No. i.] MORPHOLOGY OF VERTEBRATE HEAD. 93 



like sections figured by Marshall (No. 11), in which he repre- 

 sents the trochlear nerve as it first appears in Scyllium, I found 

 that in later stages of Acanthias embryos, the nerve resembles 

 its earliest appearance in Scyllium ; the proximal portion then 

 consisting of a mere knob of cells upon the dorsal surface of the 

 brain. This difference in the time at which the trochlear nerve 

 first appears in Scyllium and in Acanthias is explained by Van 

 Wijhe, who tells us (No. 16) that in Scyllium there are two 

 long outgrowths from the neural crest anterior to the facial 

 nerve. One lies anterior to the second (mandibular) somite ; 

 the other is situated at its posterior and lateral side. The 

 neural crest then gradually vanishes from before backwards, 

 and with its disappearance the anterior outgrowth approaches 

 the posterior, at the same time relinquishing its attachment to 

 the mid-brain. It finally comes to arise by a common root with 

 the trigeminus from the anterior end of the medulla. Thus 

 Van Wijhe failed to recognize, in the anterior prolongation of 

 the neural crest, the primary trochlear nerve, and the ramus 

 ophthalmicus superficialis trigemini seemed in Scyllium and 

 in Pristiurus simply a branch of the main trigeminal root. In 

 Acanthias, however, the trochlearis retains its original attach- 

 ment after the disappearance of the neural crest in the region 

 between this nerve root and the trigeminus. Later, when the 

 proximal part of the trochlearis atrophies, the distal part re- 

 mains and is seen by its position to be the ramus ophthalmicus 

 superficialis trigemini. 



The history of the trochlear nerve in Acanthias is as follows : 

 primarily a wide sheet of cells extends down the walls of the 

 brain from mid-brain to medulla. This sheet of cells then 

 divides into three parts, connected above and below. These 

 divisions lie opposite successive constrictions of the brain, and 

 are consequently separated from one another by two neuro- 

 meres. The two posterior parts fuse into a single nerve root, 

 the trigeminal. The anterior part retains its original attach- 

 ment to the brain, and as the dorsal wall of the brain grows, 

 becomes further removed from the trigeminus, changing at the 

 same time the line of its direction, so that ultimately, while 

 the trigeminal root meets the Gasserian ganglion in a vertical 

 plane, the trochlearis, lying in a plane nearly horizontal, meets 

 the ganglion at the point where the ramus ophthalmicus pro- 

 fundus trigemini leaves it. 



