478 C. JUDSON HERRICK 



so-called prethalamus (" epistriatum" of Johnston) and is there- 

 fore not interrupted by the di-telencephalic fissure. Column 3 

 is partially evaginated in cyclostomas and urodeles and fully 

 so in higher animals. Column 4 is not evaginated at all in cyclo- 

 stomes. In the amphibians about half of the telencephalic part 

 of this column remains in the telencephalon medium as nucleus 

 preopticus, while the remainder is evaginated to form the pars 

 ventro-medialis of the hemisphere. In the higher forms the evag- 

 inated part increases at the expense of the median part until in 

 man the small supraoptic nucleus is all that remains of the latter, 

 while the former includes the precommissural body, septum pel- 

 lucidum, tuberculum olfactorium, etc. In view of the relations 

 of the olfactory bulb to the other parts of the hemisphere in early 

 phylogenetic stages, as discussed above (p. 468), it is probable that 

 this, the oldest part of the hemisphere, is terminal with reference 

 to each of the other four parts of the secondary or evaginated part 

 of the telencephalon. This, of course, does not imply that the 

 olfactory bulbs represent the anterior end of the primitive neural 

 tube, for the latter lies in the lamina terminalis or below it, prob- 

 ably in the preoptic recess. In higher vertebrates the olfactory 

 bulb seems to belong to the pars ventro-medialis hemisphaerii; 

 but this peculiarity has been acquired secondarily by reason of the 

 preponderating importance of the efferent tract to the hypothal- 

 amus, as discussed on p. 468. 



The adult frog presents us with a typical picture of the fully 

 evaginated cerebral hemisphere reduced to its lowest morpholog- 

 ical terms. A recapitulation of the morphological characteristics 

 of its subdivisions follows : 



(1) The olfactory bulb. This we have considered in the im- 

 mediately preceding paragraphs. 



(2) The ventro-medial part. This is primarily a secondary 

 olfactory center, receiving the tractus olfactorius ventro-medialis 

 and is directly continuous behind and genetically related with 

 nucleus preopticus of the telencephalon medium and the hypo- 

 thalamus. The hypothalamus is an important diencephalic 

 correlation center and in early phylogenetic stages it sends ascend- 

 ing fibers into the pars ventro-medialis of the hemisphere. These 



