Section VIII. 

 The Porebrain. 



Tha key to the naierstanJin^, of ths vertebrate forabrain 

 ^as given by Rabl-Riickhari ('83) when he fornulatel the theory 

 of the membranous pallium for the teleost. The extension of 

 the generalization to other groups has been productive of re- 

 sults flhich fall into place in an almost schematic way. A 

 quite remarkable series is given by the fishes. This series 

 begins with a coniition in the teleost /rhere the entire roof 

 of the forebrain reiiains non-nervous, and culminate.^ in the 

 iipnoid flith a pallium having the essential morphological char- 

 acters pertaining to all brains of a higher order. 



The forebrain of Mustelus appears superficially to be 

 somewhat livergent from the direct line of the series, o<?ing 

 to the absence of conspicuous external evidence of its bilat- 

 erality. The tfo striata are fused together into a solii ba- 

 sal mass; thera is merely a broad and shallow? furrow on the 

 ventral side to mark their plane of contact. The pallium, 

 while a thickened nervous plate as in far higher types of brains. 

 is perfectly continious between the right ani left sides, anJ it 



is also confluent in the median plane with the striata below. 



(1?0) 



