MUSEUM OF COMI'AIIATIVE ZOOLOGY. . 63 



formed into pigment, though they are cousidei'ably larger than any 

 cells, even blood corpuscles, that are found in the eye (Plate II. Fig. 13, 

 gib. 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 thej'^ are relatively much shorter and 

 less distinct] in some specimens they scarcely appear at all (Plate II., 

 Figs. 5 and 13 pr'c). 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. I am 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 II. 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. In a 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, ir.). 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 

 ligamentiui 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, /3) remind one of the ciliary 



