NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 209 
and the motor components of the dorsal nerves. Furthermore, we 
must determine the primitive relations of medullary nerves, not by the 
place of exit of their fibres (i. e. by their roots), for we know these to be 
variable, but by the position of their “ Kerne” in the walls of the neural 
tube.! 
There is considerable difference of opinion as to, whether nerves 
(“roots”) arise primarily from the expanded portion of the encephalo- 
mere (or myelomere), or from the constrictions between these segments. 
As early as 1878 Marshall said, “ My investigations tend very strongly 
to prove that all the nerves arise primitively from the widest parts of 
the dilated vesicles, whether of brain or cord, and never from the inter- 
vening constrictions.” Later, McClure (89), who is in agreement with 
Marshall as well as with Orr, Béraneck, and Waters, said, “ The dorsal 
roots of spinal nerves take their origin from the apex of their respective 
myelomeres in exactly the same manner as the nerves of the medulla 
do from their respective encephalomeres.” Minot (92) criticises McClure 
for overlooking the fact that the “ neuromeres can have no genetic rela- 
tion to the ganglionic nerves.” The ground of Minot’s statement does not 
seem to me to be so self-evident as not to be in need of explanation.? 
In disagreement with McClure, Miss Platt (89) claimed that “the 
concavity in both medulla and spinal cord is the source from which the 
nerve originates,” and her conclusion, which Minot accepts, is that 
the origin from the expanded portion of the neuromere is secondary. 
In view of this difference of opinion it is of interest that Balfour (85) 
stated that in Selachian embryos the dorsal and ventral roots of spinal 
nerves alternate with each other, the dorsal roots being intersegmental 
(intersomitic) and the ventral roots segmental (somitic) in position. 
Miss Platt did not, however, in her statement of nerve relations make 
a distinction between dorsal and ventral nerves. 
1 The most serious obstacle to the use of this criterion is the difficulty of apply- 
ing it in those early stages of development when metameric relationships appear 
least modified. Martin (90 and ’91, p. 230) has noted an ontogenetic ventral 
shifting of motor “ Kerne” in the cat. 
2 It is to be regretted that McClure gave no figures of the nerve relations of 
myelomeres. Minot apparently assumes that the neuromeres are constituted 
solely in adaptation to a motor segmentation, and therefore that the neuromeres 
are segmental localizations of ganglionic cells (i.e. motor “ Kerne ”) in the wall of 
the neural tube, just as are the segmental ganglia of Annelida. It seems to me 
therefore that McClure might have met Minot’s criticism by reminding him that 
neurologists have recognized in the medulla groupings of ganglion cells which are 
in relation with sensory fibres, i. e. sensory “ Kerne ” or “ Endkerne ” (see Edin- 
ger, 96, p. 366), and may well contribute to the metameric enlargements. 
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