PRIMITIVE SEGMENTATION OF THE VERTEBRATE BRAIN. 463 
white border (see fig. 2, p.c.). This I consider the posterior 
commissure, and that it defines the backward limit of the 
primitive fore-brain. This area has a peculiar club-shaped or 
trefoil appearance, and nothing can be seen of the brain cavity 
or of the canalis centralis, which appears as a faint line, 
except where the drawing away of the brain walls, to accom- 
modate the budding out of the optic vesicles, forms an 
irregular lozenge-shaped opening into the cavity of the third 
ventricle (fig. 2, op. lu.). Directly in front of the eyes, and in 
apposition with the anterior extremity of the thalamencephalon, 
lie the olfactory vesicles (figs. 2 and 38, ol. v.). These I find 
connected with the brain by a short thick mass of cells on 
either side, the olfactory nerves, which even in the late stages 
seem to have no connection with the prosencephalon, so that 
for the Cod I am able to confirm the observations of Marshall 
(17, c) in regard to this nerve in other forms. While I can 
affirm nothing as to the persistence of the neural ridge to the 
forward extremity of the fore-brain, all my sections, even the 
earliest stages, show the olfactory pits lying on either side, and 
slightly in front of the fore-brain, and there is in no section 
the slightest trace of an olfactory lobe. The nerves themselves 
in relation and histological structure agree closely with the 
other cranial nerves, except that so far as I am able to observe 
they are developed somewhat in advance of them. My sections 
being longitudinal I am not able to state with certainty their 
precise point of origin, but I can see no reason to doubt that 
it is at the anterior extremity of the fore-brain. They certainly 
pass downward and outward and at right angles to the longi- 
tudinal axis of the head—the characteristic course, according to 
Marshall, of a segmental nerve. They consist of rounded or 
oval cells with a very few nerve-fibres. Many earlier stages 
show a distinct proliferation of the cells into the white sub- 
stance on either side, behind and above the position occupied 
by the developing nerve, which seems to support Marshall’s 
idea that these nerves have been shifted downward and for- 
ward from their original point of origin. ‘The region of brain- 
wall giving rise to them shows markedly the characteristics of 
