FIG. 6. A small part of the cytoplasm of an epithelial cell from a gill filament of 

 Fundulus heteroclitus. The cytoplasm is obviously packed with profiles of tubules 

 joined together in a dense three-dimensional lattice. The membranes limiting these 

 structures are free of particles and the whole system is properly interpreted as 

 representing the smooth ER of this cell. In one place, marked by an arrow, there 

 is a short profile of a lamellar vesicle studded with particles. These seem to be 

 extremely rare and are the only representatives of the rough ER found in these 

 cells. 



The epithelial cell, of which a part is shown here, has been identified as a salt- 

 secreting unit such as are regularly found in the gills of saltwater fishes. A similar 

 pattern of ER development is common to cells of this type where they have been 

 studied. Chloride-secreting, parietal cells of the stomach are quite similar ( Sedar, 

 1961 ; Ito, 1961 ) . A part of an adjacent epithelial cell, shown at the lower left, 

 illustrates the diversity which may be evident between two closely associated units. 

 Its cytoplasmic matrix is filled with fibrillar elements (K), perhaps keratin in 

 nature. Its mitochondria show fewer cristae than are found in the mitochondria of 

 the salt cell, and the ER is represented by a relatively few profiles. Mitochondria 

 are indicated at m, glycogen granules at gl. {Courtesy of C. W illiam Philpott.) 

 Magnification: 48,000 X. 



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