.330 



ZOOLOGY 



roof of the brain marking the anterior limit of the dien- 

 cephalon, is another commissure, the aberrant commissure (c.p.p.\ 

 which connects together the posterior and dorsal parts of the 

 parencephala ; this is not represented either in the Frog or 

 in higher Vertebrates. The mid-brain consists dorsally of two 





oval o^/^'c /o&fs (corpora bigemina, Fig. 970, o. Z.) and ventrally 

 of a mass of longitudinal nerve-fibres, the crura cerebri (V. e.), 

 passing forwards to the fore-brain. Each optic lobe contains a 

 cavity (optoccele) communicating with the iter, a narrow passage 

 leading from the diaccele to the metacoele. The cerebellum (cb.) is, 

 like that of the Frog, of small size, being a small antero-posteriorly 



