1 IG THE BRAIN OF THE TIGER SALAMANDER 



MORPHOLOGICAL LANDMARKS 



Comparative study shows that some general features of structural 

 plan run through the vertebrate series with remarkable constancy 

 and that other features undergo amazing transformations. The dis- 

 covery of the laws in accordance with which these transformations 

 are effected is the goal toward which we are working. We are con- 

 fronted with a similar, but not parallel, series of problems in the 

 study of embryological development. In so far as we succeed in our 

 search for these laws, we advance our understanding of fundamental 

 vital processes. 



The stable structural features of the nervous system are the most 

 useful landmarks for the comparative anatomist. They are expres- 

 sions of the conservative hereditary factor in morphogenesis ; but this 

 stability cannot safely be interpreted as the simple manifestation of 

 some primordial archetypical pattern, for these features are retained 

 during the course of phylogenetic history only in so far as the funda- 

 mental features of the peripheral connections and their internal rela- 

 tionships are constant, that is, because they are parts of an apparatus 

 of adjustment to environment which is common to all vertebrates. 

 Other parts of the brain are more variable because, with complica- 

 tion of the behavior pattern in higher species, more elaborate and 

 diversified mechanisms of adjustment and integration are requisite. 



In all vertebrate brains the most fundamental structural land- 

 mark is the transverse plane separating the spinal cord and rhom- 

 bencephalon below from the cerebrum (as defined in the BNA) 

 above. In Amblystoma this plane is marked externally by the fissura 

 isthmi and internally by the sulcus isthmi. The zonal arrangement as 

 described in chapter v is well defined in the rhombic brain and the 

 midbrain, rostrally of which it is obscured by various secondary 

 modifications which become more complicated as we pass from lower 

 to higher members of the vertebrate series. A second important land- 

 mark is the transverse plane separating the diencephalon from the 

 telencephalon, marked externally by the deep stem-hemisphere 

 fissure. 



These two planes also mark the positions of two strong flexures of 

 the neural tube in early embryonic stages, caused by inequalities of 

 growth of the dorsal and ventral zones of the neural tube. The first 

 of these flexures to appear is a ventral bending of the neural tube in 

 the mesencephalic region, caused by precocious enlargement of the 



