198 J. B. JOHNSTON, 
from an imperfectly lettered figure of Barbus which appears in his 
description of the Reptilian tectum) a smaller number in fishes. 
These papers are too long for me to give citation and discuss in 
detail. VAN GEHUCHTEN’s paper (92) on birds I know only from 
KÖLLIKER’S references. In the following paragraphs I shall refer 
to all these papers, comparing the different types of cells in the 
various forms with those described in fishes by VAN GEHUCHTEN, 
FUSARI, and myself. 
The four types of cells which I describe for the tectum of 
Acipenser (page 73), leaving aside the torus longitudinalis, represent 
as generalized types all the kinds of cells described in forms above 
the fishes. Cells A in Acipenser are cells with short neurites which 
break up in the middle zone of the tectum. Similar cells are found 
in Teleosts (VAN GEHUCHTEN), frog (P. Ramon), Reptiles (P. Ra- 
MON), birds (S. Ramon), and Mammals (KOLLIKER), although they 
were overlooked by FUSARI, and their neurites escaped VAN GE- 
HUCHTEN in Teleosts. In Teleosts, Reptiles, and birds some cells 
with short neurites lie in the outer half of the tectum. In all forms 
below the birds, however, these cells are always fusiform and ver- 
tical, as are the most of them in birds. Taking the fusiform cell 
of Acipenser as the primitipe type, the others may all be derived 
from it on the supposition that they have gradually migrated farther 
from the central cavity and those which have migrated farthest (stellate 
cells in birds) have become stellate simply because they are now 
near the surface and there is neither space nor necessity for the 
vertical fusiform condition. It is of the greatest importance that 
the neurites and dendrites have maintained the same disposition in 
the tectum throughout all the forms described. . Always the dendrites 
ramify near the surface while the neurites break up in the middle 
third, just outside the middle fibre zone when such exists. The 
place of end-branching of the neurites is such that they may come 
into connection with the greatest number of dendrites of cells whose 
neurites go to other parts of the brain or to the opposite side of 
the tectum. The disposition of the dendrites and neurites determines 
the function of the cells in the central system in the great majority 
of cases, at least in the lower Vertebrates. This is, therefore, the 
proper basis of classification of cells in brain nuclei. 
The cells B in Acipenser find exact counterparts in all classes 
(VAN GEHUCHTEN, P. Ramon, S. Ramon, KÖLLIKER), although they 
were not found by Fusarı. Everywhere their neurites join the most 
