RiiTER. — Parietal Eye. 



ABBREVIATIONS. 



cav, e'phy. Cavity of the epipliysis. mtic. opt. 



cbl. Cerebellum. 



ceb. Cerebrum. ml. ba. ex. 



chs. opt. Optic cliiasm. 



cl. i. Cells of zone 5 of the retina. ml>. ha. i. 



d. Ins. Cells of tiie lens. nl. 



coy. Coagulum. n/'. 



com. a. Anterior commissure. nl". 



com. p. Posterior commissure. nl'". 



com.su. Superior commissure. os par. 



con't. tis. Cwinective tissue. pig. 



cp. sng. Blood corpuscles. pre. r. 



e'Jrm. Ectoderm. 



en'lh. va. Endothelium of retinal blood- 

 vessels, rtn. 



eph. vs. Epithelium of the epiphysial sn. sng. 



vesicle. tct. thl. opt 



e'th. Epithelium. thl. opt. 



fa. trm. Lamina terminalis. va. rtn. 



Ins. I/cns. vnt.* 



lob. opt. Optic lobes. i'«. 



m. scu. Scale of the parietal eye. z". 



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. 



PLATE I. 



Fig. 1. Left face of a section through the dorsal wall of the head of Phrynoroma 

 Doiifjlassii in the sagittal plane, and consequently passing through the 

 middle of the parietal organ. Diagrammatic in unimportant details. 

 X 140. 



" 2. Transverse section through the dorsal wall of the head and middle of 

 the parietal organ of P. coronata. Diagrammatic in unimportant de- 

 tails. X 140. 



" 3. Section of a small portion of the deep wall of the parietal organ and the 

 enveloping connective-tissue capsule, to show the processes radiating 

 from the external basement membrane. X 1060. 



" 4. A transverse section of one of the retinal vessels, in which a blood 

 corpuscle is seen. X 1060. 



