MINUTE ANATOMY OF THE OLFACTORY NERVES. 211 



whilst in others the fasciculi of primitive fibres split into pri- 

 mitive fibres within their sheath, and these again are composed 

 of fibrils and a sheath of Schwann. So far as my observation 

 has extended, however, the fasciculi in question, whether they 

 are provided with a sheath or not, consist in all animals of 

 extremely fine fibrils kept in position by a finely granular 

 mass. In some animals nuclei are sometimes seen in addition, 

 disposed in regular rows between the fibrils, in consequence of 

 which the whole fasciculus is divisible into secondary fasciculi 

 destitute of a sheath. The sheath of the primitive fasciculi 

 cannot represent the sheath of Schwann, but is rather to be 

 compared, from a morphological point of view, with the neu- 

 rilemma, with which also its peculiarities- and structure may 

 perhaps agree. We may also reasonably admit this even where 

 the fibrils contain no nuclei between them, or form no second- 

 ary fasciculi, as occurs for example, according to Max Schultze, 

 in the Pike. If in this case we were to regard the sheath as 

 Schwann's sheath, we must do so also when the fibrils split into 

 secondary fasciculi within the sheath, which fasciculi, according 

 to Max Schultze, are again provided with a Schwann's sheath ; 

 w r e should be obliged to admit, in other words, that the nerve 

 fibres possessing a nerve sheath, are again enclosed in a com- 

 mon sheath of Schwann. I may also remark in addition that 

 I was unable to satisfy myself that the primitive-fibre fasci- 

 culi in many animals, especially in Plagiostomata, possess 

 any sheath at all. The history of the development of the peri- 

 pheric nervous system suggests that the olfactory nerves are to 

 be regarded as embryonal structures that remain persistent at 

 the second grade of their development, whilst the fibres of 

 Remak attain the ultimate stage. The nuclei found between 

 the fibrils of the olfactory nerves are, for the most part, true 

 cells. They are not unfrequently fusiform, and in this case 

 adhere, by means of their fine processes, very firmly to the 

 nerve fibrils. I shall have an opportunity of entering into 

 fuller details respecting them in another part of this work. 



The question that now arises is, what becomes of the nerve 

 fibrils when they have reached the epithelial layer ? Unfortu- 

 nately it can only be answered hypothetical!}'. Examinations 

 instituted on specimens stained with chloride of gold by no 



p 2 



