8 UNIVERSITY OF MISSOURI BULLETIN 



mitotically and in no case has any evidence of amitotic division 

 been observed. 



Cell division is most abundant near the level of the con- 

 striction caused by the glochidium, and the cells formed in 

 this region of active proliferation push the ones above them up 

 over the surface of the shell. The edges of this growing tissue 

 ultimately fuse over the dorsal side of the glochidium, and the 

 wall of the cyst is thus completed. 



Implantation on Gills 



Fig. 1 shows a cross-section through a glochidium which 

 has been attached to a gill filament for fifteen minutes. The 

 epithelial cells near the glochidium have already begun to 

 proliferate. In this case, as in all the other infections I studied, 

 the formation of the cyst began at once, and not, as Schmidt 

 (15) observed, twenty-four hours after the infection. As 

 stated before, the cells divide most actively just below the 

 glochidium, and in this position a mitosis may be seen on each 

 side in fig. 1. Dividing cells are rarely ever found in the upper 

 part of the cyst during the early stages of its formation, but 

 as the lower cells increase in number they push the ones above 

 them up over the surface of the glochidium. Numerous red 

 blood cells and leucocytes are often found around the glochi- 

 dium before the cyst is fully formed (fig. 1). These come 

 from vessels of the gill filament which have been torn by the 

 glochidium in the process of attaching itself. Occasionally 

 epithelial cells which have been sloughed off from the edge of 

 the wound are found with these blood cells. 



Fig. 2 is a cross-section through a glochidium thirty min- 

 utes after attachment. The numerous blood cells seen in the 

 gill tissue just below the point where the glochidium is attached 

 are due to an extensive hemorrhage caused by the tearing of 

 vessels in this region. 



The wall of the cyst continues to grow rapidly, and the 

 progress made by the end of one hour may be seen in fig. 3. 

 The glochidium usually bites entirely through the epithelium 



