uj6 C. Jiídson Herrick 



logical factors are operative but with various special modifications, and 

 the morphological pattern varíes accordingly. In soma cases (cyclosto- 

 mes, elasmobranchs) a portion of the área olfactoria has joined the 

 bulbus olfactorius in the evaginated cerebral hemisphere; in some cases 

 felasmobranchs) extensive parts of the área olfactoria are massed cióse 

 to the crus olfactorius, greatly thickened and inverted to fuse with the 

 corresponding parts of the opposite side; in other cases (teleostsj the 

 enlargement of the telencephalon médium is accomplished by lateral 

 eversión of the dorsal part. Xone of these patterns lead up directly to 

 the amphibian type of forebrain. Further study of the polypterid fishes 

 and Dipnoi will probably reveal the fundamental features of the morpho- 

 genesis of the amphibian type, but at present \ve lack the knowledge 

 necessary for such an inquiry. An examination of amphibian brains 

 and an incomplete study of their morphogenesis suggest that this form 

 of forebrain was developed from a type not representad in existing 

 Cyclostomi, Teleostomi or Elasmobranchii. 



The extinct ganoidean ancestor of the Amphibia probably possessed 

 a telencephalon whose thin and relatively undifferentiated waUs were 

 widely evaginated in fashion similar to that of recent Dipnoi. The dorso- 

 lateral convexity of this cerebral hemisphere was an undifferentiated 

 dorso-lateral olfactory área, whose lateral part recei\ed the tractus pallii 

 trom the hypothalamus and the somatic projection fibers from the 

 tlialamus and later was further specialized into amygdalo-striate complex 

 ventrally and lobus pyriformis dorsally. The dorso-medial wall of this 

 primitive hemisphere was primordium hippocampi. Prior to the ganoi- 

 dean stage this primordium lost its primitive somatic sensory components 

 (if such were present) and developed, during the ganoidean phase, exten- 

 sive correlalion tracts and complex mechanisms of association and at the 

 same time the direct olfactory connection was reduced. The process of 

 evagination of the hemisphere separated the medial primordium hippo- 

 campi from the área of distribution of the tractus pallii and thalamic 

 projection fibers and brought it into contact with the septum. This 

 topographic rearrangement left the primordium hippocampi in relative 

 physiological isolation from the olfactory bulb in front and from the 

 diencephalon behind and gave a new impulse to the differentiation of corre- 

 lation tracts with the adjacent dorso-lateral and ventro-medial sectors of 

 the wall of the hemisphere. These topographic and pliysiologic reía- 



