262 R. T. YOUNG 



on the one hand and the basement membrane on the other. The 

 ciha are probably continuous with the epithehal fibrillae and pos- 

 sibly such a continuity also exists between the latter and those 

 of the basement membrane. 



In vertical sections, it is impossible to distinguish cell bound- 

 aries in the epithelium, but in tangential ones occasional poly- 

 gonal areas may be seen which probably indicate its cellular 

 character. Treatment with silver nitrate, moreover, reveals the 

 cell boundaries very clearly. 



Directly below the epithelium is a definite basement membrane 

 which, in most of my material, appears homogenous, but in one 

 specimen shows a very evident fibrillar character. The basement 

 membrane here is very lightly stained and appears as though 

 the ground substance had been in some way removed from the 

 meshes of the fibrillae, thereby rendering them apparent. These 

 form a loose network, the meshes of which run in general parallel' 

 to the surface and stain similarly to those of the epithelium. 

 Externally, the membrane presents a scalloped appearance, which 

 may possibly be due to the insertion of the epithelium, which 

 in fixation has shrunken a little away from the membrane, 

 thus bringing its scalloped appearance more plainly into view. 

 Whether there is any anastomosis between the fibrillae of the 

 basement membrane and those of either the epithelium or the 

 parenchyma is a point which I must leave undecided. That 

 this membrane is chemically different from the other tissues is a 

 point which is indicated by its differential stain. In sections 

 strongly counter-stained with eosin, the epithelium is red or 

 pinkish while the basement membrane varies from colorless or 

 light straw color to gray or pale brown. 



The ciliary layer, epithelium and basement membrane show 

 wide variations in thickness, not only in different specimens, but 

 in different parts of the same specimen. These differences are 

 doubtless due in great measure to differences in amount of con- 

 traction of the tissues in fixation and to distortion produced 

 thereby; they may also be due in part to differences in the plane 

 of section; probably also to differences in development of the 



