284 - THE SPOROZOA 



of the pansporoblast divides repeatedly by mitosis, to form several 

 nuclei, the normal number being about ten (Fig. 90, b, c, d). The 

 protoplasm at the same time segments within the envelope into two 

 masses, the definitive sporoblasts ; this may take place before, or 

 after, the full number of nuclei are formed in the pansporoblasts. 

 The two sporoblasts when completely developed have each three 

 nuclei, since four of the ten original nuclei are cast out as 

 residuary nuclei, and undergo degeneration (Fig. 90, e). Each 

 sporoblast now begins to secrete a cuticular spore-membrane at the 

 surface, and within the membrane the protoplasm segments into 

 three portions centred round each of the three contained nuclei 

 (Fig. 90, /). Of the three corpuscles or cells thus formed, two are 

 rather smaller than the third ; the former are the two capsulogenous 

 cells; the larger corpuscle is the spwoplasm, and its nucleus divides 

 into two, a division which sometimes takes place at an earlier stage, 

 so that the undivided sporoblast may contain four instead of three 

 nuclei. 



Each capsulogenous cell gives rise to a polar capsule in the 

 following way. A clear spherical vacuole first appears near the 

 nucleus of the cell. At some point, which is not constant, in the 

 wall of the vacuole, a bud of protoplasm grows into the interior of 

 the vacuole, pushing aside the clear substance contained in the 

 latter (Fig. 90, g). The bud of protoplasm becomes a little pear- 

 shaped body, surrounded by a, clear envelope which is derived from 

 the contents of the vacuole. At first connected by a stalk with the 

 point at which it took origin, the pear-shaped body becomes free 

 through severing of this connection, and then takes up a definite, 

 specific orientation with regard to the spore as a whole (Fig. 90, h). 

 At the surface of the pear-shaped body a membrane is formed, and 

 in its interior a spirally-coiled filament is developed. The polar 

 capsule, when fully formed, has a striking resemblance to a 

 Coelenterate nematocyst, since the coiled thread can be shot out 

 upon suitable stimulation (Fig. 97). Round the polar capsule are 

 found the remains of the capsulogenous cell and its nucleus, but 

 they soon degenerate and disappear. 



When the development is completed, there are found, still 

 contained in the envelope of the pansporoblast, two spores, each 

 enclosed in a tough membrane, within which are the polar capsules 

 and a little binucleate mass of sporoplasm which represents the 

 single sporozoite. The envelope soon breaks down, and the spores 

 are then found scattered in the endoplasm of the trophozoite. 



The development above described is that typical for the Phaenocystes, 

 but even in this order there is considerable variation, and in the 

 Cryptocystes the deviations from the type above described are still more 

 pronounced. 



In the Phaenocystes, apart from individual abnormalities (Fig. 90, i), 



