A NEW CRANIAL NERVE IN SELACHIANS. 49 



the main olfactory is more lateral in position and is composed of two roots; that 

 belonging to the median nerve is on either side of the neuropore and shows a gan- 

 glionic enlargement. 



A frontal view of a Squalus embryo that had reached a length of 25 millimetres 

 is shown in Figure 10 (PL VI). At this stage the median nerve is well established, 

 as is also the main olfactory. To obtain this view, the front surface of the brain 

 was completely exposed by first removing the overlying layer of ectoderm, and then 

 sweeping away the mesoderm by the use of an artist's brush and a needle when 

 necessary. The olfactory cups have been left in position. The mark of the closed 

 neuropore is seen in the median plane, and on each side of it the new nerves (n. nov.) 

 with a ganglionic enlargement (gn.). From these positions the nerves pass laterally 

 across the front surface of the brain and join the main olfactories, being principally 

 connected with their outer or lateral roots (n. olf. I.). It is to be noted that the main 

 olfactory consists of two divisions (n. olf. I., n. olf. m.), each of which is composed of 

 smaller bundles. This condition persists in the adult, as already indicated above. 



A frontal view of the right half of the brain of an older embryo, 38 millimetres 

 in length, is shown in Figure 11 (PL VI). Part of the olfactory cup has been broken 

 away, exposing the olfactory membrane, which is already thrown into folds. The 

 new nerve (n. nov.) is relatively long and slender; its course and relationships are the 

 same as shown in Figure 10. The two chief divisions of the olfactory nerve (n. olf. I., 

 n. olf. m.) are more complex and composed of several small bundles. The olfactory 

 pit is imperfectly divided into two parts, a median and a lateral portion, and the 

 fibres of the two great divisions of the olfactory nerve are mainly distributed to the 

 corresponding portions of the pit. 



The new nerve, as it approaches the main olfactory, becomes flattened and there- 

 fore broader. It passes in front of the median division (n. olf. m.), then underneath 

 two slender bundles of the lateral division {n. olf. I.), and enters into connection with 

 the largest bundle of that division. It branches very unequally just before passing 

 behind the bundles of the lateral division. As far as I can make out by sections its 

 fibres do not anastomose, but commingle in a very intimate manner with the fila 

 olfactoria. They subdivide and pass mainly to the lateral portion of the nasal mem- 

 brane in close association with the fila olfactoria. 



Figure 12 (PL VI) is an almost ventral view of the brain of an embryo 40 milli- 

 metres long. On the left the olfactory cup has been completely broken away, while 

 only the anterior part of the right cup has been removed. The new nerve is seen, 

 as in former cases, to cross the forebrain, pass in front of the median division of the 

 olfactory, and behind two slender bundles of the lateral division, to come into con- 



