158 C. Judson Herrick 



lamina terminalis in the mid-plane remaining relatively fixed. By ihis 

 evagination two lateral lobes of the telencephalon are formed separated 

 by a sagittal fissure. This evagination is accompanied by local thicken- 

 ings in the massive walls, by an inversión of the dorsal borders of these 

 walls into the median plañe, and later by a local thickening of the lamina 

 terminalis, which grows forward and also backward as a median septum, 

 thus completing the separation of the lateral ventricles from the common 

 ventricle. The result is that the foramen interventriculare in the adult 

 has come to lie far behind the primitive lamina terminalis instead of 



Fig. II. — Ichthyomyzon. Schema of the centers of the telencephalon and dien- 

 cephalon as seen in median section of the brain. Compare figure 8 and the cor- 

 responding schema of Acipenser. figure 6. also Johnston 191 2. figure 40. The 

 dorsal and ventral longitudinal zones are marked with oblique lines, the interme- 

 díate zone with vertical lines. The área olfactoria lateralis is not indicated. 



adjacent to it as in the fully evaginated brains of higher vertebrates. 

 The size of the lateral ventricle and the degree to which its cavity has 

 been enlarged by the processes of invagination and inversión just descri- 

 bed vary in different elasmobranchs widely. This matter has been fully 

 discussed and illustrated by Johnston (I911). 



The walls of the lateral ventricles of elasmobranchs are, accordingly, 

 not exactly comparable with the fully evaginated cerebral hemispheres of 

 amphibian and higher brains. These lateral enlargements of the telence- 

 phalon of elasmobranchs in this discussion will be called the lateral lobes. 

 As compared with Acipenser, the olfactory system is enormously devel- 

 oped and the enlarged área olfactoria appears to be heaped up as cióse 

 to the attachment of the crus olfactorius as possible, this being the 



