356 J. B. JOHNSTON 
turning up into the nucleus habenulae, as is seen in the most 
dorsal sections. Compare figures 15, 16 and 19, 20. Behind the 
line mentioned there is a wholly irregular tangle of nerve fibers and 
farther back the fibers of the optic tract. Scattered among the 
tangled fibers are bipolar and multipolar neurones with long sinu- 
ous dendrites devoid of spines. The type of structure of the two 
bodies as a whole is as strikingly different as are the individual 
neurones found in them. 
In transverse sections, owing to the direct contiguity and the 
oblique overlapping of the primordium hippocampi and the epi- 
thalamus, the boundary is of course not so clear, but there is a 
very abrupt transition from one type of neurones to the other 
which strongly suggests the distinctness of the two centers. 
Tretjakoff (09) gives a very imperfect description of the ‘prae- 
thalamus’ without figures. He states that its cells are much 
like those of the thalamus and that the only afferent or efferent 
tract connected with the praethalamusis constituted by the fibers 
from the parapineal organ. These reach the praethalamus after 
crossing in the habenular commissure. The praethalamus serves 
as a rudimentary perception center for the parapineal eye. ‘‘ Eine 
andere Bedeutung des Prithalamus lasst sich bei Ammocoetes 
kaum vermuten, da ungeachtet der grossen Zahl der Zellen der 
Praithalamus von Ammocoetes, nach meinen Untersuchungen, 
keine eigenen aus- oder zu fiihrenden Bahnen hat.’? The author 
was evidently impressed with the inadequacy of the parapineal 
tract to account for so large a center with a great number of cells. 
This impresses us much more when we remember that the para- 
pineal organ and tract exist only on the left side. According 
to Tretjakoff’s description the fibers cross in the commissure to 
enter the (right) praethalamus. Hence the left ‘praethalamus’ 
is wholly devoid of afferent fibers according to this author, and 
we are led to suppose that a large center, rich in highly developed 
cells exists in Ammocoetes totally without function. The writer 
has found no evidence in his preparations that the parapineal 
tract enters the ‘praethalamus’. 
Schilling (’07) is inclined to the opinion that the ‘praethalamus’ 
belongs to the telencephalon (p. 431; Herrick cites him erroneously 
