128 JOURNAL OF COMPARATIVE NEUROLOGY. 
Many collateral branches are given off in its course, and the ulti- 
mate destination of the principal stem appears to be a bundle of 
fibres contributing to the stratum medullare profundum. 
The body of the cell is so largely filled with the nucleus 
that often only the thinnest investing film of cytoplasm is ex- 
hibited. Six cells from this layer have their structural features 
shown in Fig. 53, and one of these has the thinnest enveloping 
cytoplasm found in this class. The nucleus of a cell is regularly 
oval in contour, holding one prominent nucleolus. The chrom- 
atic network is delicate and of small mesh. 
The cytoplasm contains masses of tigroid substance which, 
in proportion to the actual magnitude of the cell, are relatively 
large in size. When the nucleus lies toward one end of the 
cell, the largest tigroids are found in the free extremity. The 
prevailing form of tigroid mass is triangular, and the several 
masses have a rather open arrangement. 
From the form of the neurone just described, we should 
infer that it is adapted to the part of a conducting medium 
between the superficial and the deeper levels of the tectum 
mesencephali. The internal structure also indicates that it is 
characterized by no slight amount of metabolic activity. Evi- 
dently, then, there is here a nervous element of considerable 
importance in the economy of the midbrain. 
Van GEHUCHTEN (’94) has described the deeper neurones 
from the optic lobe of the trout (his couche granuleuse) as being 
without dendrites, and as sending their axones peripherally 
into the superficial levels. The character of this type of neu- 
rone in Mustelus certainly corresponds the more nearly with 
the conditions found in higher vertebrates, although much sim- 
pler of course, so that, in this respect at least, the selachian is 
seen to be in the direct phylogenetic line, while the teleost is 
divergent. 
In the inner zone of the deeper layer of the tectum, at the 
level occupied by the numerous collaterals from the proper 
axones, there are to be found at intervals neurones of another 
character (Fig. 21). Such neurones are irregularly stellate as 
to general form, the dendrites and the axone radiating widely 
