MUSEUM OF COMPARATIVE ZOOLOGY. 63 
formed into pigment, though they are considerably larger than any 
cells, even blood corpuscles, that are found in the eye (Plate II. Fig. 13, 
glb. pig.). They probably merely signify that the. pigment tends to 
segregate in such nodules during its formation. On the inner surface 
processes of pigment project down among the rods, as in normal eyes, 
excepting that in most instances they are relatively much shorter and 
less distinct ; in some specimens they scarcely appear at all (Plate II., 
Figs. 5 and 13 prc.). The greater portion of the thickness of the layer 
pertains to the region between the bases of these processes, i. e. the 
terminals of the rods, and the outer surface of the layer. And it is 
hence in this portion that the increase in thickness over that found in 
normal eyes has taken place ; for in the latter, this region is relatively 
thin. Iam at a loss to know from what source this pigment has come. 
As already said, no cellular elements are to be found in the layer, so it 
is quite certain that they have completely degenerated into pigment. 
I shall return to this question in the comparative part. The layer thins 
out rapidly as it approaches the ora serrata, and is frequently thrown 
into an equatorially directed fold (Plate IT. Figs. 5 and 13, *), which 
may extend entirely around the eye, but more commonly is confined to 
one side of it. 
Immediately in front of the thinned out region just mentioned, the 
pigment thickens again somewhat, to form the pigmented portion of the 
iris. This structure, though always present, varies greatly both in form 
and size. Ina majority of specimens the pigmented portion constitutes 
the entire iris ; and in all cases it forms by far the greater portion of it. 
The dense pigment is entirely the same in structure as that in the pig- 
ment layer of the retina. In the few cases where a cellular portion is 
present, it is in small quantity, and appears to be of the nature of con- 
nective-tissue cells and fibres. Neither blood capillaries nor epithelium 
have been detected in this region (Figs. 5, 13, and 17, ör.). It fre- 
quently happens that the outer surface of the iris is in contact with the 
inner surface of the cornea (see figures), and it is thus made to appear 
as though the iris has a considerable part in addition to its pigment ; 
this, however, is undoubtedly only an appearance caused by the inner 
border of the iris having been thrust outward by some artificial means, — 
probably by the lens being in most cases moved outward, for this body 
is very loosely held in its place. In a few specimens a trace of the 
ligamentun pectinatum is present, though in most cases no trace of it 
exists. The short blunt processes of pigment occasionally seen project- 
ing toward the lens (Plate III. Fig. 17, 8) remind one of the ciliary 
