NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 239 
later stages, as does its ventral root, the hypoglossus of the adult having, 
according to Gegenbaur (’72) and M. Fiirbringer (97), only two ventral 
roots without dorsal ganglia. Since the reduction of dorsal and ventral 
roots takes place from the anterior towards the posterior, these two ven- 
tral roots of the adult hypoglossus are in all probability the posterior of 
the five roots of the embryo.* A similar process of fusion of dorsal ganglia 
with the dorsal ganglion of the vagus takes place in Petromyzon ; but in 
that animal the fusion of the ganglion — viz. that of the “ spinalartige 
Vagusanhang,” which for reasons already stated by me (’97, pp. 454, 
455) I regard as the exact homologue of the dorsal ganglion of van 
Wijhe’s eighth somite in Squalus ?— appears by a comparison of the 
results of Wiedersheim (’80), Schneider (80), Ahlborn (84°), Hatschek 
(92), Kupffer (96), and.M. Fürbringer (97) to be a variable one. 
This union of dorsal spinal ganglia with the ganglion of the vagus, taken 
in connection with the fact previously stated by me (’97, p. 453), that 
the dorsal ganglia of the glossopharyngeus and vagus lie primitively * 
median to the dorsal portion of post-otic somites, is a further link in 
the chain of evidence which shows that no fundamental distinction be- 
tween spinal and cranial nerves exists. From the foregoing account it 
will be seen that, as in the case of spinal nerves, we are able, using as 
criteria the central and peripheral relationships of the motor fibres, to 
divide cranial motor nerves (roots) into two classes, viz. : (1) dorsal 
(splanchnic) roots, having their nucleus in the lateral horn of the neural 
tube and their peripheral distribution in the musculature (ventral) of 
the visceral arches; and (2) ventral (somatic) roots, which have their 
nucleus in the ventral horn of the neural tube and their peripheral 
distribution in the musculature (dorsal) of the somites (somatic muscu- 
1 Hexanchus and Heptanchus both have five hypoglossus roots in the adult 
(M. Fiirbringer, *97). 
2 Homologized, however, by Ahlborn (’84*) with van Wijhe’s tenth somite, and 
by Hatschek (’92) with van Wijhe’s seventh somite, 
8 Goronowitsch (’92) first observed in the chick that the topographic relation of 
the vagus to the head somites is the same as that of the spinal nerves to the trunk 
somites. Sewertzoff (’05, p. 92) also states that “ Die Beziehung fer Kopfmyotome 
zu den Kopfnerven, z. B. zum N. vagus ist dieselbe, wie diejenige zwischen den 
Rumpfmyotomen und Riickenmarknerven, d. h. sie liegen nach aussen von Nerv 
(Cyclostomata, Ganoidei chondrostei, Urodela, Reptilia, Aves)” This is stated, 
however, by Kupffer ('94, ’96) no, to be a primitive relation of the post-otic myo- 
tomes in Petromyzon. My own observations and conclusious, however, differ from 
those of Kupffer (see Neal, ’97, p. 453). Miss Platt's ('97) observations on Nec- 
turus, and her conclusions likewise, confirm the conclusions of Gordnowitsch and 
Sewertzoff. 
