16 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



ends of the horsehoe toward the front (fig. 3). Over the lateral and 

 posterior portions of the eye runs the continuous layer of pigment 

 cells (fig. 4). Dorsally these are just beneath the ectodermal epithe- 

 lium; more ventrally they lie in the midst of the cells of the optic 

 ridge ; but in all regions they lie inside the continuous limiting mem- 



K W 



brane of the eye and ganglion. Along the whole inner side of the 

 horseshoe are the elongated rod-cells with their thin-walled ends 

 directed toward the center of the horseshoe, while their rods are 

 directed toward the pigment layer. Between the rods and the pig- 

 ment cells is a layer of intermediate cells. The innervation of the 

 rod-cells has not been found by any observer in the solitary form of 

 any species. 



