380 



ZOOLOGY 



SECT. 



roof of the brain marking the anterior limit of tlie 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 



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oval opiSic lobes (corjwra bigeviijia, Fig. 970, o. I.) and ventrally 

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

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

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

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

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



