THE FORM AND SUBDIVISIONS OF THE BRAIN 21 



the pars pallialis. In the pallial part no laminated cortical gray is 

 differentiated, but there are well-defined pallial fields: dorsomedially, 

 the primordial hippocampus; dorsolaterally, the primordial piriform 

 lobe; and between these a primordium pallii dorsalis of uncertain 

 relationships. 



The boundaries of the diencephalon, as here defined and shown in 

 figure 2A, are: anteriorly, the stem-hemisphere fissure and the pos- 

 terior border of the anterior commissure ridge and, posteriorly, the 

 anterior face of the posterior commissure and the underlying com- 

 missural eminence and, more ventrally, the sulcus, s, which marks 

 the anterior border of the cerebral peduncle. The inclusion of the 

 preoptic nucleus is in controversy; but, whether or not this inclusion 

 is justifiable morphologically, its relationships with the hypothala- 

 mus are so intimate that it is practically convenient to consider these 

 parts together. The four primary subdivisions of the diencephalon as 

 I defined them in 1910 are: (1) the dorsal epithalamus, containing on 

 each side the habenula and pars intercalaris, the latter including the 

 pretectal nucleus; (2) pars dorsalis thalami, which is the primordium 

 of the sensory nuclei of the mammalian thalamus; (3) pars ventralis 

 thalami, the motor zone of the thalamus, or subthalamus; (4) hypo- 

 thalamus. The mammalian homologies of these areas are clear, 

 though their relative sizes and fibrous connections exhibit remarkable 

 differences. 



The posterior boundary of the mesencephalon is marked by the 

 external fissura isthmi, the ventricular sulcus isthmi (fig. 2B, s.is.), 

 and ventrally in the floor plate a pit, the fovea isthmi [f.i.). These 

 are all more prominent in the larva than in the adult. This sector 

 includes the posterior commissure, the tectum mesencephali (pri- 

 mordial corpora quadrigemina) , the underlying dorsal tegmentum 

 (subtectal area), and the area surrounding the tuberculum posterius 

 at the ventral cerebral flexure, termed the "nucleus of the tuberculum 

 posterius." On embryological grounds and for convenience of descrip- 

 tion, this ventral area, which is bounded by the variable ventricular 

 sulcus s, is here called the "peduncle" in a restricted sense ('36, p. 

 298; '396, p. 582). This is a primordial mesencephalic structure which 

 is not the equivalent of the peduncle of human neurology. Amblys- 

 toma has nothing comparable with the human basis pedunculi, and 

 its "peduncle" is incorporated within the tegmentum of the human 

 brain. The III cranial nerve arises within the "peduncle" and 

 emerges near the fovea isthmi. The nucleus of the IV nerve is in the 



