85 



The retina has been shown by Granacher to be a one-layered columnar 

 epithelium but, owing to the specialization of portions of its cells, it seems 

 formed of several layers. The apparent layers are : an internal limiting mem- 

 brane , an inner pigment layer , a layer of rods , an outer pigment layer together 

 with an inner nuclear layer, the basement or outer limiting membrane, a clear 

 zone and the outer nuclear layer.' The retina arises from the ectoderm of the 

 center of the first optic invagination and remains continuous with the epithelium 

 of the peripheral portion of the pit , from which arises the pigmented epithelium 

 (choroidea) which lines the outer wall of the inner chamber of the eye. The 

 retinal epithelium rapidly becomes very thick , certain of its cells elongate more 

 rapidly than the others and , pushing their nucleated proximal ends through the 

 basement membrane , extend some distance into the surrounding connective 

 tissue. These are the sensory or rod cells , which become differentiated into 

 three regions, a long and slender distal "rod", a short intermediate rod-stalk 

 ("Sockel") and the projecting nuclear portion. The remaining cells become the 

 supporting cells. They form a honeycomb-like frame work (apparently a syncy- 

 tium) which surrounds the rods and stalks. The nuclei of these cells , with a 

 small amount of the cytoplasm , lie close to the basement membrane. Slender 

 cytoplasmic processes extend inward in the spaces between four or more adjacent 

 rods to the internal limiting membrane and serve to bind together both the rods 

 and the limiting membranes. Each rod secretes a pair of thick trough-shaped or 

 semicylindrical cuticular plates which invest it except at its ends; the outer of 

 which forms a small bulb whereas the inner is continuous with a short fusiform 

 expansion , the rod stalk which lies between the rod and the internal limiting 

 membrane and laterally between the nuclei of the supporting cells. 



The rods are closely packed in regular longitudinal and transverse rows , 

 and the cleft between the semicylinders of each rod is inclined at an angle of 

 45° to both rows and at right angles to the corresponding clefts of the four 

 adjacent rods. This arrangement brings together the half cylinders of every 

 alternate group of four rods in the form of a Maltese cross whose center is made 

 by a strand of the cytoplasm of the supporting syncytium and whose ends are 

 each bound by a similar strand to the arms of three other crosses. Grenacher, 

 regarding these pretty crosses as units , called them rhabdomes , but there is no 

 adequate reason for regarding them either as morphological or physiological units. 



The stalks and the intervening portions of the supporting cells are filled 

 with granular pigment which forms a thick pigment layer, the external pigment 

 layer, within the basement membrane. The nuclear portion of the sensory cell 

 extends inward from the basement membrane and forms about one third of the 



