THE VERTEBRATE RETINA 



Anterior to the ora terminalis the pigment epithelium passes over the 

 cihary body as the outermost of the two layers of the ciliary epithelium 

 and is almost unchanged except for an increase in the height of its 

 cells and the disappearance of all processes together with the spindle 

 form of pigment granule. Its continuation on the back of the iris is 



Fig. 20 — Pigment-epithelial cells, x 500. 



The horizontal line beneath each drawing shows the position of the external limiting mem- 

 brane. A portion of the lamina vitrea is shown as a heavy black line. Spaces occupied by 

 cones are marked c; those filled by rods are marked r. 



a, group of cells from an unstained, flat mount of human pigment epithelium, as seen from 

 the chorioid side. Through the clearings formed by the nuclei, some of the elongated pig- 

 ment granules in the distal part of the cell can be seen. 



b, two human pigment cells from the nasal periphery, in vertical sertion. One cell is opposite 

 a cone, and bears a delicate tubular process which ensheathes the cone outer segment {cf. 

 Fig. 19). The other cell is opposite only rods, and is devoid of processes. 



c, pigment cell of a mouse opossum, Marmosa mexicana, showing the paucity of retinal 

 pigment charaaeristic of many strongly nocturnal animals. 



d, pigment cell of an African lungfish, Protopterus eethiopicus, showing a mass of fila- 

 mentous pigment-laden processes markedly differentiated from the body of the cell. 



e, pigment cell of goldfish, Carassius auratus, light-adapted. The two or three heavy proc- 

 esses contain relatively little migratory pigment (in rod-like granules) in their tips {cf. Fig. 

 62, p. 146; Fig. 94, p. 237). 



almost devoid of pigment in those animals in which it has produced 

 a dilatator pupillas (Fig. 7, p. 15). At the edge of the pupil the layer of 

 cells doubles back upon itself and continues, now heavily pigmented, to 

 the periphery of the iris as the latter's most posterior layer of tissue. 

 There its pigmentation disappears and a clear epithelium proceeds over 



