312 DR. A. MILNES MARSHALL. 



indebted to Mr. Edon, of the South Kensington Museum of 

 Pisciculture, to whose care and experience I owe the success- 

 ful hatching of the ova. 



The early stages of development of the olfactory nerve are 

 unfortunately even more difficult to investigate in Teleosteans 

 than in either the chick or dogfish ; and my observations on 

 these stages are exceedingly imperfect. The earliest stage at 

 which I can speak with any confidence as to the existence of 

 an olfactory nerve is shown in fig. 29, which represents a 

 transverse section through the anterior part of the head of a 

 trout embryo on the twenty-seventh day after the fertiliza- 

 tion of the ova. The section passes through the forebrain 

 (f.b.), and through the olfactory pits [olf-); on the left side 

 of the section a small mass of cells, somewhat more com- 

 pactly arranged and more deeply stained than the mesoblast 

 cells, connects the upper part of the forebrain with the 

 olfactory pit. This mass of cells (i) I believe to be the 

 olfactory nerve, mainly from its relation to what is un- 

 doubtedly the olfactory nerve a few days later. I do not 

 wish, however, to speak at all positively on this point. 



Between the thirtieth and fortieth days the olfactory 

 nerves, though still extremely short, can be easily recognised. 

 Though my observations are far from complete, they suffice to 

 establish the following points for both the salmon and 

 trout : 



1. The olfactory nerves appear before the cerebral hemi- 

 spheres, and are at first connected with the dorsal side of 

 the forebrain. 



2. The nerves are, from the earliest period at which their 

 existence can be determined with anything like certainty, 

 solid ; i.e. there is no olfactory lobe. 



3. The connection between the olfactory nerve and the 

 epithelium of the olfactory pit is acquired at a very early date. 



Plate XIV, fig. 33, is taken from a transverse section 

 through the head of a salmon embryo two days after 

 hatching. The section, which is a little oblique, passes on 

 the left side through the eye (o. c.) with the superior (r. s.) 

 and inferior [r.i.) recti muscles; on the right side through 

 the olfactory pit [olf.) The forebrain (_/.i.), which lies in 

 the centre of the section, is seen to have a small vesicular 

 cavity in its upper part; its roof is thin, its floor and sides 

 very thick. From the lower part of its sides a pair of nerves 

 (i) arises; these nerves run downwards for a short distance 

 towards the ventral surface, then turn directly outwards, and 

 the nerve on the right side is seen to divide into two 

 branches, which can be readily traced to the thickened 



