PARIETAL EYE OF PETROMYZON. 



141 



a long, fibrous, colorless rod suggestive of those seen in Apus. (Figs. 99 and 100.) 

 A layer of nuclei and fibers is seen below the columnar cells. Its outer wall, in 

 its central portion, consists of similar cells and rods. They have been regarded 

 as forming an imperfect lens, but their histological structure indicates that they 



Fig. 100. — The parietal eye vesicle of a young lamprey, 6mm. long. A. Sagittal section; B. cross-section. 



Fig. ioi. — Plan of the parietal eye vesicle with its nerves, ganglia, and epiphysis, seen from the neural surface. 



A, Young Limulus; B, young lamprey. 



represent the remnants of visual cells, although they are not so well developed as 

 those on the floor of the sac. The two sets of rods meet in the middle of the sac, 

 their distorted ends forming a distinct cleavage band. On the periphery of the 

 eye the walls consist of a single layer of short, columnar cells. 



