NEAL : NERVOUS SYSTEM IX SQUALUS ACANTHIAS. 207 



seen bv 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 Wijlie's), become difFei'entiated 

 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.^ 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 ('8V) said : " Es bleibt auch im Auge zu behalten, dass die 

 gesammte Xeuromerie secuudarer Xatur ist : sie ist nur eine Wieder- 

 hohlung aller vor ihr entstandeneu ^letamerien des Kbrpers. Eine 

 primare Metamerie, wie sie z. B. im dorsalen Mesoderm vorliegt, ist 

 weder im centralen, noch im periferischen Nerven-system vorhanden,^ 

 und wenn im Rumpfe die Xeuromerie mit der primaren Mesomerie 

 iibereinstimmt, so reicht diese Eigenschaft im Allgemeinen nur so weit, 

 als die Xerven sich innerhalb des primar segmentirten Mesoderms be- 

 findeu, und sie hort auf, wenn die Xerven in solche Organe eintreten, 

 die ausserhalb der ]\Iesomeren liegen, oder die in einer anderen nicht 

 segmentalen Metamerie entwickelt sind." 



In the trunk, the arrangement both of myelomeres and nerves is 

 clearly metamerie, 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 cutauei) and in the intestine (sensor and motor sympathetic 

 fibres). 



In a study of the simple, and 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 muse, rectus posterior (Hatschek). 



2 Compare Froriep ('94). 



