62 
J. BEARD. 
insight into the structural elements of the retina. In others 
no traces of pigment are to be found (fig. 9). 
This again, as in the Ammocoetes, is an indication of the 
variability of the organ. 
When I here speak of pigment I mean black pigment. 
Ahlborn, following Wiedersheim’s discovery of grey-white pig- 
ment in Ammocoetes, found in all his Petromyzon only white 
pigment (No. 1, p. 233). I do not dispute these discoveries, 
which appear to me well authenticated. As I have never 
investigated perfectly fresh Petromyzon or Ammocoetes I have 
never seen this white pigment, but as Ahlborn always found 
grey-white pigment and no black, and asin my sections 
if pigment could be recognised at all it was always 
black, I think we may safely assume that in all those cases 
of Petromyzon or Ammocoetes where I have found no pigment, 
the grey-white pigment of Wiedersheim was originally present, 
but was dissolved out in the process of preparing the sections. 
The front wall of the vesicle iu adult Petromyzon is very 
little different from what we saw in Ammocoetes. It contains 
no pigment and is usually somewhat folded (figs. 1, 8, and 9). 
It is composed of long cylindrical celis, and can hardly be said 
to form a lens. 
As in Ammocoetes the cavity of the vesicle is filled by a 
coagulable fluid (fig. 9, c.f.) which in sections is drawn into 
threads which appear to connect the two walls of the vesicle. 
Of course the importance which Ahlborn attached to these 
connections is negatived by their nature. 
With low powers the posterior wall of the vesicle or retina 
is seen to be made up of three layers : an inner layer of rods 
(fig. 9, r .) which also contains the pigment ; following this a 
layer of “ nuclei ” (fig. 9, n 1 .), and outside this a somewhat 
granular striated layer, which contains a few ganglion-cells 
(fig. 9, n 2 .). Outside of all is the connective-tissue investment 
of the eye. 
The retina as figured in figs. 1, 8, and 9 would be compar- 
able to the retina of Varanus or Ilatteria as described by 
Spencer. However, it is interesting and important to examine 
