274 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



nection with them ; and (4) of the development of at least a part of the 

 musculature innervated by them from splanchnic mesoderm. Thus 

 there is to-day a distinct conflict as to the morphology of the eye-muscle 

 nerves, one party to the conflict being supported by histological evi- 

 dence, the other by embryological. The assumption by His ('88), that 

 the eye-muscle nerves develop as processes of medullary cells (neuro- 

 blasts), — which is involved in his contention that they are the serial 

 homologues of ventral spinal nerves, — has never hitherto received the 

 requisite embryological confirmation. In fact, the latest embryological evi- 

 dence concerning the development of the oculomotorius and trochlearis 

 seems quite irreconcilable with the view of Schneider ('79), van Wijhe, 

 and His. In regard to the latter nerve, Hoffmann ('89, p. 338) says, 

 if one disregards the fact that no ectodermal fusion takes place, *' so 

 gleichtdie Anlage des Trochlearis in sehr jungen Entwicklungsstadien 

 [of Lacerta] vollkommen der eines segmentalen Kopfnerven, besonders 

 der des Trigeminus." Froriep also finds that the trochlearis possesses 

 in early stages a ganglion, and is differentiated from neural-crest cells in 

 situ. Miss Piatt ('91% p. 2.59) likewise states that " in Acanthias the 

 development of the trochlearis in all essential respects so completely 

 corresponds to that of the trigeminus and facialis, that like them it must 

 be considered to combine primarily those dorsal and ventral elements 

 which have separate roots in the nerves of the trunk. It can, therefore, 

 not be regarded as the ventral root of another segmental nerve." More- 

 over, Kupff"er ('95, '96) finds the trochlearis to possess in Ammoccetes 

 both dorsal and ventral roots. 



With regard to the oculomotorius, the conclusions of embryologists 

 are even more conflicting. "While Dohm ('91) finds that this nerve is 

 formed by the migration of cells from the ventral wall of the midbrain, 

 and considers it a motor nerve, Miss Piatt ('91") states that she has 

 shown the oculomotorius to be " undoubtedly originally sensory." Her 

 observation that the nerve develops from the ganglion toward the brain 

 has been confirmed by both Mitrophanow ('93) and Sedgwick ('95). 

 Nevertheless the evidence which has been stated by me in division VI. 

 shows conclusively, as I believe, that all the eye-muscle nerves, oculo- 

 motorius, trochlearis, and abducens, develop, like ventral spinal nerves, 

 as processes from neuroblasts lying in the ventral horn of the medul- 

 lary tube. Therefore, from their development, as well as their adult 

 histological structure and relationships, the eye-muscle nerves must be 

 regarded as the serial homologues of ventral spinal nerves. Finally, 

 with the accumulating evidence given by many investigators, — among 



