382 ROYAL SOCIETY OF CANADA 
figure 11 present a typical epithelial character, whilst others are pro- 
jecting partly into the canal and were probably arrested in the process of 
further thylosal development. In figure 12 the cavity is bounded by a 
fairly regular epithelium, none of the cells of which jut out into the 
space as in figure 11. In this manner definite cysts are formed, the 
cavities of which are remnants of the original canal. 
That these cysts are limited in extent is seen from the fact that a 
little lower down the whole passage is blocked by irregular thylosal cells. 
Figure 13 is a drawing from the same region as that represented in 
figure 12, but taken at a lower focus, and shows the complete blocking 
up of the cavity. 

Fig. 13.—Pteris aquilina, same as fig. 12, but at a lower focus. Showing canal 
completely blocked with thyloses, Th. X 367.5. 
The origin and possible function of these vessels and cysts is not 
very clear and can only be conjectured. 
The vessels probably represent thyloses which began to grow into 
the canal at an earlier period than the other thyloses which surround 
them, and, having plenty of space to develop in, grew greatly in length 
and breadth and assumed a circular outline. This would also explain 
the usually greater thickness of their walls. The epithelial cells, on 

