I42 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
the olfactory epithelium (see sense organs), and into this a portion of 
the cavity (ventricle) of the hemisphere may extend (fig. 146). 
Considerable differences exist in the olfactory lobes. In some cases they are 
directly continuous with the hemispheres, but they may be prolonged, each having 
two portions, a narrower stalk, the tractus olfactorius, and a distal enlargement, 
the bulbus olfactorius. The true olfactory nerve takes its origin from the end 
of the bulb or its homologue (for details see cranial nerves). 
The ventral zone and posterior part of the dorsal zone of the fore- 
brain, after the differentiation of the telencephalon, forms the thala- 
mencephalon (’twixt-brain, diencephalon). Its sides, the optic 
Fic. 147.—Half of model of brain of embryo pig, 15 mm. long. (Compare with fig. 
146, 3.) ¢, cerebrum; cb, cerebellum; cs, corpus striatum; i, infundibulum; is, isthmus; 
fi, interventricular foramen; m, mesencephalon; mo, medulla oblongata; ¢, thalamus. 
thalami, remain without marked modification, but its floor and roof 
form median outgrowths, corpus pineale, epiphysis, etc., above, in- 
fundibulum below—to which reference will be made again later. 
In the mid-brain there is little modification except a thickening of 
the walls forming a pair of prominences, the optic lobes, or corpora 
bigemina (in mammals two pairs of lobes, corpora quadrigemina) 
on the dorsal surface. The mid-brain of the adult is also called the 
mesencephalon (fig. 146). 
In the hind-brain the great modifications occur again in the dorsal 
zone. Its dorsal portion extends itself upward and backward as a 
broad lobe which tends to arch over the rest of the hind-brain. This 
outgrowth forms the cerebellum or metencephalon, while the 
remainder of the hind-brain constitutes the medulla oblongata or 
myelencephalon (fig. 146). 
Thus there arise in the adult brain five regions—telencephalon, 
thalamencephalon, mesencephalon, metencephalon and myelencephalon 
—derived from the primitive three. These usually retain in the interior 
