THE DEVELOPMENT OF LEPIDOSIREN PAllADOXA. 449 



Studnicka lias recently asserted that " Der Vorderliirn 

 aller Crauioten ist paarig angelegt/' ^ Lepidosiren fur- 

 nishes nothing to throw doubt on this generalisation^ and the 

 general tendency of modern research, especially upon forms 

 in which the topographical relations are not liable to be 

 distorted by the presence of a large mass of yolk with its 

 accompanying exaggerated cerebral flexure, seems on the 

 whole to support it. If true it ought to involve the rejection 

 of His's brain regions telencephalon and diencephalon, as not 

 representing true morphological entities. The old expressions 

 thalamencephalon and prosencephalon are much more accu- 

 rate, but it seems questionable whether it would not be 

 better to drop the latter name and merely use the descriptive 

 term "hemispheres," as this conveys no erroneous suggestion 

 that we have to do with anything more than a local hyper- 

 trophy of the thalamencephalon wall upon each side. 



The enormous importance of the primitive fore-brain — that 

 brain reo-ion which assumes its definitive characters so extra- 

 ordinarily early in development, and which appears to be 

 already marked off in Amphioxus,^ can hardly be exag- 

 gerated. Compared with it the hemispheres, however im- 

 portant they may be physiologically, are morphologically of 

 relatively little account. 



It appears to me, biassed possibly by my observations of 

 what happens in Lepidosiren, that too much morphological 

 weight is attached to the velum. It is looked upon as being 

 the landmark which indicates in the brain roof the boundary 

 between two important brain regions, and structures lying 

 anterior to it are regarded as belonging to the secondary 

 fore-brain. In Lepidosiren we have seen that the velum is 

 a paired structure, also that it arises simply as a prolongation 

 backwards from each lateral plexus. The evidence of Lepi- 

 dosiren pointing towards the secondary fore-brain being 

 primitively paired and to the lateral plexus being similarly 



' 'S. B. Bohmiscli. Ges./ 1890, xv. 



- Kupffer, 'Studien zur vergl. Eiitwickluiigsgoscliiclite des Kopft's der 

 Kraiiiuten,' Heft 1, S. 71. 



