352 H. S. DAVIS 



IV. GENERAL DISCUSSION AND REVIEW OF LITERATURE 



Auerbach ('10) has recently given a very complete review of 

 the literature on the Myxosporidia and it is, therefore, super- 

 fluous to go into a full discussion of it at this time. 



1. Polymorphism 



That the polysporous and disporous forms of trophozoites 

 described in the preceding pages are one and the same species 

 is, I believe, certain. At first I naturally took them for entirely 

 distinct species, but as the work progressed it became more and 

 more evident that only the assumption that both forms belong 

 to the same species would account for the facts. In the first 

 place the two forms are practically always found together. More- 

 over, the spores in both cases are practically identical, and this 

 is, of course, the strongest evidence for considering the two types 

 of trophozoites only different forms of the same species.* The 

 development of the spore in the two types is very similar, differ- 

 ing only in unessential details. Also I would lay especial stress 

 on the fact that although the chromosomes in the polysporous 

 forms are much larger than in the disporous, the number and 

 shape is the same in both cases. In both forms the chromo- 

 somes can be grouped in three pairs differing somewhat in size, 

 the relative sizes of the chromosome pairs being the same. Fur- 

 thermore, both types of trophozoites ingest the erythrocytes 

 of the host, a phenomenon known to occur in only one other 

 species of Myxosporidia. These two species form, so far as I 

 know, the only examples of the ingestion of solid food in the 

 entire group of the Sporozoa. 



On the other hand I do not wish to minimize the fact that in 

 appearance and structure the two types of trophozoites are un- 

 like. In the examination of living material numerous examples 

 of small trophozoites were seen which could not with certainty 

 be assigned to either form. In cases where the ectoplasmic 



* It is well known that the structure of the spore is very constant in the 

 Myxosporidia, each species having its own characteristic spore which shows little 

 variation. So constant is the form of the spore that it is the principal character 

 used in distinguishing between the different species. 



