762 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
be larger and more rapidly developed in the embryos of demersal ova than in pelagic 
forms, though in the clupeoids with both demersal and pelagic ova the spacious nature 
of the otocysts is a characteristic feature. 
It is difficult to follow the changes in the structure of the ear in the transparent 
living fish, on account of its complexity. The semicircular canals seem to be developed 
primarily from thickenings of the cellular lining of the auditory sac, like the nervous 
cushions, a soft hernia being produced which grows inward as an increasing ridge, in 
which a cavity is formed, as shown in PI. VI. fig. 10, can. Viewed from the side, at 
a later stage the semicircular canals protrude into the chamber of the otocyst as three 
short knobbed processes directed inward from the margin. The median and inferior 
canals end abruptly in the middle of the chamber. Frequently the otoliths, instead of 
lying apart, each in a depression of the auditory floor, may shift, so as to lie towards the 
same part of the otocyst, e.g., in the anterior depression, as in Pl. VI. fig. 7, and Pl. XII. 
fig. 7. At times three otoliths occur, and when two are present, as is normally the 
case, there is usually a marked disparity in size, LeresouLLer remarking that in 
Perca fluviatilis the posterior otolith acquires a diameter triple that of the anterior 
(No. 93, p. 632). 
In preparations very deeply stained with hematoxylin-the otoliths not only show the 
usual glistening crystalline structure with radial striations (ote, Pl. VI. figs. 2, 3, 4, 9), 
but less numerous concentric striations, and a very marked dark central core surrounded 
by an external stratum, which stains more faintly (PI. VI. fig. 11). 
A further stage in the development of the Teleostean ear is observed in the young 
flounder (Pl. XV. fig. 8), in which the disparity of the otoliths and the complex 
condition of the auditory chamber are well shown. 
Olfactory Nerves and Pits.—The olfactory pits are distinguishable on the sixth day 
or later, z.e., about the time that the heart’s pulsations commence. They are produced 
by a paired thickening of the sensory epiblast (ep*, Pl. IV. fig. 17) in front of the upper 
part of the hemispheres. Each soon forms a flattened oval sac of slightly elongated cells 
(ol, Pl. IV. fig. 2), beyond which a small portion of the fore-brain (fb) extends (Pl. IV. 
fig. 1). A depression commences from the outside, and each nasal sac becomes a cup- 
like structure, whose cells are now fusiform and radially arranged (o/, Pl. IV. fig. 17). 
The flattened corneous layer is no longer present at the two points where the pits are 
formed, and as they become deeper and the walls of each sac increase in thickness, they 
may be brought into close contact with the anterior fore-brain, upon whose front they 
seem to lie in the living embryo (Pl. VI. figs. 6, 7, 8, 10; see also Pl. XII. figs. 1, 3, 
7, and Pl. XIII. figs. 1, 3, 5, 6, 7). So small is the space at this time separating 
the sacs from the brain that it is difficult to detect the nerve-strand which connects 
them. Horrman, however, made out the origin of the olfactory nerves as minute pro- 
liferations of the wall of the anterior fore-brain (No. 69, p. 87). This minute outgrowth, 
on reaching the nasal sac, coalesces with the proximal surface of the nasal pit. No olfactory 
lobes are at this time discernible ; indeed, MarsHatt doubts whether in the Teleosteans 
