EARLY DEVELOPMENT OF HEMISPHERES 283 
It would seem, therefore, that from the terminal ridge caudally 
to the fovea isthmi we are dealing not with the non-nervous floor 
plate of His, but with a nervous tissue continuing the basal plate 
of one side with that of the other. So far as Amblystoma is 
concerned, it is probable that the fundamental arrangements of 
the longitudinal columns of His in the rostral end of the neural 
tube were as indicated by Kingsbury (720) in his figure 6. These 
rostral relations may be stated as follows: The floor plate of His 
terminates at the fovea isthmi, about which arches anteriorly 
the basal lamina, the continuity of which is maintained by the 
ventral portion of the neural plate lying between the fovea and 
preoptic recess. Arching about the basal lamina still more ros- 
trally and separated from it by the sulcus limitans, is the alar 
lamina. The continuity of the two lateral alar laminae is repre- 
sented in the midline by the terminal ridge. The roof plate 
whose rostral portion is the lamina terminalis, therefore, does not 
meet the floor plate, but is separated from it by the structures 
above mentioned. 
EVAGINATION OF HEMISPHERES 
The general form and arrangement of the constituent elements 
of the forebrain of Amblystoma are well known through the 
researches of Herrick, Johnston, and others. As a result of their 
work, and more particularly that of Herrick (’10, ’21 a), we know 
that the forebrain of Amblystoma has a fairly well-defined organi- 
zation. ‘Through his study of Amblystoma larvae of varying 
stages and of the adult, Herrick has made fairly definite the fol- 
lowing parts of the hemisphere. It can be divided into a number 
of relatively distinct nuclei. The most important of these is the 
olfactory bulb lying at the rostral end of the hemisphere. In 
addition, four regions are described called, respectively, the ven- 
tro-medial, ventro-lateral, dorso-medial, and dorso-lateral parts. 
These converge forward into a relatively undifferentiated nucleus 
olfactorius anterior at the base of the olfactory bulb. 
The ventro-medial part is the septum (in the broad sense), 
characterized by the ventro-medial olfactory tract, the medial 
forebrain bundle and their connecting tracts. The ventro-lat- 
