328 



On the Olfactory Nerve, tyc, of Vertebrates. [Feb. 13, 



points only being noticed which are of special interest in connexion 

 with the conclusions arrived at in the preceding part of the paper. 



The olfactory pits appear at almost the same time as the visceral 

 clefts ; or, to speak more accurately, they first become conspicuous 

 objects at, or very shortly after, the time when the anterior visceral 

 clefts become open to the exterior. This occnrs about stage K in the 

 dogfish, and about the fiftieth hour in the chick. 



In their early stages the olfactory pits present a striking resem- 

 blance to the visceral clefts in position, shape, size, and general rela- 

 tions ; their external apertures elongate and become slit-like, and the 

 direction of the slit, like that of the visceral clefts, is at right angles 

 to the longitudinal axis of the head. These facts are best illustrated 

 by the study of whole embryos, and of longitudinal vertical sections.* 

 They come out with great clearness in all the types of vertebrates 

 examined, but with especial distinctness in the axolotl and salmon. 



The development of the Schneiderian folds presents several points 

 of great interest, which can be most favourably studied in the Elasmo- 

 branchs. Attention has already been directed by Balfourf to the very 

 early appearance of these folds. The important point, so far as the 

 present question is concerned, is that these Schneiderian folds appear at 

 the same time as, or very shortly after, the first rudiments of the gills. 

 In addition to this identity in time, there is also identity in structure ; 

 in both cases development consists in the formation of a series of equal, 

 closely apposed folds, mainly epithelial, but involving the underlying 

 mesoblast to a certain extent. These folds are in the two cases — gills 

 and Schneiderian folds — of the same width, the same distance apart, 

 have epithelium of the same thickness and same histological character, 

 involve the mesoblast to exactly the same extent, and in exactly the 

 same manner ; in a word, are structurally identical. 



In the later stages the Schneiderian folds, like the gills, receive a 

 very abundant supply of blood-vessels ; and the relations of these 

 vessels to the folds, which are very peculiar and characteristic, are 

 identical in the two cases. Even in the adult Elasmobranch there is a 

 remarkable histological resemblance between the gills and the nose. 



The facts above recorded concerning the development of the olfactory 

 nerve and olfactory organ point towards the same conclusions as to 

 morphology of these structures, viz., that the oifactory organ is the 

 visceral cleft ; that the olfactory nerve is the segmental nerve supply- 

 ing that cleft in a manner precisely similar to that in which the hinder 



* For figures of whole embryos illustrating the points referred to, vide Parker, 

 " On the Structure and Development of the Skull in Sharks and Skates," "Trans. 

 Zool. Soc.," vol. x, part iv, 1878, PI. 25, fig. 1 ; PI. 39, figs. 1 and 2 ; PI. 40, fig. 1 ; 

 and Balfour, op. cit., PI. 7, Stage L. 



f Op. cit., p. 184, and PI. 44. fig. 14. 



