DISSECTION OF TBE AMPHIBIAN BRAIN. 421 



With the scalpel previously dipped in alcohol, or with the fine 

 scissors, cut off obliquely the latero-caudal angle of the left hemi- 

 sphere. This exposes the corresponding procoelia, and blowing 

 into it inflates all the other parts. With the scissors, remove the 

 entire latero-dorsal wall of the hemisphere, noting its extension, 

 rhinocoelia, into the base of the Lohus olfacborius. 



Blow gently upon the mesal wall of the hemisphere at about its 

 middle, and note the presence of an orifice through which air passes 

 into the other coelise ; this is the left porta or "foramen of Monro." 

 Note the continuity of the hemisphere wall at all other points. 



Wyman mentions (34, 15), Ijut does not figure, another openinf; from the procoelia. 

 dorsad of the thalamus ; this, as he suggests, would probably correspond with the rima or 

 " fissure of Bichat." It could not appear until after the removal of the pia, and we have 

 not satisfied ourselves of its existence. 



Pass the beaded bristle through the porta toward the opposite 

 hemisphere, and note that it enters the other procoelia, as indicated 

 by the protrusion of its wall at some point. 



Just cephalad of the porta is an elliptical raised surface which 

 is thought by some (Wyman, 34, 15) to represent the striatum of 

 the higher Vertebrates, but there is doubt upon this point. 



In Menobranchus the porta is partly filled by a plexus which extends cephalad in the 

 procoelia ; this is the proplexus, which may be snipped off with the scissors, but never 

 pulled upon. The porta is much longer than in the frog, but there is no thickening of 

 the mesal wall of the hemisphere like the supposed striatum of the frog. 



With the scalpel, remove the lateral prominence of the left opti- 

 cus, and note that a somewhat expanded lateral portion of the 

 mesoccBlia communicates by a constricted orifice with the mesal 

 portion and so with the corresponding expansion in the right opti- 

 cus. Then remove the dorsal wall of the entire mesoccelia with the 

 scalpel and scissors, and note the marked folding, the valvula, by 

 which the cerebellum is continuous therewith. Pass a bristle ven- 

 trad of the cerebellum into the mesocoelia, and then sinistro-cepha- 

 lad through the diacoelia so as to emerge in the left procoelia. 



Slice off the caudal part of the diacoelian roof, including the part 

 known as postcommissura. Note that the walls (thalami) are quite 

 thick and nearly in contact, but that the interval is a nearly simple 

 vertical fissure and not a wide and partly divided cavity like the 

 mesocoelia. Pass a bristle caudo-ventrad so as to enter the hy- 

 pophysis. 



