THE MYXOSPORIDIA, OR PSOROSPERMS OF FISHES. 79 



formed by the parasite. Tlielolian now believes a true membrane to 

 be absent, the i3seudo-membrane being merely the denser, most external 

 layer of the ectoplasm, peculiarly modified (coagulated and contracted) 

 under the action of the fixing and hardening reagents. It can then 

 take on the aspect of a membrane, the resemblance being sometimes 

 even further heightened by its exhibiting very definite stria3. 



Sections of a barbel's intestine showed connective tissue spaces 

 each inclosing a niyxosporidium with an often very well differentiated 

 external zone which presented a very distinct striation. Although at 

 first regarding this as a confirmation, Tlielolian, after a more thorough 

 examination, varying the observation methods and studying a great 

 number of sections of difierent myxosporidian species, became con- 

 vinced that these pseudo-membranes are artificial productions, the 

 result of a rouglier action of the reagents on tbe more exposed external 

 ectoplasmic layers, which action accentuates their differentiation and 

 exaggerates their characters. In fact this membraniform layer can be 

 seen to become continuous, without a line of demarcation, with the 

 ectoplasm proper. 



Further, a similar appearance was sometimes observed in sections of 

 the ifike's urinary bladder, where (the myxosporidia being free and 

 motile) there can be no question of a cyst membrane. Moreover, the 

 distinction is much more apparent in sections after the action of re- 

 agents (under which conditions the limit of the 2 layers is clearly indi- 

 cated and marked by a continuous, often very pronounced, line) than 

 in fresh preparations. 



Thelohan' says that, as Biitschli remarks, the age of the cyst can 

 be inferred from its size, the less advanced cysts being larger with a 

 central zone containing the older spores and an outer one containing 

 nuclei and spores in jjrocess of formation. The oldest cysts are small, 

 contain no nuclei, and spore formation has ceased, only developed spores 

 being x^resent. 



SPORE FORMATION 



GENERIC RELATIONS. 



This process exhibits differences which not only serve as ordinal 

 characters, but which appear also to stand in some sort of relation to 

 generic lines. 



Thus in Glugea we have polysporogenetic pansporoblastic spore for 

 mation within a myxosporidium, the pansporoblast not subpersistent 

 as a sporojihorous vesicle. 



In P/mtoj)/tora we have polysporogenetic pansporoblastic spore forma- 

 tion, no myxosporidium (completely transformed into pansporoblasts?), 

 the pansporoblast subpersistent as a sporophorous vesicle. 



»Aniial. de Microgr., 1890, ii, p. 204. 



