporozoites, as in i, arise, and 

 begin the cycle afresh. 



(Schuherg, Schaudinn, and Siedlecki, and, because of their theoretical 

 importance, I should like to summarize tlie essential points. 



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

 centipede, Lithodhu^; in Fig. 131 tlie 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 (a). There it grows to a spherical 



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