290 H. SAXTON BURR 
The subsequent changes in the larval forebrain are confined 
to the elaboration of the primordium hippocampi and the choroid 
plexuses. Two stages in the transformation of the central gray 
of the pars dorso-medialis into the characteristic scattered cellular 
arrangement of the primordium hippocampi and the elaboration 
of the vascular plexus are shown in figures 25 and 26. 
DISCUSSION 
The above study of the early morphogenesis of the rostral end 
of the neural tube in Amblystoma confirms in large measure the 
interesting and suggestive theory of Kingsbury concerning the 
anterior relations of the six longitudinal zones of His. In this 
form at least the evidence tends to show that the roof and floor 
plates of His do not meet at the neuropore, nor are the basal and 
alar plates separated in front of the notochord by the floor plate. . 
Rather, the relations in this region show that the floor plate of 
His ceases at the fovea isthmi, the midventral portion of the 
neural plate between the fovea and the preoptic recess being oc- 
cupied by the rostral continuity of the basal plate and that por- 
tion stretching from the preoptic recess to the lamina terminalis 
by the rostral continuity of the alar plate. In other words, the 
hypothalamus is made up entirely of basal-plate material and 
the terminal ridge, eventually occupied by anterior commissure 
fibers, is derived from the alar plate. In addition, the roof plate 
of His, instead of reaching ventrally the preoptic recess, stops at 
the rostral limit of the terminal ridge, the lamina terminalis being 
formed by the fusion of the lateral lips of the neuropore. 
The significance of the above to the theory of the evagination 
of the hemispheres becomes evident when we consider Herrick’s 
conception of the organization of the walls of the telencephalon. 
In his 1910 paper he concluded from a study of the morphology 
of the forebrain in Amphibia and reptiles, that “each cerebral 
hemisphere is naturally divided into four parts which correspond 
respectively with the four primary laminae of the lateral wall of 
the neural tube whose evagination produced the hemisphere’’ 
(p. 498). The lamination so described, while evident in the 
structurally defined hemisphere, is not so obvious in the stages 
