THE TELENCEPHALON IN CYCLOSTOMES aad 
on p. 473) and points out that it is separated from the nucleus 
habenulae by a deep ventricular sulcus. He did not distinguish 
the characteristic cells of the ‘praethalamus,’ but describes the 
passage through it of the bundles of the taenia thalami which 
give off collaterals to it. 
None of the authors who have studied the petromyzont brain 
have used methods adequate to the differentiation of the types of 
cells characteristic of the so-called praethalamus and the parts 
adjacent to it. No method is so well adapted to this purpose as 
the Golgi method and the description of this region given by the 
writer in 1902 stands as the most complete heretofore given. That 
description gave, however, a very incomplete account of the facts 
shown in my preparations and the deficiency is to some extent made 
up in the figures accompanying this paper. The ‘praethalamus’ 
of authors is sharply distinguished from the epithalamus’ and 
thalamus not only by the ventricular sulci but by the well marked 
characteristics of its cells and by the course of fibers which traverse 
it. 
Between this body and the roof of the hemisphere there is in 
the same way a clear difference in the type of cells. This has . 
been sufficiently illustrated earlier (’02 a) and there is no dispute 
among authors upon this point. 
The fiber tracts related to the primordium hippocampi 
These fiber tracts have been very imperfectly understood. 
Both Schilling (’07, p. 482) and Tretjakoff (’09, p. 731) state that 
no tracts connected with this nucleus were seen. 
The stria medullaris is perhaps less complex in petromyzonts 
than in the higher fishes, but at least four bundles related to the 
telencephalon are present. (Fibers connecting the epithalamus 
with other parts of the diencephalon will not be considered here). 
One of these bundles comes from the medial olfactory nucleus, 
enters the primordium hippocampi near its rostral end above the 
foramen and seems to pass over the dorsal ventricular surface of 
this body to enter the nucleus habenulae (figs. 14 to 1G 2b, 26; 
35). It is by no means certain that this is a continuous tract. 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 22, NO. 4 
