218 bulletin: museum of comparative zoology. 



the glossopharyugeus so much that they now lie opposite encephalomere 

 VII. The two nerve Anlagen, however, usually remain connected with 

 each other dorsally by a thin cellular strand. This strand is wanting 

 in some cases, or may be present on one side of the embryo only. 

 Dohrn ('90) has also stated that the separation of the seventh and ninth 

 nerves is due to the crowding caused by the ear capsule, and he held 

 that the connecting strand of cells was evidence of the original conti- 

 nuity of the neural crest on the dorsal side of the ear. Behind the 

 glossopharyugeus the neural crest extends in unbroken continuity into 

 the trunk, but only its anterior portion, which forms the ganglionic 

 Anlage of the Urvagus, extends ventrally between the mesoderm of the 

 side plates and the superficial ectoderm into the region of the phaiynx. 



In embryos with 38 or 39 somites (Plate 3, Fig. 15) the thalamic 

 portion still extends as a compact cellular cord from the region of con- 

 striction between forebrain and midbrain to a point above the eye, where 

 it unites with the line of ectodermal cells which in later stages forms the 

 ophthalmicus profundus trigemini. This nerve, because of its relations 

 with the trigeminus, " primary trochlearis," and " thalamic " nerves, is 

 regarded by Marshall ('82) and Miss Piatt ('91) as a commissural nerve 

 connecting the three nerves mentioned. It has also been regarded as an 

 independent nerve (van Wijhe, '82, M. Filrbringer, '97), and as a ramus 

 dorsalis either of the trigeminus or the oculomotorius. The acustico- 

 facialis Anlage, opposite encephalomere Y, is still in continuity with that 

 of the glossopharyugeus by means of a cellular cord dorsal to the audi- 

 tory invagination, while the cells of the glossopharyugeus and vagus 

 Anlagen no longer appear to be continuous dorsally, as they were in the 

 previous stage. 



At a stage of development when the embryo possesses 42 to 44 somites 

 (Plate 3, Fig. 16), and when two visceral clefts are formed, both the 

 thalamic and trochlear portions of the trigeminus Anlage are much re- 

 duced. In an embryo with 48 somites the thalamic portion consists of 

 a strand or cord of cells which extends dorsally from the ophthalmicus 

 profundus, at a point just above the eyestalk, toward the region of con- 

 striction between primai-y forebrain aud midbrain, where the two cel- 

 lular strands coming from opposite sides of the head unite above the 

 wall of the brain. Because of this union, Coggi ('95) has considered 

 this portion of the trigeminus Anlage as a connective " nerve," uniting 

 the lateral halves of the ophthalmicus profundus. Its position in Tor- 

 pedo, according to Coggi, is anterior to the thalamencephalon. If 

 Coggi is correct, its position in Torpedo is clearly different from that 



