46 



the entrance to the cavity, and hanging into the interior. The arrange- 

 ment of these flaps is such, that on the surface, there are two rounded 

 openings. The two great divisions of the olfactory nerve, already 

 noted, apparently have reference to the two portions of the membrane 

 within the olfactory cup. The fibers of the new nerve are largely as- 

 sociated with those of the lateral division of the olfactory nerve. 



One histological feature of the new nerve makes its identification 

 relatively easy — that is the presence of rounded cells intermingled 

 with its fibers. The fibers also take a difierent tone in staining. In 

 the smaller subdivisions the rounded cells cease, and the fibres are 

 distinguished from the fila olfactoria with great difficulty, nevertheless, 

 small branches of the new nerve (in combination with the fila olfac- 

 toria), can be successfully traced between folds of the nasal membrane 

 and into the nasal epithelium. I have had several series of sections 

 for examination, variously stained in alum- and borax-carmine, Czor- 

 ker's alum-cocchineal, iron-ha^matoxylin, Hermann's fluid, and one in 

 Weigert's hsematoxylin. In one series, a fortunate differentiation 

 with iron-hsematoxylin, has efiected a distinction in appearance between 

 the minute branches of the fila olfactoria and those of the new nerve. 

 The specimen was 180 mm long. In this series, the fibres of the 

 new nerve were clearly traced between the five first antero-lateral folds 

 of the nasal membrane. Other less important fibers were lost in the 

 maze of fibres in attempting to follow their distribution. The nature 

 of the connection of these fibers with the cells in the nasal epithelium 

 could not be determined. Fig. 10 B shows in a general way the course 

 of the nerve in the olfactory cup as seen in a single section. Under 

 the high power of the microscope, by the study of serial sections, 

 finer fibers were traced between the folds of the nasal membrane and 

 into the 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 ana- 

 tomical relations of the nerve in the adult stages of other selachians. 



In Heptane h us cine reus. — In this primitive form the brain is 

 of the elongated type. As seen from above, all the parts are distinctly 

 separated, there being no largely developed part overlying the others. 

 It may be compared without difficulty with the brain of Squalus. The 

 new nerve connects with the mesial surfaces of the prosencephalon, 

 within the median furrow, by two unequal roots as in Squalus. The 

 roots enter somewhat nearer the dorsal than the ventral surface. A 

 spindle-formed ganglion is found within the median furrow. 



The course of the nerve to the olfactory cup (Fig. 11) is a little 



