THE FORM AND SUBDIVISIONS OF THE BRAIN 19 



aspects and the distribution of endocranial arteries and veins. The 

 outHnes of the brain were drawn from specimens dissected after 

 preservation for 6 weeks in 10 per cent formahn. One of these is 

 shown here (fig. 86 A). Figure IB is drawn from a dissection made by 

 the late Dr. P. S. McKibben, showing the sculpturing of the ven- 

 tricular surfaces. Figures lA and 85 are drawn from a wax model in 

 which there is some distortion of the natural proportions. Not all the 

 differences seen in these pictures and in the proportions of sections 

 figured are artifact, for the natural variability of urodele brains is 

 surprisingly large (Neimanis, '31). Brains of larval stages have been 

 illustrated by many authors and in my embryological papers of 

 1937-39. 



The somewhat simpler brain of the mudpuppy, Necturus, has 

 been described in a series of papers as completely as available mate- 

 rial permits, and comparison with the more differentiated structure 

 of Ambly stoma is instructive. The sketches shown in figures 86B and 

 C illustrate the differences between the form of this forebrain and 

 that of Amblystoma. The monograph of 1933 contains a series of 

 diagrams ('33&, figs. 6-16) of the internal connections of the brain of 

 Necturus similar to those of Amblystoma shown here (figs. 7-24). 

 In 1910 I described the general features of the forebrain of A. 

 tigrinum, with a series of drawings of transverse Weigert sections, 

 no. lie, which has subsequently been used as the type specimen. 

 Though this paper contains some errors and some morphological 

 interpretations which I now regard as outmoded ('33a), most of the 

 factual description has stood the test of time, and additional details 

 and reports on other parts of the brain have been published in a 

 series of papers. 



The most conspicuous external fissures of the brain of Amblystoma 

 are: (1) the longitudinal fissure separating the cerebral hemispheres; 

 (2) the deep stem-hemisphere fissure; (3) a wide dorsal groove sepa- 

 rating the epithalamus from the roof (tectum) of the midbrain; (4) 

 the ventral cerebral flexure or plica encephali ventralis, which is a 

 sharp bend of the floor of the midbrain, where it turns downward and 

 backward into the "free part" of the hypothalamus; and (5) the 

 fissura isthmi, extending downward and forward from the anterior 

 medullary velum between midbrain and isthmus. The middle part of 

 the fissura isthmi is at the anterior border of the auricle, which is more 

 prominent in the larva than in the adult ('14a, figs. 1-3). Here in the 

 adult it lies near the posterior border of the internal isthmic tissue, 



