NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 259 
Myxinoids this nerve possesses motor fibres (J. Müller, P. Fürbringer, 
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 mesocophalic 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 IT. 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 
in 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 compa rability of encephalomere IT 
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. RELATIONS OF ENCELPHALOMERD is 
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 Platt in discovering “rudimentary” nerves, but until 
we have a better eriterion 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. 
2 Possibly the skeletogenous element of the ventral portion of this segment is to 
be found in the “maxillar Lippenknorpel” of Gegenbaur, 
