108 PHYSIOLOGICAL SERIES. 



With the exception of the medulla and cerebrum, all parts 

 are very poorly developed. The medulla is similar in form to 

 that of a low Shark, with wide open fossa rhomboidalis and 

 no sharp boundary towards the cord. The hemispheres, in- 

 cluding the olfactory bulbs which are at the best only slightly 

 indicated, form the largest part of the brain, and show signs of 

 incipient pseudo-occipital lobes. But their size is somewhat 

 deceptive, as the walls are relatively thin throughout. They 

 eon-ist of an outer molecular and an inner cellular layer, and 

 show no signs of cortical formation, unless possibly a small 

 aggregation of cells in the outer layer of the median wall should 

 be regarded as a rudimentary hippocampal cortex. 



The hemispheres are united by an anterior commissure, in 

 wlm-h two portions can be distinguished: one between the basal 

 parts the true anterior commissure; and the other connecting 

 the olfactory regions of the pallium a hippocampal commissure. 



The hemispheres are connected to the thalamencephalon by a 

 large basal tract on either side. The ganglia habenula3 are 

 always well marked and externally visible, though not so strong 

 a- in the Cyclostomi and Dipnoi. The pineal gland is ves- 

 tigeal. The optic lobes are only developed to any extent in tin- 

 Aim, and in them have a many-layered tectum. The roof 

 nucleus, in connection with the trigeminal nerve, is particularly 

 strong in Urodeles. The cerebellum, although small, is com- 

 1 of three layer- nuclear, intermediate and molecular, but 

 an internal fibrous layer, owing to the small size of the cerebellar 

 tracts, is not differentiated. 



URODELA. 



D. 122. A Menopoma (Mcim^oina alleglianiewis) with the brain 

 and >pinal cord exposed from above. This specimen, although 

 -omewhat damaged. >ho\vs clearly the chief superficial cha - 

 racters of the Urodele brain. The cerebral hemispheres are 

 separate in the mid-line nearly to their posterior extremities. 

 Each is large and oval in shape, and projects backwards to 

 -iinir exent over the lateral parts of the tlialamencephalon, 

 tor-mini; a rudimentary p-endo-oe.-ipital lobe. Independent 

 olfactory bulbs are absent, but two strong double olfac- 

 tory nerve-, ari-c from the antero-ventral parts of each 

 hemisphere. 



