RiTTBR. — Parietal Eye. 



ABBREVIATIONS. 



7IHIC. opt. Spot marking the position 

 of the parietal organ. 



mb. ba. ex. External basement mem- 

 brane, [brane. 



mh. ba. i. Internal basement mem- 



nl. Nucleus. 



nl'. Nuclei of zone 2 of retina. 



nl". Nuclei of zone 4 of retina. 



nl'". Nuclei of zone 6 of retina. 



OS par. Parietal bone. 



pig. Pigment. 



pre. r. Processes radiating from the 



external basement mem- 

 brane. 



rtn. Retina. 



sn. sng. Blood sinus. 



tct. thi. opt. Roof of the optic thalamus. 



thl. opt. Optic thalamus. 



va. rtn. Retinal blood-vessels. 



vnt.'^ Third ventricle of brain. 



vs. Epiphysial vesicle. 



z". Second zone of retina. 



PLATE IV. 



Figs. 10, 11, 12. The left faces of three sections of P. coronata, parallel with the 

 sagittal plane, — Figure 12 nearly median, Figures 10 and 11 to the 

 left of it. Figure 10, farther to the left of the median plane than 

 Figure 11, passes through the longest part of the epipliysial vesicle. 

 Figure 12 is more highly magnified, to show the histological structure. 

 Figs. 10 and 11 X 40. Fig. 12 X 90. 



