IMPLANTATION OF THE GLOCHIDIUM ON THE FISH 9 



with the result that the ventral border of the shell comes in 

 contact with the underlying mesodermal tissue, which may, 

 therefore, contribute to the formation of the lower part of the 

 cyst and in which one often sees an indication of slight prolifer- 

 ation (fig. 3). 



Fig. 4 represents a section through a glochidium which 

 has been attached to the gill for three hours. The tissue grow- 

 ing up on either side is meeting above the glochidium. 



After the complete implantation of the glochidium the 

 cells composing the cyst continue to divide, and the region of 

 proliferation is no longer limited to the lower part of the cyst, 

 but mitoses may often be found higher up around the glochi- 

 dium, as is shown in fig. 5, which is a section through a glochi- 

 dium forty-four hours after attachment. The cyst has become 

 thin and compact and the cells are closely pressed together, 

 especially next to the glochidium where the nuclei of the cells 

 flattened against the shell become elongated and oval in out- 

 line. Numerous mucus cells may -be seen in the outer part of 

 the cyst. 



Fig. 6 is a section through a glochidium which has been 

 attached to the gills of the fish for twelve weeks. The total 

 period in this case was thirteen to sixteen weeks, so that the 

 metamorphosis is at this time almost completed, and the tissue 

 of the cyst has assumed the appearance which it retains until 

 the liberation of the glochidium. The accumulation of lymph 

 has brought about the formation of large intercellular spaces 

 filled with this fluid. These spaces gradually increase in size 

 and the epithelial cells become greatly elongated and con- 

 nected by long protoplasmic processes. Many of the large 

 spaces between the cells contain red blood cells and leucocytes. 

 This vacuolated appearance is most marked just below the 

 glochidium and in the outer part of the cyst, while the tissue 

 next to the glochidium still remains compact and the cells lie 

 closely pressed together. The gill-filament for some distance 

 around the parasite is affected by this accumulation of lymph 

 and presents much the same appearance. Even after the inter- 



