THE TELENCEPHALON IN CYCLOSTOMES 365 
is of course directly adjacent to the eminentia thalami and Pro- 
fesssor Herrick hasapparently thought that more was to be included 
in the unevaginated primordium hippocampi than the writer 
intended. This structure is the continuation of the fimbria- 
border of the hemisphere into the dorsal border of the telencephalon 
medium until it meets the eminentia thalami. This small rem- 
nant of the primordium is seen in a model of the forebrain of 
Necturus standing vertically rostral to the eminentia thalami and 
separated from the latter by a groove. Figure 36 will show more 
clearly the relations of this structure. Deep fibers related to the 
cells of this unevaginated body enter the hippocampal commissure 
as already described. 
As the evagination proceeds the caudal part of the hemisphere 
soon extends rostrad beyond the level of the interventricular 
foramen and the lamina terminalis. This is already true in selach- 
ians and in amphibians and reptiles the hemisphere protrudes far 
rostrally (fig. 37). In selachians, it has already been pointed out 
that the sulcus limitans hippocampi continues rostrally beyond 
the laterally placed olfactory peduncle and encircles the rostral 
wall of the hemisphere to end at the neuroporic recess (figs. 38, 
39). This portion of the sulcus was called the medial sulcus 
limitans hippocampi. As compared with cyclostomes, the con- 
dition in selachians has resulted from the continued evagination 
which has carried the olfactory bulbs far laterad and has brought 
the greater part of the primordium hippocampi into the roof of 
the hemisphere. The region of the medial olfactory nucleus has 
bulged forward beyond the lamina terminalis, and the sulcus which 
separates the primordium from the medial olfactory nucleus in 
front of the interventricular foramen in Petromyzon comes in: 
selachians to lie in the medio-rostral wall of the hemisphere. In 
both classes it runs from the neuroporic recess into the lateral 
ventricle and marks the line of separation of the same structures. 
It is properly called the sulcus (or zona) limitans medialis. In 
amphibians the conditions are essentially the same (fig. 37). 
Professor Herrick’s view as to the evagination of the hemispheres 
and the relation of his four columns to them is summarized in 
the following paragraph (’10, p. 477): 
