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 PL 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, Lereboullet remarking that in 

 Perca jluviatilis the posterior otolith acquires a diameter triple that of the anterior 

 (No. 93, p. 632). 



In preparations very deeply stained with hsematoxylin the otoliths not only show the 

 usual glistening crystalline structure with radial striations (oto, 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 (PL 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, i.e., about the time that the heart's pulsations commence. They are produced 

 by a paired thickening of the sensory epiblast (ep 2 , 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. l). A depression commences from the outside, and each nasal sac becomes a cup- 

 like structure, whose cells are now fusiform and radially arranged (ol, 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. Hoffman, 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, Marshall doubts whether in the Teleosteans 



