371 J. T. CUNNINGHAM. 



whether there is but one layer of cells or more than one. A 

 nucleus is often visible in the cells. In sections the epithelium 

 is often badly preserved ; its cells seem to be very delicate and 

 easily destroyed, and the presence of the concretions here also 

 obscures the cell-outlines. When the epithelium is well pre- 

 served and stained it has the following appearance. Near its 

 base the nuclei of the cells are seen surrounded by the concre- 

 tions, and the vertical boundaries of the cells are also visible. 

 Towards the cavity of the gland the epithelium terminates in a 

 number of clear rounded projections ; in sections I have never 

 been able to discover the cilia. It is evident that the rounded 

 projections are the vesicular cells still attached ; it is these 

 which drop off so easily and are seen free in teased prepara- 

 tions. Their remains can also be often seen in sections in the 

 lumen of the gland, and the process of secretion is evidently 

 effected by these cells becoming detached in the renal cavities, 

 and then breaking up and allowing the concretions to escape. 

 But there is evidently another layer of cells in the epithelium 

 beneath the vesicular projections, cells which contain concre- 

 tions but are not yet vacuolated, and which show a nucleus ; 

 these take the place of the mature secretory cells when they 

 fall into the lumen, and go through the same course of deve- 

 lopment as these. Von Jhering has observed the stages of 

 this development in the kidney cells of Tethys,^ in prepara- 

 tions of the fresh kidney. I have not been able to trace the 

 process in this way in Patella. Very often the renal epithe- 

 lium in sections does not show any vesicular projections, these 

 having disappeared in the course of preparation ; but the pro- 

 jections present variations in size and prominence which prob- 

 ably correspond to variations in secretory activity. 



The structure of the two renal organs in Patella is exactly 

 the same ; the difference in colour which they present to the 

 naked eye is due to the fact that the urinary concretions are 

 more numerous in the right kidney than in the left. The cavities 

 of the two kidneys do not communicate. 



I have also cut some series of sections through the nuchal 

 1 Von Jhering, ' Morpli. Jalirbucli,' Bd. 2. 



