DEVELOPMENT OF A MYXOSPORIDIAN 353 



processes are absent, the small polysporous trophozoites are 

 often strikingly like the larger vegetative trophozoites of the 

 disporous form. However, when stained there is no difficulty 

 in distinguishing between the two types since as regards the nu- 

 clei they are very distinct. In a very few instances I have found 

 individuals in which the nuclei were not typical for either type, 

 and it may be that they were transition forms, but such tropho- 

 zoites occurred so rarely in my preparations that I have been 

 unable to satisfy myself as to their significance. Possibly transi- 

 tion forms occur only at certain seasons, but this must, for the 

 present, remain pure conjecture. 



Auerbach ('09 and '10) finds that Myxidium bergense forms 

 both monosporous and polysporous trophozoites, but there is no 

 such difference in the appearance of the two forms as in S. 

 dimorpha. According to the same author (Auerbach '09a) 

 Zschokella may be either monosporous or disporous, while 

 Amerizew ('11) finds that the trophozoites of a species of Myx- 

 idium in the gall bladder of Cottus scorpius may form from one 

 to three spores. 



2. Multiplicative reproduction 



Doflein ('98) has distinguished between multiplicative and 

 propagative reproduction in the Myxosporidia. The former 

 results in the increase of the organism within the host, or in other 

 words in autoinfection, while the latter furnishes a means by 

 which the parasite may pass from one host to another. 



Plasmotomy is held by most writers to be the common method 

 of multiplicative reproduction. However, in S. dimorpha, 

 while plasmotomy has been observed in individuals after being 

 removed from the bladder, there is no evidence that this is a 

 common occurrence under normal conditions. In sections and 

 Giemsa smears I have never seen multinucleate individuals 

 which showed any evidence of having recently divided. In 

 the disporous forms the character of the nuclei is such that plas- 

 motomy, if it occur, should be easily detected; while the large 

 numbers of this form which are practically always present indi- 

 cate that some form of multiplicative reproduction is common, 



