THE TELENCEPHALON IN CYCLOSTOMES SH 
The pyramidal body and the dendrites studded with knobbed 
spines are so characteristic of these neurones that the resemblance 
to the hippocampal cells in Acipenser, Amia and Rana is at once 
striking and unequivocal. Examples of these cells in each of the 
forms named are shown in figures 30 to 34, for comparison with the 
cells in Lampetra. The bodies of the neurones in Lampetrastand 
near the ventricle and their dendrites divide into a few relatively 
straight branches which traverse the thickness of the wall, often 
reaching the outer surface (figs. 30, 31). 
When we look for the boundary between the primordium 
hippocampi and the epithalamus, the internal structure as seén in 
horizontal sections seems to furnish the necessary data. First, 
there is no difference in the internal structure of the whole dorso- 
median ridge bounded by the foramen and the sulcus limitans 
hippocampi. Everywhere it is filled by the peculiar type of 
neurones just described and nowhere is there any change of finer 
structure which would lead us to say that any two or more parts 
of it represent different functional centers. However, the moment 
the sulcus limitans hippocampi is passed in any direction we come 
upon neurones of types wholly different from those of this ridge. 
This can not be accidental; it must be the expression of functional 
differentiation. 
The neurones of the so-called striatum have been described and 
figured in my earlier paper (’02 a, figs 18, 19). They are bipolar 
or multipolar cells with irregularly curved and branching dendrites 
free from spines. The neruones of the nucleus habenulae have 
also been figured (02 a, fig. 16; this paper, figs. 17 to 20). They 
are, like those in Acipenser, small cells with short very irregular 
dendrites often with enlarged tips bearing tufts of small branches. 
The subhabenular region is broadly continuous with the primor- 
dium hippocampi but the boundary line is very distinct in Golgi 
sections. The type of neurones peculiar to the primordium hip- 
pocampi stops abruptly along a line drawn latero-caudad from 
the sulcus limitans hippocampi (see figures of horizontal sections, 
18, 19, 20). At the same time along this same line end abruptly 
the parallel fibers coursing lengthwise through this ridge. At 
this line the fibers are cut off in horizontal sections because they are 
