A NEW CRANIAL NERVE IN SELACHIANS. 45 



nerve are mainly, but not entirely, distributed to the outer lateral portions of the 

 olfactory membrane, inasmuch as the chief branch of the nerve takes a course to 

 that portion of the olfactory cup. Here the fibres can be traced between the folds 

 of the olfactory epithelium. The nature of their termination within that membrane 

 or their possible connection with cells of it will be especially difficult to determine. 

 After the divisions of the fibres become very minute they are easily confused with the 

 smallest bundles of the fila olfactoria and the tracing of individual fibres is very 

 uncertain. Small bundles of the fibres of the new nerve can, however, be traced 

 between the folds of the olfactory membrane and as far as the base of the epithelial 

 cells. Since writing the above my attention has been called by one of my students, 

 Miss Effie Thayer, to a series of sections in which a fortunate differentiation with 

 iron-haemotoxylin staining has made a distinction in appearance between even the 

 small fibres of the fila olfactoria and those of the new nerve. I am indebted to her not 

 only for making the sections, but for tracing these fibres, thus seen with greater distinct- 

 ness than was possible in other sections, into contact with the olfactory epithelium. 



The next question in reference to this nerve would naturally be, What is its 

 embryonic history? This has been worked out in Squalus acanthias, and the main 

 facts will be given, after considering the anatomical condition of the nerve in the 

 adult stages of other selachians . 



2. In Mustelus canis — The brain of the smooth hound, Mustelus canis, seen 

 from above is represented in Figure 2 (PI. V). The olfactory cups are relatively 

 of enormous size, showing that the olfactory apparatus is doubtless of great impor- 

 tance in the life of this animal. The tractus olfactorius is shorter than in Squalus, 

 and the forebrain is not divided by a median furrow as in that selachian. The new 

 nerve is more easily detected in Mustelus than in Squalus, for it lies more freely in 

 the membrane of the cranial cavity and is not so closely applied to the tractus. Its 

 point of entrance into the brain-wall is on the ventral surface as shown in Figure 3. 

 Here the nerve enters a small depression on the base of the brain, considerably in 

 front of the optic nerve. From this point it passes towards the olfactory cup, in a 

 course nearly parallel to the tractus, but it does not come to lie close to that structure. 

 As seen from above, it traverses the relatively short space between the anterior wall 

 of the forebrain and the bulbus and then crosses obliquely the median division of 

 the fila olfactoria (Fig. ,2 n. olf. m.) and enters the furrow between the two great 

 divisions. Just as it reaches this furrow it branches unequally, the main branch 

 taking a similar course to that described for Squalus. The nerve is, therefore, 

 mainly distributed to the antero-lateral portions of the olfactory membrane. No 

 well-marked ganglion was observed on the course of the nerve in Mustelus. 



