142 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 



FIG. 147. Half of model of brain of embryo pig, 15 mm. long. (Compare with fig. 

 146, 3.) c, 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 tdencephalon, 



thalamencephalon, mesencephalon, metencephalon and myelencephalon 



derived from the primitive three. These usually retain in the interior 



