232 THE BRAIN OF THE TIGER SALAMANDER 



scribed in several species of amphibians by Rudebeck ('45). The adult 

 structure of these epithelial organs and the related meninges and 

 blood vessels I described in 1935. The ventral medial wall of the 

 adult (fig. 2) between the anterior commissure ridge and the tuber- 

 culum posterius is thin but nervous, except for the dorsal wall of the 

 infundibulum. It has a massive thickening — the chiasma ridge — 

 midway of its length. The arrangement of the chiasma ridge, the 

 anterior commissure ridge, and the adjoining preoptic recess and pre- 

 commissural recess is peculiar in the amphibian brain (fig. 2 and p. 291). 



The massive side wall of the adult diencephalon is composed of 

 four columns of gray substance, which project into the third ven- 

 tricle as longitudinal ridges separated by deep sulci. As shown here in 

 figures 1 and 2, these are epithalamus, dorsal thalamus, ventral 

 thalamus, and hypothalamus. This adult configuration is achieved 

 by a series of local pulses of proliferation and differentiation of cells, 

 which shift in location and rate of growth from stage to stage, as indi- 

 cated by changes in the sculpturing of the ventricular surface. 



Rudebeck ('45) illustrates embryonic stages of Necturus, Triturus, 

 and the anuran Pelobates which are younger than any of my models 

 of Amblystoma. His pictures show that, in each of these species in 

 stages preceding hatching of the eggs, the first ventricular markings 

 to appear in the rostral part of the neural tube take the form of a 

 series of transverse sulci. The most anterior of these, his "sulcus sub- 

 palliahs," separates the subpallial from the pallial parts of the hemi- 

 sphere. Next follows von Kupffer's sulcus intraencephalicus anterior 

 (p. 117), which separates the hemisphere and preoptic nucleus from 

 the diencephalon. Posteriorly of this sulcus is a high transverse ridge, 

 the di-telencephalic ridge, the adult derivatives of which apparently 

 are the habenula and those structures ventrally of it which are listed 

 in our next chapter as bed-nuclei of the di-telencephalic junction- 

 nucleus of Bellonci, eminentia thalami, nucleus of tr. olfacto- 

 habenularis (p. 239), and part of the preoptic nucleus and hypo- 

 thalamus. This ridge is bounded spinalward by his "sulcus 

 diencephalicus ventralis," the dorsal part of which persists as the 

 sulcus posthabenularis and the ventral part as the sulcus ventralis 

 thalami plus the sulcus hypothalamicus. Posteriorly of the di- 

 telencephalic ridge is an undeveloped area, giving rise to the adult 

 pars intercalaris diencephali and the major parts of the dorsal 

 thalamus, ventral thalamus, and hypothalamus. 



After swimming is well established (about Harrison's stage 37), 



