410 



THE PROTOZOA 



The development of Sphceractinomyxon (Fig. 170) begins with a uninucleate 

 amcebula (Fig. 170, A), .which represents the planont phase, and doubtless 

 multiplies by fission, since these parasites, though rare generally, occur abun- 



Fio. 170. Stages in the development of Sphaeractinomyxon stylci. A, Amoebula 

 with a single nucleus ; B, binucleate amcebula ; 0, the two nuclei of the 

 preceding stage have each divided, and the body is divided into four cells, 

 two peripheral envelope-cells and two central germinal cells ; D, the space 

 enclosed by the 'envelope- cells has become greatly enlarged, and the two 

 germinal cells have divided into four ; E, the germinal cells have increased in 

 number by repeated division into sixteen, which as gametes are copulating 

 in eight pairs ; F, O, two stages in the fusion of two gametes ; H, the zygote 

 has divided into two cells ; in one of these (on the left), which will form the 

 germinal mass of the spore, the nucleus is at this stage undivided ; in the 

 other (on the right), which will form the accessory structures of the spore, the 

 nucleus has divided to form six nuclei ; /, further stage of the preceding : 

 in the germinal cell (on the left) the nucleus has divided into several nuclei 

 of unequal sizes ; the sporal tissue (on the right) is represented by six cells, 

 three peripheral, the parietal cells, and three central, the capsulogcnous cells ; 

 J, ripe spore, enclosed by a sporocyst composed of three valves meeting in 

 sutures ; the germinal mass (g.), separate in the preceding stage, has migrated 

 into the interior of the spore, and contains an immense number of nuclei ; 

 at the upper pole of the spore are seen the three polar capsules (p.c.). After 

 Caullery and Mesnil (769). 



dantly in the infected animals. The planont phase is succeeded by a binu- 

 eleate stage (Fig. 170, B), the origin of which is uncertain, but which most 

 probably arises from an association and plastogamic fusion of two planonts, 

 and which represents the spore-forming phase. 



