1 66 



C. Judson Herrick 



The steps in the progressive enlargement and eversión of the área 

 olfactoria can be readily followed in the series of forebrain forms from 

 the more primitive cartilaginous ganoids (Acipenser) to the more highly 

 specialized teleosts. In Amia the telencephalon médium has assumed 

 the typical teleostean form and in the embryological development of this 

 brain the steps in this process of differentiation are recapitulated. 



In Amia 7 mm. long there is no evagination of the olfactory bulbs. 

 At 15 mm. the olfactory ventricles have appeared 1 Johnstonj. In speci- 



F'ig. 18. — Squalus acanthias. Schema oí the centers of the telencephalon and 

 diencephalon as seen in median section of the brain. Compare figures 6 and 11 

 and the similar schema of Johnston '1912, fig. 41 ). The dorsal and ventral longi- 

 tudinal zones are marked with oblique lines, the intermedíate zone vith vertical 

 Unes. The latter is extended forward as far as direct thalamic connections are 

 believed to exist. The área olfactoria lateralis is the rostral member of the inter- 

 medíate series of centers, and it is in functional connection with the área somática 

 by the olfactory somatic correlation tract (fig. 16, tr. ol. sotn.). Its limits are not 



indicated. 



mens of 25 mm. i'figs. 2 1, 22) cross sections show a form of lateral walls 

 of the telencephalon médium very similar to that of adult Acipenser 

 (figures 3, 4, 5). In both of these cases the lateral walls show considerable 

 thickening and a tendency toward eversión of the dorsal lip. The more 

 rapid growth of the ventricular border of the wall results in the convexity 

 of this surface bulging into the ventriculus commune and a divergence of 

 the dorsal borders. In the transition from this condition to that of the 

 adult (fig. 23) this inequality of extent of medial and lateral surfaces of the 

 wall is increased until the attachment of the membranous roof (taeniai is 

 carried around to the ventro-lateral aspect. The significance of this ever- 

 sión is considered more in detail beyond. 



