72 



PROTOZOA 



sinks into it with a little water, to form a nutritive vacuole. 

 Fission is the commonest mode of reproduction, and temporary 

 plastogamic unions are not uncommon. Arising from these true 

 conjugations occur, two and two, as described by Schaudinn. A 

 gelatinous cyst wall forms about the two which are scarcely more 

 than in contact with their rays withdrawn. Then in each the 

 nucleus divides into two, one of which passes to the surface, and 

 is lost (as a "polar body"), while the other approaches the 



Fig. 19. — Actinosphaerium eichornii. A, entire animal with two contractile vacuoles 

 {c.vac) ; B, a portion mucli magnified, showing alveolate cytoplasm, pseudopodia 

 ■with axial rods, non-nucleate cortex [cort), multiple nuclei {nu) of endoplasm [med), 

 and food- vacuole {chr). (From Parker and Haswell.) 



corresponding nucleus of the mate, and unites with it, while at 

 the same time the cytoplasms fuse. Within the gelatinous cyst 

 the zygote so formed divides to produce two sister resting spores, 

 from each of which, after a few days, a young Actinophrys escapes, 

 as may take place indeed after encystment of an ordinary form 

 without conjugation. 



The axial rods of the pseudopodia may pass either to the 

 circumference of the nucleus or to a central granule, correspond- 

 ing, it would appear, to a centrosome or blepharoplast ; or again. 



