DEVELOPMENT OF A MYXOSPORIDIAN 359 



S. diniorpha each pansporoblast eventually forms twelve — not 

 fourteen — cells and a careful study of a large number of pansporo- 

 blasts has failed to show any trace of 'residual nuclei.' It seems 

 strange that there should be such fundamental differences in 

 the process of sporulation in forms so closely related to each 

 other as the species under discussion, but in the present state of 

 our knowledge it is impossible to harmonize the discordant 

 accounts. 



4. Mode of infection 



I have no direct evidence of the method by which S. dimorpha 

 gains entrance to a new host. The investigations of Auerbach 

 ('09, '10) on Myxidium bergense and Erdmann ('11) on C'hlor- 

 omyxum leydigi have shown that, in all probability, the infec- 

 tion of new hosts takes place through the digestive tract. The 

 sporozoite becomes free in the intestine and, as an amoebula, 

 wanders actively up the bile duct to the gall bladder. Probably 

 the infection of new hosts by S. dimorpha is accomplished in 

 the same way, although the universal occurrence of the parasite 

 in the urinary bladder of Cynoscion regalis is difficult to ex- 

 plain as the result of purely accidental infection. 



There can be little doubt that the spores pass to the exterior 

 soon after becoming free from the mother trophozoite, for in 

 no case were large numbers of spores found in the bladder, al- 

 though trophozoites in all stages of sporulation were abundant. 

 Spores of S. dimorpha, when placed on the slide without previous 

 exposure to sea water, and mixed with a drop of fluid from the 

 pyloric caeca of the host, usually germinated within five to fif- 

 teen minutes. In most cases of germination the valves of the 

 spore membrane separated along the sutural line followed by 

 the emergence of the sporozoite which crept out, by an active 

 amoeboid movement (fig. 47). In some cases the sporozoites 

 were observed to disintegrate shortly after emergence (prob- 

 ably due to the action of the digestive fluid) ; others remained 

 intact for the length of time they were under observation. In 

 most cases, but not all, the filaments were extruded from the 

 polar capsules on the addition of the caecal fluid. A few spores 



