MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 301 



With regard to the earliest stages in the development of 

 the olfactory nerve, I have, unfortunately, heen unable to 

 make any satisfactory observations, for all the specimens 

 younger than Balfour's stage k were in bad condition. The 

 chief points I wished, if possible, to determine were — firstly, 

 whether the neural ridge extends to the anterior end of the 

 fore brain in Elasmobranchs, as I have already shown it to do 

 in the chick ;i secondly, whether the olfactory nerve is de- 

 veloped from this ridge ; and, lastly, the exact date of 

 appearance of the olfactory nerve. On all the.se points I 

 have, owing to the unsatisfactory condition of my specimens, 

 failed to obtain reliable evidence. 



Plate XIV, fig. 19, represents a section through the 

 head of a dogfish embryo at stage m of Balfour's nomen- 

 clature; the section is made in a plane transverse to the 

 longitudinal axis of the head, and passes through the fore 

 brain {f. h.), the olfactory sacs (olf.), and the olfactory 

 nerves (I). 



This figure, which is taken from one of an excellent series 

 of preparations in perfect histological preservation, illustrates 

 several features of considerable interest — (1.) In the first 

 place it will be noticed that the fore brain presents no 

 trace whatever of a division into cerebral hemispheres ; in 

 other words, that the olfactory nerves come into existence 

 before the cerebral hemispheres, and are therefore con- 

 nected at first with the forebrain, and not with the hemi- 

 spheres. As confirmation of this point, I may repeat that 

 fig. 19 is taken from an embryo at stage m, while Balfour 

 has already shown, and my own observations are in complete 

 accordance with his on this point, that until stage o there is 

 no trace whatever of a division of the forebrain into cerebral 

 hemispheres.- 



(2.) There is no trace of an olfactory lobe or vesicle. 

 This is a point of considerable importance, and one on which 

 I desire to lay stress. The figure shows that at stage m 

 the olfactory nerves are solid, and present no trace of a 

 central lobe or vesicle, either at their roots or at any part of 

 their length. 



(3.) The olfactory nerve at stage m agrees closely in its 

 general relations and in its histological characters with the 

 other cranial nerves, either at the same or at slightly younger 

 stages. Like these, it arises from the upper part of the 



> 'Quarterly Journal of Microscopical Science,' January, IS 78, pp. 

 13— IG. 

 - 'Elasraobrauch Fishes,' p. 178. 



