360 H. S. DAVIS 



failed to germinate or extrude their polar filaments after being 

 exposed to the caecal fluid for three hours, when the experiment 

 was discontinued. 



It appears probable, therefore, that the free spores, when taken 

 into the intestine of the host, germinate, and the sporozoites, as 

 free amoebulae, actively make their way into the urinary bladder. 

 Whether the amoebulae then go through an intercellular stage 

 as has been found by a number of investigators to be the case in 

 other species, it is impossible to say. While I have noted a 

 number of instances in which the epithelial cells of the bladder 

 contained an intracellular parasite I am not at all certain that 

 the parasite w^as a stage of S. dimorpha. The urinary bladder 

 and Wolffian ducts of C. regalis are often infected with a species 

 of Leptotheca in addition to S. dimorpha, and while there is no 

 difficulty in distinguishing the free forms of the two species, ow- 

 ing to their very different structure (especially as regards the 

 nuclei), I am by no means certain that it is equally easy to dis- 

 tinguish the intracellular forms. Certainly the nuclei of the 

 intracellular parasites are quite different from those of the 

 free-living trophozoites of S, dimorpha. 



5. Fertilization 



Finally comes the question: Is there any evidence of a sexual 

 process in the life-history of S. dimorpha? We have already 

 seen that no sexual phenomena are associated with sporulation 

 in this species, so that if there be a sexual process, it must occur 

 at some other stage. Although the two nuclei of the sporo- 

 plasm are still distinct in the fully matured spore, it is probable 

 that they fuse later (possibly at the time of germination), as 

 has been found to be the case in so many species of Myxosporidia. 

 Such a fusion would be a case of endogamy, but there is some 

 evidence that exogomy also occurs. 



In Giemsa smears there are occasionally seen small mono- 

 nuclear cells which are quite different in appearance from the 

 mononuclear trophozoites shown in figures 8 and 9. Not only 

 are they much smaller, but the amount of cytoplasm is relatively 



