NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 207 
seen by van Wijhe in Galeus. The somatic value of the post-otic head 
somites is indisputable. The pre-otic somites, five in all, are also in my 
opinion homodynamous with trunk somites. They are segments of the 
dorsal mesoderm (with the possible exception of the “ anterior”), which, 
as exemplified in the third somite (van Wijhe’s), become differentiated 
into myotome and sclerotome. While the “anterior” and the fourth 
somites become rudimentary and develop no muscle fibres, the eye 
muscles are differentiated from the median and lateral walls of the first, 
second, and third. The eye muscles of Selachii are therefore somatic 
in their origin, not splanchnic,t as has been held by Hatschek (’92) 
and Kupffer (94). It will furthermore be shown that the nerves 
which supply them are serially homologous with ventral spinal nerves, 
It is to the consideration of the nerve relations that I now pass. 
VI. The Relation of Neuromeres to Nerves. 
Ahlborn ('84*) said: “Es bleibt auch im Auge zu behalten, dass die 
gesammte Neuromerie secundirer Natur ist: sie ist nur eine Wieder- 
hohlung aller. vor ihr entstandenen Metamerien des Körpers. Eine 
primäre Metamerie, wie sie z. B. im dorsalen Mesoderm vorliegt, ist 
weder im centralen, noch im periferischen Nerven-system vorhanden,? 
und wenn im Rumpfe die Neuromerie mit der primären Mesomerie 
übereinstimmt, so reicht diese Eigenschaft im Allgemeinen nur so weit, 
als die Nerven sick innerhalb des primär segmentirten Mesoderms be- 
finden, und sie hört auf, wenn die Nerven in solche Organe eintreten, 
die ausserhalb der Mesomeren liegen, oder die in einer anderen nicht 
segmentalen Metamerie entwickelt sind.” 
In the trunk, the arrangement both of myelomeres and nerves is 
clearly metameric, being correlated with the segmentation of the meso- 
derm. Related to each mesodermal somite is a ventral nerve (motor 
root), which arises from segmentally arranged groups of ganglionic cells 
in the anterior (ventral) horn of the spinal cord, that is, from each myel- 
omere. Into the posterior (dorsal) horn pass the’ fibres of the dorsal 
nerve, which have their peripheral distribution in the skin of that seg- 
ment (rami cutanei) and in the intestine (sensor and motor sympathetic 
fibres). 
In a study of the simple, ..d it has been assumed primitive relations 
in the trunk, it is important to consider not only the peripheral distribu- 
1 With the exception of the musc. rectus posterior (Hatschek). 
2 Compare Froriep (94). 
