484 



E. A. MINCHIN. 



fig. 21) independently as consisting each of a single perforated 

 cell, placing the internal gastral cavity in communication with 

 the outer world through the body-wall. We differed, how- 

 ever, in our views as to the origin of these pore-cells. Bidder 

 regarding them as derived from metamorphosed " endodermal " 

 collar- cells, while I described them as arising from immigrated 

 cells of the " ectodermal " flat epithelium. More recently 

 Dendy (1893, p. 214) has thrown doubt on these statements, 

 and is inclined to regard the pores as '' inter-cellular and 

 not intra-cellular in nature/' '^simply gaps between cells.'* 

 It is evident, therefore, that Dendy has never studied the 

 pores in surface views of material suitably preserved and 

 mounted, for a single glance at such a preparation would be 

 sufficient, I think, to dispel his doubts for ever. On the other 

 hand, so long as the matter is only studied in series of sec- 

 tions, chopped up in paraffin from material thrown into alcohol 

 and stained in borax carmine, it will be possible to doubt 

 anything. 



No fact in the histology of Ascons is more easily demon- 

 strable, even to the tyro in these matters, than that each pore 

 is, in the expanded condition, a single perforated granular 

 cell, of a peculiar and easily recognisable type. Compare 

 especially figs. 10, PI. 38, and 39, PI. 41 ; also figs. 1, PI. 38, 

 18 and 18a, 19 and 19«, and 20, PI. 39, and figs. 49, 50, and 

 52, PI. 42. Seen in surface view the pore has two openings, a 

 sm.aller external or dermal aperture [derm, ap.) on a level with 

 the flat epithelium, and a larger internal or gastral aperture 

 [gastr. ap.) on a level with the collar-cells. The dermal open- 

 ing perforates a thin membrane which is usually more or less 

 free from granules, and stretches like a tympanum across the 

 cavity of the pore-cell. The gastral opening is surrounded 

 by a thick and granular wall, smooth on the inner side, but 

 running out into points and processes on the outer side — a con- 

 figuration due, as can be seen at once from fig. 39 on PI. 41, 

 to the pressure of the adjacent collar-cells. The projecting 

 points fit into the interstices between the collar-cells, each 

 depression between two projections being the impression made 



