I70 BULLETIN OF THE BUREAU OF FISHERIES. 



epithelium is forced to slide up over the surface of the shell on all sides, until the free 

 margins meet and fuse over the back of the larva, as may be understood by reference 

 to figures 59 to 61, plate xv, and 35 to 38, plate xi. 



So rapid is the overgrowth, especially in the case of implantation on the gills, that 

 it would seem that something more than the mere mechanical irritation produced by the 

 glochidium is concerned in causing the proliferation of the epithelium. We have, 

 therefore, carried out a series of experiments with a view to determining whether or not 

 a chemical stimulus is provided by the larva, and by using various methods have studied 

 the action of glochidial extracts on the epithelium of both fins and gills. The results 

 have been entirely negative, although the question has by no means been settled by the 

 experiments which have been thus far attempted. By further improvements in the 

 technique, some of the difficulties involved in the investigation, which is still in progress, 

 may be overcome. 



The process of implantation and cyst formation may be readily observed on the fila- 

 ments of an excised gill, which under favorable conditions will live long enough in a 

 dish of water to enable one to see the glochidium completely covered by the proliferated 

 epithelium. Figure 54, plate xiii, drawn from the living excised gill, shows the distal 

 end of a single filament bearing a glochidium of Unio complanatus which has become 

 nearly covered by the walls of the cyst. In this case the gill was cut from the fish two 

 hours after the infection and the drawing was made an hour later; immediately after 

 the excision of the gill this particular glochidium was hardly half covered. The same 

 glochidium was kept under observation, and two hours later (five hours after the infec- 

 tion) the sketch was made which is reproduced in figure 55, plate xiii. By this time 

 the cyst, which is seen to have very thick walls, was completed, and formed a prominent 

 mass near the end of the filament. Shortly afterwards the tissues of the gill began to 

 disintegrate, but for at least three hours they remained alive and the proliferation of the 

 epithelial cells proceeded rapidly, the entire process of cyst formation taking place in a 

 perfectly normal manner. 



The histological changes which the epithelium undergoes in the formation of the 

 cyst have been studied in this laboratory by Miss Daisy Young, and, as her results will 

 soon be published in detail, only a brief reference will be made in this place to the 

 essential points involved in the cellular changes occurring during implantation of the 

 glochidium. 



Figure 59, plate xv, shows a very early stage, 15 minutes after attachment, in the 

 formation of the cyst on the fin of a fish which had been infected with the glochidia of 

 Sytnphynota complanata. The section is taken transversely through the glochidium 

 and the free border of the fin on which the parasite has a firm grip. The mass of 

 tissue, consisting of epithelial cells, connective tissue, and blood vessels in the mantle 

 chamber of the glochidium, is the edge of the fin which w-as inclosed between the valves 

 when attachment was effected. Already the proliferation of the epithelium is beginning 

 in the neighborhood of the constriction, where two mitoses may be seen on the right in 

 the figure. At the edges of the wound caused by the closure of the shell some of the 



