SPOROZOON PARASITES OF FISHES. 1 99 



there are numerous deep fibers containing many large cells, which vary in size and have 

 conspicuous nuclei. (Fig. i8, pi. xxi.) These are confined by the sarcolemma to a 

 very few fibers and extend for a long distance through them. A small cavity only is 

 excavated about each cell. They are usually isolated, though two or more may occupy 

 the same cavity. The sarcoplasm in such cases is much atrophied, being uniformly 

 granular or homogeneous. A sharp line of demarcation exists between the infected 

 and uninfected parts of the muscle fiber, the former being degenerate and the latter 

 striated and normal. Situated amongst the fibers containing the Protozoa are others 

 lacking them but atrophied in a typical manner, the sarcoplasm being broken into 

 irregular fragments. There are several other foreign and unnatural structures in the 

 sections just referred to, about which the details are given on page 203. Muscle fibers 

 packed with blood tissues and degenerate nuclei have not been found in any of the 

 sectioned tissues which contain unmistakable cases of Myxosporidia; but no special 

 significance has-been attributed to this fact. 



Smears of gill filaments stained with Giemsa stain present the following conditions : 

 Both normal and degenerate tissues are encountered. In some places the cartilage 

 supporting structures have been attacked and are partly disintegrated. The general 

 external form of the supporting tissue, including the surrounding connective tissue and 

 epithelium, are, as a rule, partly maintained; but elsewhere the degeneration is com- 

 plete. EpitheHum and connective tissue cells disappear completely, leaving the elastic 

 fibers and blood elements. Here, as elsewhere, the nuclei of the latter are most persistent, 

 especially those of the erythrocytes. A large portion of the expressed fluids is composed 

 of an acidophile substance containing odd-shaped portions of the fused nuclei. The 

 spaces between the chromatin threads of the latter having become much dilated, fuse 

 and form large masses of network. These are mechanically separated on crushing the 

 tissue. Such masses of nucleic acid or degenerate chromatin have unbroken connections 

 with the normal blood in the arteries or veins of the less disturbed tissue. Where the 

 blood emerges from partly degenerated blood vessels, they are filled with atrophied 

 erythrocyte nuclei. It seems probable that very large masses of homogeneous eosinophil 

 material, which are constantly associated with the degenerate gill tissue, are derived 

 from haemoglobin, lecithin, etc., of the stroma. 



Myxospores abound in these degenerate gill tissues, especially in the purulent 

 residues of degeneration where nothing else remains recognizable. They also occur 

 deep in the connective tissue near the cartilage and amongst the capillaries. The 

 spores, developing spores, sporoblasts, and pansporoblasts, in all stages, are clearly 

 defined, apparently unaffected by the conditions where tissue cells have become wholly 

 atrophied. This fact, together with the great abundance of m>'xospores and developing 

 myxospores, both occurring in considerable clusters, prove beyond question that the 

 primary cause of necrosis in this case is the Myxoboliis. No bacteria or other possible 

 agents have been encountered. 



BACTERIA ASSOCIATED WITH ATROPHIED TISSUES. 



The small bacillus above referred to (p. 195) varies greatly in size. The smallest 

 (fig. 8, pi. xx) measure less than 0.711 in thickness and 1.5// in length. The large ones 

 (fig. 9, pi. xx) average i.5« in thickness and 7/( in length. The former are homogeneous 

 when stained. The latter frequently appear to have very conspicuous granules just 



