86 



length of the entire cell. The nuclear segments are supported b\' richly vascular 

 connective tissue. The proximal portions of these segments contain the nuclei , 

 which are arranged at various levels so that they seemingly form two or three 

 layers. 



The above description is generally accepted , but the essential part of the 

 sensory cell, — the percipient structure — has been, and perhaps still is. little 

 understood. Hesse (1900) finds that the neuroflbrillae which form the fibers of 

 the optic nerve , and which seem to him the percipient structures , arise near 

 the distal end of the rods and extend the entire length of the sensory cells. 

 Merton (1905), however, finds that the neuro-fibril, entering the nuclear seg- 

 ment , broadens into an irregular and often split lamella which extends to the 

 base of the rod stalk; and that the rod contains a cytoplasmic network — 

 mistaken by Hesse for the continuation of the neurofibril — which is the path 

 of migration of the pigment. 



The pigment migration has been extensively studied by Rawitz , Hesse and 

 Hess, who have shown that the retina of a Cephalopod which has been long 

 in the dark contains a single layer of pigment, — the external pigment layer, 

 and that in the light the pigment granules migrate inward from this layer and , 

 massing in the expanded inner ends of the rods , form the inner pigment layer 

 which Hes beneath the internal limiting membrane and which is connected 

 with the external pigment layer by a small amount of pigment scattered along 

 the rods. 



Hess finds that the fresh retina always appears dark lirown or black and 

 that a red hue tends to predominate in the upper, a brown shade in the lower 

 half of the eye. On the contrary , a I'etina preserved after remaining long in 

 the dark , ajjpears gray : after exposure to light , dark brown or black ; while 

 after an exposure for one hour to changing light, there appears a horizontal 

 gray band about 4 mm. broad between the darker upper and lower halves of 

 the retina. With increased light this band becomes narrower and finally dissappears. 



The dark color of the fresh retina is due to its transparence , which allows 

 the pigment to be seen at any depth , while the opaqueness of the preserved 

 retina does not allow the pigment to be seen unless it lies near the surface. 



The retardation of the migi'ation of pigment to the distal part of the rods 

 of the horizontal band seems to indicate that the cells of this region are less 

 sensitive to an excess of light. 



In addition to this merely protective pigment, Hess finds that the Cephalo- 

 pod retina contains a purple pigment which is very similar to , perhaps identical 

 with, visual purple. 



