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 Marshall'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 (Balfour's stage 0) 

 (No. 100, vide pi. 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 Marshall, 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, 

 PI. IV. fig. 16), and these latter as they increase in bulk approach, as in PI. IV. 

 fig. 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 

 (PI. 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 Marshall 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. Marshall could 

 not make out any ganglionic enlargement (No. 100, p. 312); but Beard 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, pi. 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, PI. XXIV. 

 fig. 4; also vide Marshall, No. 100, pi. 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 Marshall has undoubtedly given accurately the facts of 

 the early development of the first pair of nerves, which, however, Huxley 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 (PI. 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 (PI. XV. fig. 8). 



Optic Nerves and Vesicles. — One of the earliest features in the development of the 



* Vide section xiii. p. 254. 



