Ritter. — Parietal Eye. 



ABBREVIATIONS. 



Spot marking the position 

 of the parietal organ. 



External basement mem- 

 brane, [brane. 



Internal basement mem- 

 Nucleus. 



Nuclei of zone 2 of retina. 



Nuclei of zone 4 of retina. 



Nuclei of zone 5 of retina. 



Parietal bone. 



Pigment. 



Processes radiating from the 

 external basement mem- 

 brane. 



Retina. 



Blood sinus. 



Roof of the optic thalamus. 



Optic thalamus. 



Retinal blood-vessels. 



Third ventricle of brain. 



Epiphysial vesicle. 



Second zone of retina. 



Fig. 8. Left face of a sagittal section through a portion of the epiphysis, a short 

 distance above its connection with the brain in P. Douglassii. It is in 

 part diagrammatic, though the outlines of the figure as a whole, and 

 of most of the foldings of the epithelium, were drawn with the 

 camera. From the same individual as Figure 1. X 312. 

 " 9. Similar view of a sagittal section from the same individual, to show the 

 relation of the epiphysis to the brain and the blood sinus. X 30. 



