50 A NEW CRANIAL NERVE IN SELACHIANS. 



nection, finally, with the large outer bundle of the lateral division. On the left side 

 the principal blood-vessels (va. sng.) are indicated. In the other sketches they have 

 been omitted. The ganglionic enlargement has not been represented in Figure 12. 



The brain of an embryo 47 millimetres long is seen from above in Figure 14. On 

 the right side the surface of the olfactory cup and the various bundles of the olfactory 

 nerve have been exposed by dissection, while on the left the tissue covering these 

 parts has not been removed. The new nerve (n. nov.) is seen on both sides, and on 

 the right side its connection with the lateral division of the olfactory is represented. 

 This lateral division (n. ol}. I.) has been broken free from the olfactory lobe in order 

 to show better the point of union. 



In Figure 13 is seen the brain of an embryo 68 millimetres long viewed from 

 above. A shallow furrow now divides the forebrain into right and left portions. 

 The epiphysis rising from the roof of the thalamencephalon is shown in the median 

 plane. As in Figure 14, a dissection of the chief bundles of the olfactory nerve has 

 been made to show the course of the new nerve and its point of union with the outer 

 bundle of the lateral division of the olfactory. On the left side the nerve after partly 

 crossing the olfactory lobe disappears in the tissue at the base of the olfactory cup. 

 The new nerve branches very unequally just between the two great divisions of the 

 olfactory, a condition not shown in the figure. Centrally, the fibres penetrate the 

 brain-wall near the median furrow. 



The brain of an embryo about 150 millimetres long is shown in Figure 15. This 

 is the stage at which the young are designated as "pup" by the fishermen, and is 

 about the size reached in Squalus before being freed from the oviduct of the parent. 

 The olfactory lobes are now fully formed; the olfactory cups are large and directed 

 forward. The forebrain is divided by a median furrow which, in front, extends 

 from the dorsal to the ventral surfaces and separates the anterior end of the brain 

 into two portions; farther back the furrow is simply a shallow fissure. The new 

 nerve (n. nov.) passes into the furrow in front; near the point where the two sides of 

 the brain unite it enters the brain substance. 



The level at which it enters is in some specimens slightly nearer the dorsal than 

 the ventral surface, and in other specimens nearer the ventral surface. The position 

 and the course of the nerve-fibres within the brain substance is shown in Figures 17, 

 18, and 19, and has been already described (p. 44). 



Within the furrow and also on the front surface of the brain the new nerve is 

 flattened. It becomes rounded in cross-section as it comes into the angle between 

 the brain-wall and the olfactory lobe, and becomes flattened again on the surface 

 of the lobe. After crossing about two-thirds the diameter of the lobe it enters the 



