neal: nervous system in squalus acanthias. 259 



Myxinoids this nerve possesses motor fibres (J. Mtlller, P, Flirbringer, 

 Price), and its segmental value as a dorsal nerve seems thereby estab- 

 lished. The fact that the fibres of the ophth. profundus V enter the 

 brain at a point posterior to encephalomere III, instead of anterior to 

 it, as they should in order to conform to my scheme of segmental rela- 

 tions, appears to me no serious objection. That they enter the brain 

 at a point posterior to that at which the motor fibres innervating the 

 mandibular musculature enter, and in consequence cross these fibres in 

 the mesocephalic ganglion, is to be explained by the tendency, especially 

 of the sensor cranial nerves, to enter the brain as near the otic capsule 

 as possible (see Ahlborn, '84"), and by the more conservative relations 

 of the motor fibres (roots) generally. 



In my preliminary paper I placed tentatively the so called " thalamic " 

 nerve as the possible dorsal nerve of encephalomere II. Now, how- 

 ever, I question the correctness of this opinion. We certainly need 

 something more than a strand of neural-crest cells which persist for 

 some time in a region of constriction between encephalomeres, but 

 which never assume fibrillar relation with the neural tube, to warrant us 

 hi assuming that we have to do with a nerve} 



The development and relations of van Wijhe's first somite and of the 

 oculomotorius leave no doubt that in them we have the somatic ele- 

 ments of a metamere. Probably no ventral or splanchnic portion of the 

 mesoderm of this segment exists, consequently the r. ophthalmicus pro- 

 fundus possesses no splanchnic fibres.^ In my opinion it is doubtful if 

 the hypophysis may be regarded as evidence of an ancestral visceral cleft 

 between segments I and II. 



However, I hold that the structural comparability of encephalomere II 

 with hindbrain encephalomeres, together with the evidence of its rela- 

 tion with a segmental ganglion, and of its connection with somatic muscu- 

 lature by means of a ventral motor nerve, strongly favors the view that 

 it is serially homologous with hindbrain encephalomeres. 



g. Eelatioks of Excelphalomere I. 



, That which I regard as the first cephalic segment of Craniota consists 



of an encephalomere (primary forebrain) which has been shown to be 



1 Kupffer excels Miss Piatt in discovering "rudimentary" nerves, but until 

 we have a better criterion for a nerve than a cellular strand there is no reason 

 why the number of "rudimentary" nerves should not be much larger than it is 

 at present recognized to be. 



■^ Possibly the skeletogenous element of the ventral portion of this segment is to 

 be found in the " maxillar Lippenknorpel " of Gegenbaur. 



