No. i.] MORPHOLOGY OF VERTEBRATE HEAD. 103 



ties are represented in the resulting single cavity, they are surely 

 to be looked for at the line where the lateral cavities have fused 

 with the central space. The four premandibular eye-muscles 

 arise from the dorsal wall of the lateral portions of the preman- 

 dibular cavity, approximating closely, in their place of origin, the 

 line of fusion between the paired cavities and the central space. 

 In view of these facts it is difficult to understand the ground of 

 one of the chief objections advanced by Rabl (No. 15) against 

 considering the premandibular, the mandibular, and the third 

 head cavities homologues of the body somites. He says, 

 "Wahrend ferner die Muskulatur der Urwirbel ausschliesslich 

 aus der medialen Wand entsteht, nimmt sie im Vorderkopf zum 

 grossten Theil aus der lateralen und zum kleineren Theil aus 

 den hinteren Wand der sogenannten Somite den Ursprung." 



10. The trochlear and trigeminal nerves arise as a continu- 

 ous sheet of cells, extending from the neural crest down the 

 walls of the brain. The two nerves soon separate. The troch- 

 learis is then seen to arise anterior to the cerebellar division of 

 the brain, and the trigeminus posterior to this division. The 

 proximal cells of the trochlearis, i.e. the cells which connect 

 the nerve with the brain, soon disappear, leaving the distal cells 

 of the primary nerve still connected with the trigeminal gan- 

 glion at the point of origin of the ramus ophthalmicus profundus. 

 From these cells a nerve is formed which extends forwards over 

 the anterior prolongation of the mandibular cavity. This nerve 

 is surrounded anteriorly by a mass of deeply staining cells, 

 which lie above the forming superior oblique muscle. They are 

 the remains of the primary trochlearis, and from them cells are 

 proliferated towards the brain to meet the small fibrous out- 

 growth of the permanent trochlearis. Thus a portion of the 

 original trochlear nerve, which was an outgrowth from the neu- 

 ral crest, does not entirely disappear, but is known later as the 

 ramus ophthalmicus superficialis trigemini, and the permanent 

 trochlearis is formed by the growth of ganglion cells towards the 

 brain, and of nerve fibres from the brain. 



11. Anterior to the mid-brain there is an outgrowth from the 

 neural crest, which extends downward back of the eye, meeting 

 the ganglion line ventrally in the neighborhood of the ciliary 

 ganglion. This outgrowth persists after the disappearance of 

 the neural crest. It is carried forward by the growth of the 



