196 ^ JEREMIAH S. FERGUSON 



licles in Mustelus which were well filled with colloid was .077 mm., 

 w^hile in a similar number of follicles which were either devoid of 

 colloid, or nearh^ so, the height of the epithelium averaged .087 mm. 



Each epithelial cell possesses a fairly distinct cell-wall. Many 

 cells appear to have a well marked cuticular border which appears 

 to be more highly refractive than the endoplasm, but wherever the 

 colloid lies in contact with the surface of the cell the cuticular 

 border is obscured. I have also noticed that it is less pronounced 

 in the thinner sections so that I am inchned to regard it as an 

 optical diffraction line rather than a true cuticular membrane.. 

 The conformation of the free ends of the epithelial cells tends to 

 confirm this opinion. These cells project slightly into the lumen of 

 the follicle by means of a somewhat convex free border so that the 

 height of a cell is greater in its axis than at its margin. Thus the 

 lower portions of a cell will, in the thicker sections (.010 mm. or 

 more), show through the taller central or axial portions and so 

 account, at least, for a portion of the cuticular appearance. 



The exoplasmic membrane is specially distinct in the epithe- 

 lium of the Elasmobranch thyroid gland. Baber ('81) called atten- 

 tion to this fact, and desciibed it as an intercellular network 

 enveloping the cells and connecting the lumen of the follicle with 

 the surrounding tissue spaces. If the lining epithelium of the 

 follicle be cut parallel to the surface the resulting sections will 

 show the membrane as a distinct mosaic within whose meshes the 

 cells are apparently contained. It appears to me that this mosaic, 

 which is distinct from the intercellular colloid observed by Lang- 

 endorf (see page 200), is rather to be regarded as a cell membrane 

 than as an intercellular substance, for there are many portions 

 where in thin sections a narrow intercellular space is distinctly 

 apparent and is bounded on either side by the exoplasmic mem- 

 brane of adjacent cells. Occasionally the cells are separated by 

 wider intervals through which the follicular lumen is placed in 

 direct communication with the surrounding tissue spaces. 



There appears to be no distinct basement membrane upon 

 which the follicular epithelium may rest. At intervals the cells 

 are invested with a very small amount of loose connective tissue 

 (fig. 11), but in large part the epithelium rests directly upon the 



