494 B. A. MINCHTN. 



Had the drawing (fig. 41) been continued as far again to the 

 right, collar-cells would have appeared in the section. 



If now we follow up the layer of granular epithelium lining 

 the oscular rim in the direction of the margin of the osculura, 

 it is found to lose its distinctive characters by degrees, be- 

 coming more and more similar to the epithelium of the dermal 

 surface. PI. 41, fig. 42, shows an epithelial cell from about 

 halfway up the oscular rim, obviously intermediate in its 

 characters between the cells figured in figs. 38 and 40 respec- 

 tively. Finally, near the actual margin of the osculum the 

 epithelium is quite of the ordinary type, and is continuous 

 where it turns the edge with the general flat epithelium 

 covering the exterior. There is, in fact, a gradual transforma- 

 tion in the oscular rim of ordinary cells of the flat epithelium 

 into porocytes, the changes consisting of (1) increase in size 

 of the cells ; (2) increase in the number and size of the 

 granules; (3) slight increase in the size of the nucleus, and 

 corresponding decrease in its staining powers. 



If, on the other hand, we follow the porocytic epithelium 

 downwards instead of upwards, we find as we approach the 

 limits of the collar-cell layer (PI. 41, fig. 38) the porocytes be- 

 coming more compact and definite in their outlines, and hence 

 less like epithelial cells. Lower still (figs. 38 and 39) we find 

 them lying between the collar-cells, and there becoming 

 gradually transformed into the familiar pores. In other 

 words, the collar-cell layer continually extends its upper limit 

 by proliferation of its cells, and as it does so the porocytes 

 become surrounded by and enclosed amongst the collar-cells. 

 As soon as this occurs they spread out greatly and form a large 

 cavity, open towards the gastral space, and by perforation of 

 the outer wall of this cavity they acquire a dermal aperture. 

 In fact, they go through just the changes which a contracted 

 pore cell goes through when it becomes expanded. 



In all the Ascons of the genus Clathrina studied by me, 

 and apparently also in the species of Leucosolenia, cer- 

 tainly in Ascandra falcata, the same state of things is 

 to be found. I found the oscular rim in tiie species blanca, 



