444 H. M. BERNARD. 



literature is, however, full of scattered observations, and it is 

 possible that some of them may have been overlooked by me 

 or far too briefly noticed. I do not pretend to have mastered 

 the whole of the literature on the retina. I do not wish, 

 therefore, to make any claims to priority, but simply to 

 describe my observations, referring briefly to those of former 

 students, so far as I know of any covering the same ground. 



And here I should add that, while confining myself in this 

 paper solely to the Amphibia,^ these researches have extended 

 over other Vertebrates, and that the general conclusions 

 arrived at are not drawn solely from the facts here described. 



Enveloping Membranes. — While the existence of the 

 membrane investing the inner limb of the rod requires no 

 demonstration, it has been much disputed whether the outer 

 limbs possess any such envelope or not. Apart from the fact 

 that such a covering is difficult to demonstrate, it is possible 

 that the conception of the rod as a cuticular structure may 

 have strengthened the doubt. It has long been known that 

 the outer limbs of the rods can be made to divide up trans- 

 versely into discs, and that on such a dissolution no investing 

 membrane can be seen (cf. Max Schultze's figures, 'Arch, 

 mikr. Anat.,' Bd. iii, pi. xiii, figs, lie, etc.). 



MerkeP found membranes wherever he looked for them 

 except in the Amphibia, while Landolt^ figured very thick 

 homogeneous membranes covering the rods (frog and newt) 

 and the cones (newt). I am not aware that these have ever 

 been confirmed, and I doubt their existence. He figures 

 them even passing in between outer limb and ellipsoid. 

 Something like what he figures may be seen in my fig. 

 13^ c — i, the significance of which will be discussed later on. 

 In the meantime I may state that I do not regard the thick 

 rind there shown as an outer covering. 



1 The following forms have been examined: — Ran a temporaria, Bufo 

 vulgaris, Molge cristata and M. vulgaris, Salamandra maculosa, 

 and Siredon pisciforniis. 



» 'Arch. Anat. u. Phys.,' 1870, pi. xiv. 



» 'Arch. mikr. Anat.,' Bd. vii, 1871, p. 81, pi. ix. 



