NERVOUS SYSTEM. VERTEBRATA. 109 



Thethalamencephalon and mesencephalon differ little from 

 one another in diameter, and form together a narrow cylin- 

 drical neck between the hemispheres and the broad anterior 

 end of the medulla. A large hole between the hinder extre- 

 mities of the hemispheres indicates the spot from which the 

 conical vascular paraphysis has been removed ; its posterior 

 border is formed by two slight thickenings, the ganglia and 

 commissura habenulse. The optic lobes are very weak, and 

 do not differ materially from those of Protopterus. The 

 cerebellum is quite rudimentary a mere narrow band of 

 nervous tissue forming the anterior border of the rhomboid 

 fossa. The medulla is flattened from above downwards. 

 In front it is broad, but gradually narrows posteriorly, and 

 merges imperceptibly into the cord. Its lateral walls 

 diverge widely in front and bound an extensive rhomboid 

 fossa. 0. C. 1311 F. 



D. 123. The. head of a larval Axolotl (AmUystoma tigrinum) 

 showing the brain. It differs little from that of Meno- 

 poma except for the stronger development of the optic lobes. 

 In this specimen the paraphysis and ganglia habenulae 

 are very clear. 



Stieda, Zeits. wiss. Zool., Bd. xxv. 1875, p. 285. 



D. 124. A Menobranchus lateralis with the brain exposed from 

 above. It agrees in all essentials with that of Menopoma. 



0. 0.1311 E. 



D. 125. The head of a Proteus anguinus showing the brain. The 

 olfactory bulbs are separated from the hemispheres by a 

 slight constriction. The ganglia habenulse are remarkably 

 large ; close behind them lies the minute epiphysis. The 

 optic lobes, as might be expected in a blind creature, are 

 scarcely differentiated. 



ANURA. 



D. 126. The head of a Bull Frog (Rana catesbiana) with the brain 

 exposed from above. 



This brain, though formed on a similar plan to that of 

 a Urodele, differs from it in the relatively greater breadth 



