EARLY DEVELOPMENT OF HEMISPHERES 281 
Now the ventral lip of the neuropore is the region at the rostral 
end of the neural tube where the material which makes up the 
lateral lips of the neuropore is continuous from side to side 
across the midline. This material constitutes the rostral portion 
of the alar plate, and hence we may conclude that in the terminal 
ridge the alar plate of one side becomes continuous with that of 
the other. In other words, the assumption of Kingsbury (720) 
that the alar plate of the neural tube forms an arch about the 
anterior end of the neural plate may be considered as correct. 
The terminal ridge represents the median portion of the alar 
plate with its caudal boundary marked by the preoptic recess 
where the sulcus limitans of one side becomes continuous with 
that of the opposite side. 
The further suggestion of Kingsbury that the basal plate is 
likewise continuous from side to side across the median line 
arching around the fovea isthmi, the anterior limit of the 
floor plate of His, seems to be substantiated by the following 
facts. In figures 7 to 12 are shown a series of critical cross- 
sections through the brain of Amblystoma showing the relations 
and structures, respectively, of the lamina terminalis, the terminal 
ridge, and the chiasmatic ridge. The gross relationships are 
shown in figures 7, 8, and 9 and the microscopical structure in 
figures 10,11, and 12. It is evident from these figures that from 
the point where the lamina terminalis merges with the terminal 
ridge the midventral line of the brain is occupied, not by typical 
floor-plate material, i.e., non-nervous supporting tissue, but 
throughout its length until the fovea isthmi is reached, by typical 
nervous tissue. Only one exception can be made to this general 
statement and that is that part of the floor plate which is to form 
eventually the roof of the hypothalamus. In the older stages it 
is reduced to a non-nervous lamina. This, however, has no 
significance when it is noted that this region in its early develop- 
mental stages does not differ from the structure of the terminal 
ridge and the chiasmatic ridge, so far as can be determined from 
microscopic sections. The thinning of this area is shown in 
figures 14, 16, and 18. 
