488 C. JUDSON HERRICK 



These relations characterize the dorso-medial part as primor- 

 dium hippocampi and the cortex hippocampi of higher forms 

 differentiates within it. But in the frog it is clearly much more 

 than an olfacto-hypothalamic correlation tissue. Though struc- 

 turally very distinct from the dorso-lateral part, the functional 

 connection between these parts is most intimate. If we adopt the 

 interpretation of Elliot Smith ('08, p. 529) that the pars dorso- 

 medialis of the frog brain is the primordium of the mammalian 

 hippocarcpus and the pars dorso-latcralis that of pyriform lobe, 

 as I think we must, it can only be with the reservation that the 

 homology is incomplete and the parts here are very imperfectly 

 differentiated from each other. All parts of the amphibian hemi- 

 sphere are under the physiological influence of the olfactory 

 bulb to some extent; i.e., there is no somatic pallium devoted 

 wholly to non-olfactory correlations. On the other hand, there 

 is, I believe, no part of the dorsal or pallial wall of the hemisphere 

 which is not influenced to some extent by the somatic sensory 

 ascending elements in the lateral forebrain tract. The homologies 

 suggested above when given their proper limitations may be 

 expressed as follows : The predominating structural and physiolog- 

 ical features of the medial border of the dorsal wall of the amphi- 

 bian hemisphere are hippocampal in type, while those of the lateral 

 border are those of the pyriform lobe. The dorsal tissue between 

 these borders is undifferentiated. From it the son.atic pallium 

 of higher vertebrates arises. It would be an error to consider 

 that the amphibian primordium hippocampi is exactly comparable 

 with the mammalian cortex hippo can. pi, though more sin ply 

 organized. On the contrary, it is adapted to serve all the forms 

 of cortical association which the animal possesses — olfacto- 

 gustatory, olfacto-tactile, olfacto-optic, etc., — but always pre- 

 dominately olfactory. With the differentiation of neo-pallial 

 (i.e., non-olfactory) cortical association centers in mammals, the 

 cortex hippocampi (particularly the fascia dentata) becomes more 

 nearly purely olfacto-receptive and the correlation tissue is sepa- 

 rately developed in neighboring association centers of the gyrus 

 cinguli, etc. 



The precise history of this differentiation will doubtless be 



