166 COMPARATIVE ANATOMY 



network of blood-vessels more nearly resembles that of Elas- 

 mobranclis than that of Amphibians. Lobi inferiores are present. 

 Nervous and glandular portions can here also be recognised in the 

 hypophysis. 



The well-marked mid-brain is indistinctly paired in Ceratodus, 

 but is unpaired in Protopterus. 



The cerebellum is relatively much smaller than in Elasmo- 

 branchs and Teleosts, though better developed than in Urodeles : 

 it gives rise to a valvula cerebelli. 



Amphibia. The prosencephalon of Amphibians is distin- 

 guished from that of I)ipnoans by a higher development of the 

 pallium, which, however, even in the latter group, is differ- 

 entiated into an external layer of nerve fibres and an internal 

 cellular layer. The basal ganglia (corpora striata) are less 

 marked, and merely form a more or less prominent thickening 

 of the wall of each hemisphere projecting into the lateral 

 ventricle. 



The Amphibian brain does not, however, lead towards that 

 of Reptiles. Although the prosencephalon is more highly differ- 

 entiated than in lower forms, the thalamencephalon and mesen- 

 cephalon are simpler than in Fishes; and, on the whole, the 

 brain of Amphibians is less complicated than that of any other 

 Vertebrates. 



In Urodeles the individual parts are more elongated and 

 separated from one another than in Anurans, and the thala- 

 mencephalon is therefore more freely exposed. The hemispheres 

 are almost cylindrical and are separated from one another by the 

 pallial fold as far back as the anterior commissure, 1 as in Pro- 

 topterus; while in the Armra (Figs. 137 and 138, A) they are fused 

 together for a short distance anteriorly, where they are continuous 

 with the olfactory lobes. The thalamencephalon and optic lobes 

 are much broader in Anurans than in Urodeles. The cerebellum 

 consists simply of a small transverse fold, and is especially rudi- 

 mentary in Urodeles. 



The infundibulum and hypophysis are well developed, but a 

 saccus vasculosus is no longer so distinct as in Fishes, though traces 

 of it can still be recognised. The epiphysis does not extend 

 beyond the skull in Urodeles, but in Anuraii larvaa it reaches the 

 integument, undergoing reduction later, when the bony skull-roof 

 is formed ; indications of its extracranial portion can, however, 

 sometimes be recognised even in the adult (the " brow-spot " in 

 e.g., Rana temporaria) : thus its intracranial portion does not 

 represent the entire epiphysis. A parietal organ appears to be 

 entirely wanting in all Amphibians with the exception of some 

 few Anura in which traces of it have been described. 2 



In the Gymnophiona the olfactory lobes and hemispheres are 



1 The dorsal part of the anterior commissure has been said to represent a 

 rudimentary corpus callosum (comp. note on p. 174, and Fig. 138, A). 



2 A parietal foramen was, however, present in the Palaeozoic Stegocephala 

 and other extinct Amphibians. 



