20 THE BRAIN OF THE TIGER SALAMANDER 



some distance posteriorly of the ventricular sulcus isthmi; but, like 

 the latter, it really marks the anterior border of the isthmus, as will 

 appear in the description of the development of the isthmic sulcus 

 (p. 179). 



The obvious superficial eminences on the dorsal aspect of the brain 

 are the small cerebellum, the dorsal convexity of the roof of the 

 midbrain (tectum mesencephali) , the habenular nuclei of the epi- 

 thalamus, and the two cerebral hemispheres. Posteriorly of the 

 habenulae in the early larvae is the membranous pineal evagination, 

 which in the adult is a closed epithelial vesicle detached from the 

 brain except for the few fibers of the parietal nerve. The lateral aspect 

 of the thalamus, midbrain, and isthmus is a nearly smooth convexity, 

 posteriorly of which is the high auricle, composed of tissue which is 

 transitional between the body of the cerebellum and the acoustico- 

 lateral area of the medulla oblongata. This auricle contains the pri- 

 mordia of the vestibular part of the cerebellar cortex (flocculonodular 

 lobe of Larsell), and most of its tissue is incorporated within the 

 cerebellum in mammals. On the ventral aspect there is a low emi- 

 nence in front of the optic chiasma, which marks the position of the 

 very large preoptic nucleus, and a similar eminence behind the 

 chiasma formed by the ventral part of the hypothalamus. The latter 

 is in the position of the human tuber cinereum but is not exactly 

 comparable with it. Most of the hypothalamus is thrust backward 

 under the ventral cerebral flexure as the pars libera hypothalami. 

 The large pars glandularis of the hypophysis envelops the posterior 

 end of the infundibulum and extends spinal ward from it, not an- 

 teriorly as in man. 



The primary subdivisions of the human brain as defined from the 

 embryological studies of Wilhelm His are readily identified in adult 

 Amblystoma, as shown in the median section (fig. 2A). 



At the anterior end of each cerebral hemisphere is the very large 

 olfactory bulb, the internal structure of which shows some interesting 

 primitive features (p. 54; '246). The bulbar formation extends back- 

 ward on the lateral side for about half the length of the hemisphere, 

 but on the medial side only as far as the anterior end of the lateral 

 ventricle (figs. 3, 4). Bordering the bulb is an undifferentiated an- 

 terior olfactory nucleus, and posteriorly of this the walls of the lateral 

 ventricle show early stages of the differentiation of the major sub- 

 divisions of the mammalian hemisphere — in the ventrolateral wall a 

 strio-amygdaloid complex, ventromedially the septum, and dorsally 



