MORPHOLOGICAL STUDIES. 
59 
the sides (figs. 3, 5, and 10, /.), and might perhaps be compared 
to a slightly convex lens. The only thing that can be made 
out in its structure is a fairly large number of rounded nuclei 
lying in a certain amount of protoplasm. Cell boundaries 
are here not to be made out in the Ammoccetes. 
The posterior wall (figs. 3, 5, and 10, re.) presents structures 
which one may compare with a retina, such as that described 
by Spencer in Hatteria or Varanus (No. 14, p. 177). 
This portion is much thicker than the anterior wall, and its 
widest part is in the middle. It is figured in Schwarz’s three 
specimens (in figs. 3, 5, and 10, re.). It presents from within 
outwards a layer of longish rods ( rd .), the free ends of which line 
the cavity of the vesicle. Without the rods is a layer of nuclei 
(«., figs. 3, 4, and 10), and beyond these a second more scanty 
layer of scattered nuclei (« 2 .)is met with. The rods are elongated 
cells, whose nuclei lie near their bases. In the three Ammo- 
coetes mentioned the rods are more or less enveloped in a deep 
black pigment, which extends to their bases, and even slightly 
into the layer of nuclei beyond. The internal row of nuclei 
are, like the fewer external nuclei, rounded, and could not be 
traced into connection with the rods, though probably such 
connections exist. The protoplasm in which the outer nuclei 
rest is granular and fibrillar in appearance. Thus the retina 
of the parietal eye of Ammocoetes presents practically the same 
structure as that of Hatteria or Varanus figured by Spencer 
(No. 14, PI. XIV, figs. 3 and 6). If the reader will com- 
pare these figures with my figs. 3, 5, and 10, he will, I think, 
be convinced of the agreement. 
I have previously stated that more usually the parietal eye 
retina of Ammocoetes presents no pigment. I have figured a 
longitudinal section (fig. 4) of such an unpigmented parietal 
eye, and this is typical of most Ammocoetes. While it presents 
in other respects the same characters as the three specimens 
described above pigment is very nearly but not quite absent ; 
there are a very few minute dots, which are figured at p. s. } 
fig. 4. 
Where the pigment is absent it is not possible in specimens 
