Functional factors in tho morphology of the forcbrain of fishes i8i 



originally purely olfactory in function by ascending somesthetic and optic 

 systems from the thalamus for correlation. The descending projection 

 neurons lie within this mixed olfacto-somatic área. In Acanthias no fibers 

 reach the área olfactoria lateralis directly from the thalamus, but tissue of 

 somatic type is developed far forward in telencephalic territory and this 

 área is connected with the área olfactoria lateralis by an olfacto-somatic 

 correlation tract. The neurons of efferent discharge lie near the boundary 

 between the área olfactoria lateralis and the área somática. 



Johnston expresses the opinión (IQII, p. 3lj that the área somática 

 of fishes foreshadows the neopallium, or non-olfactory cerebral cortex, of 

 mammals rather than the corpus striatum. In fact, nowhere does he 

 recognize the existence of a primordium of the corpus striatum in ñshes. 

 In view of the fact that the striatum seems to be a lower and more gene- 

 ralized type of somatic sensori-motor correlation center than the cortex, 

 this seems to be a curious omission. 



Taking all the comparative, embryological, and physiological evidence 

 into account, it seems probable that the striatum complex is at least as 

 oíd phylogenetically as the general cortex (if not older). That the corpus 

 striatum aróse in the Amphibia within the ventral sector of the área 

 olfactoria lateralis is clear from the conditions which I have elsewhere des- 

 cribed (1921 a). In Urodela there is a common olfacto-striatal primor- 

 dium, but in Anura a true striatum and a true amygdala have emerged 

 from this generalized primordium. These structures lie clearly in the 

 subpallial part of the hemisphere and cannot be regarded as primordia of 

 any true cerebral cortex. The cortical primordia lie farther dorsally. 



The corpus striatum of Anura and higher vertebrates not only aróse 

 within the ventral portion of the área olfactoria lateralis of Urodela, but 

 it retains ¡ts intímate relationship with the lateral olfactory system 

 (through the amygdala and lobus pyriformis) in reptiles and mammals. 

 It is to be expected that this relationship will be foreshadowed in fishes 

 as well as in Urodela; and this is, in fact, the case, for the área olfacto- 

 somatica is the common primordium of the strio-amygdaloid complex 

 and perhaps of the lobus pyriformis as well. 



The dorsal zo)ie. — In cyclostomes the dorsal border of the telen- 

 cephalon médium (área olfactoria dorsalis) is very intimately related with 

 the epithalamus and the two regions seem to form a single massive 

 column with but slight interruption at the line of attachment of the velum 



