100 The American Naturalist. [February 
classify the V. and VIL-VIII. nerves in contrast to the IX. and 
X. All four are connected with the organs of the lateral line, 
while the spinal nerves take no part in the latter structures. The 
motor roots of all four spring from the lateral column, and pass 
into the ganglia, while no motor fibres go into the spinal ganglia. 
In Selachians, at the time the sensory roots of the glossopharyn- 
geus and vagus enter thé medulla oblongata, there appears in this 
region a folding or furrowing of the walls of the neural tube, 
similar to that seen in the spinal cord. In this segmentation the 
roots of the IX. and X. nerves correspond in position to the 
: furrows separating the metameres, just as the furrows in the 
metamerism of the spinal cord answer to the sensory nerve-roots. 
The probability that the vagus is a polymere whose components 
were originally similar to spinal nerves, the similarity of the V. 
and VII—VIII. nerves to the IX. and X.in development and 
functional differentiation, and the fact that the neural ridge can 
be traced anteriorly into the VII.—-VIII. anlage, render Rabl’s 
hypothesis of unsegmented cranial mesoderm untenable. 
Dohrn’s recent contributions ('904) to our knowledge of 
primitive cranial segmentation must be regarded as epoch- 
making. In embryos of Torpedo marmorata, stage F of Bal- 
four, 12-15 myotomes are found anterior to the glossopharyn- 
geal region. Rabl refused to refer any segmentation to this 
region. Van Wijhe found four somites, These 12-15 myotome 
pass ventrally into the lateral plates, which “form the cranial 
cœlom, and out of which come the “head-cavities.” In stage 
G the myotomes are considerably coalesced, and the more te z 
development goes on the more the obliteration of myotome 
boundaries. Van Wijhe’s mandibular somite is made up of 3 eo 
myotomes, the hyoid of 3, and the fourth somite of pe 3 
The segmentation recognized by Van Wijhe is thus appar- 
ently secondary. The myotomes of the head are throughout 
comparable to the myotomes of the body. The cranial motor 
nerves show a metamerism. The III. nerve arises by si 
separate fibres, and innervates the muscles of the premadi 
head-cavity, which is a multiple of myotomes. The KE 
originates also as a multiplex of fibres, and innervates "© 
