MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 303 



and containj as before^ prolongations of the lateral ven- 

 tricles."! 



It will be noticed that, while in fig. 21 the olfactory lobe 

 projects out at right angles to the brain, and the olfactory 

 nerve arises from its extreme tip, in fig. 22 the olfactory 

 lobe is bent downwards, so as to lie against the side of the 

 cerebral hemisphere, and the olfactory nerve no longer arises 

 from its apex, but slightly from its dorsal surface. From the 

 condition here represented to that of the adult the changes 

 are unimportant. 



The earlier stages of the olfactory nerve I have not been 

 able to work out satisfactorily, for reasons already mentioned. 

 In fig. 15 the nerve is represented in longitudinal and ver- 

 tical section at stage m. It is easily recognisable at stage 

 L, and I have also succeeded in satisfying myself of its 

 existence as far back as siage k. 



Fig. 14 represents a transverse section through the ante- 

 rior part of the head of an embryo at the commencement 

 of stage K ; the section passes through the forebrain, and 

 through both olfactory pits ; on the right side a small mass 

 of cells (i) in contact with the bottom of the pit is stained 

 rather more deeply than the surrounding mesoblast cells. 

 From comparison with the condition of what is undoubtedly 

 the olfactory nerve at slightly later stages, I consider it very 

 probable that these cells form part of the olfactory nerve, 

 but cannot, of course, speak with any certainty on this point. 

 Apart from the insufficient material at my disposal, the inhe- 

 rent difficulties of the investigation are very great, for at 

 these early stages the olfactory nerve consists entirely of cells, 

 which differ but little from the surrounding mesoblast cells ; 

 the nerve is also exceedingly short, owing to the close proxi- 

 mity of the olfactory pit to the brain, while a new difficulty 

 is introduced by cranial flexure, which is increasing rapidly 

 about this time, and so causes a constant shifting in the 

 relations of the surrounding parts to one another. 



My investigations, then, lead me to give the following 

 account of the development of the olfactory nerve inElasmo- 

 branchs. The nerve arises at some period earlier than stage 

 K ; it is at first connected with the upper part of the side 

 of the forebrain ; between stages l and o its root shifts 

 downwards to a certain extent towards the ventral surface ; 

 the nerve itself is, from the earliest period at which it can be 

 recognised, solid ; the earliest trace of an olfactory lobe ap- 

 pears at the commencement of stage o as a shallow depres- 

 sion of the inner wall of the forebrain opposite the root of 

 1 Op. cit., p. 179. 



