THE SPOROZOA 289 



have seen, the nucleus, at first single, divides twice to produce four 

 nuclei (p. 285 supra). The sporoplasm generally fills the spore 

 completely, but in Ceratomyxa it is relatively small, and lodged in 

 one valve of the shell (Fig. 93). In the family Myxobolidae the 

 sporoplasm contains a peculiar vacuole, enclosing a substance which 

 stains a reddish brown with iodine, and exhibits some of the 

 reactions of glycogen (Fig. 99). No such vacuole is found in the 

 other families of Phaenocystes, but in Glugeidae a clear vacuole is 

 almost constantly present at the broader (posterior) extremity, 

 which does not, however, exhibit any characteristic reactions. 

 Besides this the sporoplasm of Glugeidae often contains numerous 

 fine fatty globules. 



(d) Development of the Spores; Infective Processes. The develop- 

 mental period which intervenes between the ripe spores and the 

 youngest trophic stages, is the least known period of the life-cycle, 

 as in all other Sporozoa, but the observations of Thelohan and 

 Doflein make it possible to form a tolerably clear idea of the events 

 that take place. The spores are set free from the parent trophozoite, 

 apparently, by the death and disintegration of the latter in all cases, 

 and we have next to consider the paths by which they leave the 

 body of the host. In this process we may distinguish conveniently 

 between natural and non-natural modes of exit. Thus in the case 

 of species which live in the gall or urine of their hosts, the spores 

 doubtless pass out of the body by natural channels. In the case 

 of tissue-infecting forms, if the cysts are formed near the surface of 

 the skin or the lining of the alimentary canal, they may excite sup- 

 puration, and so work their way to the adjacent surface when the 

 abscess bursts, setting free the contained spores. In both these 

 cases the death of the host is not necessary in order that the spores 

 may be set at liberty ; in the first case the host is not inconvenienced ; 

 in the second case a certain amount of damage is done, but not 

 necessarily enough to destroy the host, which may live to harbour 

 other parasites and to disseminate more spores. But in the case of 

 species inhabiting more deeply situated tissues, the spores can only 

 be set free by the death and disintegration of the host, and if this 

 event be too long delayed, the result is fatal to the parasite ; that 

 is to say, the spores, if retained too long within the body of the 

 host, pass their prime and die, as is so frequently the case in other 

 Sporozoa which cannot leave the body of the host by natural means. 

 The spores never, apparently, develop further in the host in which 

 they are formed. 



The spores when set free sink to the bottom of the water, when 

 the host is an aquatic animal, or in other cases, as in that of the 

 silkworm, are left lying about in the host's usual haunts. The 

 infection of a new host is apparently always a casual one. The 

 ingenious experiments of Thelohan [113] showed that the spores must 



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