DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 763 
(Salmonoids) he examined true olfactory lobes ever are formed. At any rate, he cannot 
regard them, and justifiably so, as embryonic structures (No. 100, p. 313). The proximity 
of the olfactory pits and the brain renders the determination of such a point in the 
minute Teleostean forms here considered very difficult; but MarsHa.t’s conclusions 
admit of little question. In the Elasmobranch-embryo the olfactory lobes are not 
distinguished until almost all the features of the adult are attained (BALrour’s stage O) 
(No. 100, vide pl. xiv. figs. 24, 33, 34), and in the chick they cannot be made out 
until the seventh day (No. 17, p. 162). There is no trace of these lobes in Rana during 
the earlier stages, according to MarsHALt, and the nerve-strand passing to the olfactory 
pit is very short. 
A similar condition is found in Teleosteans ; a solid strand of cells passes from the 
roof of the fore-brain, before it shows any trace of external division, to the pits (ol, 
Pl. IV. fig. 16), and these latter as they increase in bulk approach, as in Pl. IV. 
fic. 17, and come into such close proximity to the fore-brain (fb) that an actual 
reduction in length of the primitive nerve results, so that it is barely distinguishable 
(Pl. VI. fig. 6). The histological character of these primary olfactory strands supports 
the view that they are merely diverticula from the brain, in which organ no fibres are 
yet formed, for the first pair of nerves have a similar solid cellular structure. This 
structure MarsHaLu found to be retained, when the other cranial nerves had assumed 
the fibrillar character. It is remarkable that the olfactory nerves, which are amongst 
the earliest to be given off, retain their primitive structure longest. MarsHa.i could 
not make out any ganglionic enlargement (No. 100, p. 312); but Bearp in some later 
researches found that, as in Rana, a ganglion does arise in connection with the epiblastic 
thickening forming the pit, and that the olfactory nerve itself is also split off from 
the skin (vide his figs. of Rana and Rhodeus amarus, figs. 3 and 4, pl. viii. No. 22). 
The dorsal position of the nasal pits is interesting, as in the Elasmobranchii and Aves 
these structures are on the under side of the head. The nerves shift down from their 
first position, and are found to connect with the fore-brain ventrally (1, Pl. XXIV. 
fiz. 4; also vide Marsuaut, No. 100, pl. xiv. fig. 33). Of course in the Teleosteans this 
transference is much reduced, as the fore-brain does not grow so extensively as the hinder 
portions of the brain; but MarsHatt has undoubtedly given accurately the facts of 
the early development of the first pair of nerves, which, however, HuxLry considered 
to be developed late, and to have but one paired connection with the brain, and that a 
ventral connection (No. 74, p. 71). This ventral origin is secondary, and comparatively 
late, but it is very much later before the basal swellings known as the olfactory lobes 
are clearly indicated (Pl. XXIV. fig. 4). 
In the early forms treated of in this section the division of the original single nasal 
opening into two was not followed. It is readily observed in the wolf-fish (Anarrhichas) * 
and in the young flounder (Pl. XV. fig. 8). 
Optic Nerves and Vesicles.—One of the earliest features in the development of the 
* Vide section xiii. p. 254. 
