276 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



mentioned as existing between the fibrils a fine granular substance, 

 which osmic acid differentiated, and he suggested that it was a cement- 

 ing substance. I can confirm that observation to this extent, that in the 

 vom Rath preparations the mantle takes a solid, gray appearance, which 

 is quite distinct from that of the vitreous humor and the axis. In 

 some cases the fibrils are obscure. 



The swelling of the mantle in a mixture of one part weak Flemming's 

 solution and nine parts water, or in a dilute physiological salt solution, 

 presents another aspect of the case. In the preparation thus macerated 

 the axis remains of the normal diameter and length ; but the mantle 

 (still showing fibrillae) is frequently found to be so elongated that the 

 axis of the rod appears to lie in a water-filled cavity a half or a third 

 longer than itself. In such a case we should expect that the mantle 

 fibrils would be free to withdraw and remain with the axis, unless they 

 were held fast in a resistant substance. Since they do not, it seems that 

 they must be imbedded in a matrix which swells up in the presence of a 

 fluid which is more dilute than the normal fluids. 



There is still another observation which bears on the question. It 

 has been mentioned in the general account of the eye that the lens may 

 rest directly against the rods. Hence one sometimes finds rods which 

 are distorted from the usual shape by the pressure of the lens against 

 them. Although the pressure is thus considerable, one can observe 

 that the mantle has its normal thickness and the fibrillae are, as usual, 

 at right angles to the surface of the mantle — not bent or pressed flat 

 against the axis, as one would expect them to be if they protruded freely 

 into the vitreous humor. These considerations make it probable that 

 the mantle is not merely a border of protruding fibrillar brushes bathed 

 by the vitreous humor. The fibrils seem to lie in a cap of formed sub- 

 stance, which is chemically distinct from the vitreous humor and arises, 

 not from the cells which secrete the vitreous humor, but from the rod- 

 axis, which is merely a prolongation of the sensory cell itself. In a sense, 

 therefore, there seems to have been a grain of truth in the idea of the 

 older investigators as to the nature of the rod-mantle. But the view 

 that it is cuticular is so inadequate, as compared with the conception of 

 it as a border where recipient fibrillae find free end, that it must be 

 abandoned in favor of the later view. Not by virtue of its secreted 

 matrix is the mantle the light-recipient portion of the rod, but by virtue 

 of the neurofibrillae which terminate in that matrix. The essential fea- 

 ture of the mantle does not seem to be that its fibrils shall end free 

 in the vitreous humor, like the cilia of Protozoa in their surrounding 



