GLOCHIDIA — FACTORS UNDERLYING ENCYSTMENT 489 



If an excised gill filament of a bass is agitated with glochidia 

 in a watch-glass, many larvae become attached within a minute. 

 The tissue bordering the edge of the valves begins to proliferate, 

 and in a few hours the glochidium is overgrown or encysted. In 

 one instance observed, cyst formation was so rapid that the large 

 larva of Lampsilis luteola was enclosed in two hours. There is 

 one marked difference, however, between ordinary encystment 

 and that on an excised filament. In the latter, cellular prolifera- 

 tion does not stop when the glochidium is evenly enclosed by 

 overgrown tissue; it continues until large, unsymmetrical cysts 

 result.^ Superfluous new growth also is the rule in the repair of 

 simple cuts on excised filaments (fig. 15). 



Cyst formation appears to be fundamentally a response of a 

 reparative nature on the part of the gill tissues. To restore the 

 epithelial continuity the glochidium is overgrow^n as a foreign 

 body. That the reaction is not a response to some chemical or 

 vital influence emanating from the glochidium has been proved 

 by experiment: 



Thin aluminum- or lead-foil was cut into oblong strips 0.15 mm. wide 

 and 0.40 to 0.60 mm. long. When bent, these form a minute V- or 

 U-shaped clip, smaller than many glochidia (p. 465). In fact, practice 

 enables one to make clips smaller than can be handled successfully with 

 ordinary instruments. Under binocular enlargement, such clips were 

 attached by needles to excised gill filaments of the bass, Micropterus 

 salmoides, firmly clamping the tissue. As figures 7 to 14 show, the 

 clips become overgrown by stages counterfeiting glochidial encystment. 



The stimulus operative in cyst formation, therefore, is be- 

 lieved to be essentially mechanical.^ It appears, nevertheless, 

 that there is some regulatory factor superimposed over the simple 

 reparative tendency, and that when a filament is separated from 



^ It is interesting to record in this connection that the cysts on fishes made 

 immune by repeated infection (Reuling, '19), are characteristically large and 

 swollen. 



* The reduction of the terms of encystment to the mere overgrowth of an 

 abrading foreign body by host tissue may prove inadequate in some cases. It 

 will be recalled that parasitic trematodes and copepods on the gills of fishes 

 remain unencysted. Dr. A. D. Howard ' s observed the glochidia of Quadrula 

 heros only partially encysted after sever -'s' attachment to Necturus, which, 



however, is not the natural host for thi; ^el. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 33, NO. 2 



