272 PEOCEEDINGS OP THE ACADEMY OF [1884. 



chiasma as the lamina cinerea ; that lying in front, as the lamina 

 terminalis. This construction cannot be applied here with 

 accuracy, owing to the unusual position of the praecommissura, 

 in the brain-floor, instead of in the anterior median wall. Yet 

 for comparative purposes it is best to retain this interpretation. 

 It gives us an unusually extended prosocoelia, or ventriculus 

 communis loborum, which we find is a distinctive feature also of 

 the Amphiuma and Menobranchus brain. The supracommissura 

 may be considered as the upper posterior boundary of the proso- 

 coelia, separating it arbitrarily from the diacoelia, as the post- 

 commissura does the dia- from the mesocoelia. At all events, the 

 supraplexus clearly belongs to this cavity rather than to the 

 diacoelia. 



The general subject must be discontinued here, to be resumed 

 in connection with the brain of the Menobranchus, in a subse- 

 quent paper. 



Morphological Laboratory, Princeton, Oct. 20, I884. 



EXPLANATION OF PLATE VI. 



Illustrating the Brains of Menopoma and Rana. 



Encephalic segments Eh. — Rhinencephalon ; Pr. — Prosencephalon ; Di. — 

 Diencephalon ; Me. — Mesencephalon ; Up. — Epencephalon ; Met. — 

 Metencephalon. 



General Abbreviations. 



a. — ^Prosocoelia, cavity of the primitive prosencephalon. 



a. hph. — Anterior lobe of hypophysis. 



a c c— Branch oi Arteria carotis cerebralis. 



cbl. — Cerebellum. 



ch. — Optic chiasma. 



en. ce. — Canalis centralis. 



dc. — Diacoelia, third ventricle. 



dpx. — Diaplexus, choroid plexus of the third ventricle. 



end. — Ependyma. 



h. — Habenulse. 



hem. and hem'. — Section and external surface of right hemisphere. 



hph. — Hypophysis. 



i. cm. — Inferior commissure of infundibulum. 



I. — Longitudinal fibre courses, cut transversely. 



med. — Medulla oblongata. 



mse. — Mesocoelia, iter. 



mtc. — Metacoelia, fourth ventricle, 



mtpx. — Metaplexus, tela vasculosa of the fourth ventricle. 



my. — Myelon, spinal cord. 



