464. BERTRAM H. WATERS. 
a true neuromere as determined by Orr, and is, I think, the first 
or olfactory neuromere, as from its crest occurs the prolifera- 
tion of cells already mentioned. TI cannot be certain in regard 
to this point, as lack of early stages renders accurate deter- 
mination of the boundaries of the medullary plate impossible, 
and differentiation between primary and secondary fore-brain 
extremely difficult. The point is animportant one, and deserves 
further careful investigation. 
Immediately back of what, for convenience, I have termed 
the first neuromere, and in a line with the forward portion of 
the orbit, the brain-wall on either side begins to bulge out 
into broad, somewhat bluntly rounded diverticula (fig. 3, 
op. lu.). These are the remnants of the optic vesicles, the 
distal portions having been constricted off to form the second 
nerves. Now, while none of my Cod sections give any reliable 
evidence of a neuromere at this point, I will show that in 
Amblystoma there is certainly a second neuromere, and that 
the optic diverticula hold a curiously significant and close re- 
lation to it. Behind the optic vesicles the brain assumes its 
narrow and uniform tract-like appearance (fig. 3). The closely 
compacted oval cells show little or no lumen, and are enclosed 
on either side by a narrow border of white cortical matter 
(figs. 2and 8, cm.). At a point somewhat behind a line drawn 
through the axis of the eyes, these borders join across the 
brain by a narrow band of white fibres—the posterior com- 
missure,—thus marking the junction of the fore- and mid- 
brain. The distance from the posterior commissure to the 
termination of the second neuromere is about one third the 
entire neural length from before backward to this point. It 
is not possible in this space to observe any nerve or any pro- 
liferation of cells, however slight, neither can the cells of the 
brain-walls be said to have any radial arrangement. However, 
the condition of my material is quite unsatisfactory, permitting 
no clear observation of cell boundaries ; and from the fact that 
there is just sufficient room at this point for another neuro- 
mere, and that in Amblystoma I have been able more satis- 
factorily to prove its existence, I have called this the third 
