NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 257 
(5) In Torpedo it innervates musculature (musc, rectus posterior) de- 
rived from two somites, viz. van Wijhe’s third and fourth (Sewertzoff, 98). 
I am not able, however, to offer direct evidence that the nerve has part of 
its nucleus in encephalomere IV. I am therefore not able to exclude 
the possibility that the ventral root of a post-otic somite has been substi- 
tuted for the pre-otic ventral nerve which once innervated somite 3. 
That such a substitution of the fibres of a ventral nerve of one segment 
for those of another may take place ontogenetically, I have the following 
evidence. I find that in a Squalus embryo of 50 mm. the ventral nerve 
of van Wijhe’s 7th somite has become very rudimentary, while fibres 
from the ventral nerve of the Sth somite extend to the musculature de- 
rived from the 7th somite, which in this stage forms the most anterior 
segment of the lateral musculature. Now, if the ventral root of the 7th 
somite atrophies before the adult stage is reached, and if the muscula- 
ture derived from this somite remains the first segment of the lateral 
trunk musculature of the adult, as has been stated by van Wijhe (82) 
and Hoffmann (94), the conclusion seems unavoidable that we have to 
do here with a substitution of a posterior nerve for one farther anterior. 
Moreover, in Petromyzon we have evidence that the first five post- 
otic myotomes of the lateral trunk musculature are innervated by the 
ventral nerves of the last two of the corresponding somites, i. e. the 4th 
and 5th post-otic, which in my opinion are homologous with the 4th and 
5th post-otic somites of Squalus (van Wijhe’s Sth and 9th). Here also 
the conclusion seems to me to be warranted that there has been a phy- 
logenetic, if not an ontogenetic, substitution of the nerves of posterior 
segments for those of more anterior segments.' We may therefore 
infer, with a considerable degree of probability, that a similar substi- 
tution of a post-otic nerve for a pre-otic one may have occurred phy- 
logenetically in the case of the abducens, Such evidence, however, 
seems to render unwarrantable the assumption of a primary and in- 
separable connection of motor nerve and muscle. Furthermore, the 
evidence that the motor nerves develop as axis-cylinder processes of me- 
dullary cells given by His (’89) for spinal nerves, and by myself in this 
1 See Neal (’97, Figure 2, p. 446) for evidence that the fibres of a post-otic ven- 
tral nerve (hypoglossus auctorum) extend into the pre-otic region with the muscle 
they innervate. It would seem a very easy matter for such fibres to come into 
nervous connection phylogenetically with the eye muscles, and especially the 
posterior of these, with which in Petromyzon they are very closely connected. 
Hatschek (’92) stated that the muse, rectus posterior becomes connected with the 
anterior of the post-otic myotomes. See evidence given by M. Fürbringer (’97) 
and Neal (’97) upon this question. 
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