STtJDlES IN THE RETINA. 445 



That enveloping membranes occur in the outer limbs of 

 Amphibian rods is certain, both on theoretical grounds and 

 because they can be demonstrated. 



As we saw in Part I of this paper (this Journal, p. 29), the 

 rods are primarily protoplasmic vesicles protruded from the 

 retina. The walls of the vesicles are of extraordinary delicacy 

 and transparency, and it will be a triumph of microscopic 

 technique when retinas can be fixed so as to show them 

 intact. They are best seen in their very earliest stages of 

 protrusion, before any rods are formed and the pigment is 

 only just being forced away from the retina by their increase 

 in number and size. From this early stage we traced them 

 through their principal form-phases till they became normal 

 rods, and all these phases were not only consistent with their 

 being long, membranous sacs, but even confirmatory of this 

 conception of their essential structure. Lastly, the persist- 

 ence of the membrane covering the inner limbs has, as we 

 have seen, long been an established fact. 



But granted that in all the earlier stages of the rods we 

 have a wall to the vesicle — a wall which persists in the inner 

 limb, — we have still to ask whether that is the case with the 

 outer limb when the rod is complete. May not the proto- 

 plasmic wall merge in the substance which fills up the 

 interior of the outer limb and lose its individuality, so that it 

 would be impossible to speak of any investing membrane ? 

 This is, of course, quite possible ; and, moreover, it is certain 

 that even if it preserved its individuality one would rarely 

 expect to demonstrate the existence of such a delicate film of 

 transparent protoplasm round the outer limb of the amphibian 

 rod, with its usually refractive contents. Actual observa- 

 tions, however, show clearly that the protoplasmic wall does 

 retain its individuality, and that to the last the rod is a thin 

 protoplasmic vesicle filled up with matter, the origin and 

 nature of which will be discussed in the following pages. 



As demonstration of this persistence of the protoplasmic 

 wall of the vesicle, I will call attention to PI. 30, fig. 2, which 

 is taken from the retina of a toad. All the rods in this 



