94 PLATT. [Vol. V. 



Sec. 9, PI. VI, shows the trochlearis in horizontal section at 

 the time when two gill clefts have broken through, and when 

 the Gasserian ganglion is attached by its trigeminal root to the 

 side of the medulla. Along the line where the trochlearis arises, 

 the mid-dorsal wall of the brain is elevated, indicating a rapid 

 proliferation and migration of cells. This crest disappears at 

 either side of the nerve root, and the upper surface of the brain 

 becomes quite level. It will be noticed that the cells of the 

 trochlear closely underlie the ectoderm. Many sections indi- 

 cate that they maintain a closer relationship to the ectoderm 

 than that of mere propinquity. Soon after the stage repre- 

 sented in sec. 9, the primary root of the trochlearis begins to 

 atrophy. This takes place in a somewhat irregular manner. 

 Fragments of the nerve are to be found at varying points of its 

 original course, for a long time after the nerve in its entirety 

 has ceased to exist. The most common positions for these 

 fragments are at either side of the mid-dorsal line of the brain 

 (resembling Marshall's figure of the first appearance of the 

 trochlearis in Scyllium), or at a point about half-way between 

 the original attachment of the nerve and the Gasserian ganglion. 

 They are frequently asymmetrically placed on opposite sides of 

 the embryo, and may be wanting in either of the above-men- 

 tioned positions, or found in both. Sooner or later the frag- 

 ments disappear, and all that is then left of the original nerve is 

 a line of cells extending forwards from the Gasserian ganglion. 

 In whatever way the primary trochlearis breaks down, these 

 cells are constantly left. They are the homologues of the 

 trigeminal branch of the ophthalmicus superficialis, described 

 and figured by Van Wijhe. In Galeus, as in Acanthias, this 

 nerve does not arise, as is the case with Scyllium and Pristiurus, 

 from the ganglion root of the trigeminus, but in common with 

 the ramus ophthalmicus profundus from the anterior portion of 

 the Gasserian ganglion. The cells thus left by the trochlearis 

 lie above the anterior part of the mandibular cavity, and give 

 rise to a nerve which runs forwards over the wall of this cavity, 

 and to a mass of deeply staining cells, surrounding the nerve 

 anteriorly. As the superior oblique muscle develops, these cells 

 lie upon its dorsal surface. 



In describing the disappearance of the primary trochlearis in 

 Torpedo, Dohrn (No. 8) mentions an isolated mass of cells which 



