432 C. JUDSON HERRICK 



2) an evaginated portion, the cerebral hemispheres, which in 

 this case are composed of the olfactory bulbs only. The primitive 

 endbrain contains an unpaired ventricle, the common endbrain 

 ventricle, which is always bounded in front by the terminal plate 

 and in forms above the fishes is the terminal part of the third 

 ventricle as the latter cavity is usually defined. Each cerebral 

 hemisphere contains one of the lateral ventricles which com- 

 municates with the common ventricle by an interventricular 

 foramen (of Monro) . 



The endbrain of every vertebrate is organized on this plan, 

 with, however, the greatest diversity of detail. As we pass 

 from lower to higher forms, there is in the broad view a progres- 

 sive incorporation of more and more of the tissue of the original 

 unevaginated primitive endbrain into the evaginated cerebral 

 hemispheres, though always, even in the adult human brain, 

 there is a small residue of unevaginated endbrain tissue between 

 the optic chiasma and the terminal plate (the preoptic nucleus) 

 and a residual portion of the common ventricle in the preoptic 

 recess (Johnston, '11, p. 50; '11 a, p. 496; '12, p. 367). 



The different veretebrate phyla present a varied assortment 

 of deviations from the paradigm or schematic pattern just 

 described, but in no case is the pattern fundamentally changed. 

 Some illustrations of these lines of divergent specialization will 

 be cited. 



TYPES OF FOREBRAIN IN FISHES AND AMPHIBIA 



In the adult sturgeon, Acipenser, the departure from the 

 schematic type is very slight, consisting mainly in the thickening 

 of the side walls of the primitive endbrain, thus providing space 

 for the neurons of the correlation centers which receive the great 

 descending olfactory tracts from the olfactory bulbs. These 

 relations are illustrated in figure 4, which presents a diagram- 

 matic longitudinal section of the forebrain of the sturgeon so 

 taken as to reveal the relations of the brain walls to the con- 

 tained ventricles (Johnston, '01). Figure 5 shows a transverse 

 section through the primitive endbrain. 



