156 Journal of Comparative Neurology. 



Holding this view, I have recently^ made an attempt to 

 solve the head problem, and while my observations were made 

 primarily upon the nervous system in Selachian embryos, my 

 theoretical conclusions have been controlled by the study of 

 the actual relations of other organ systems and also by the 

 study of embryos of all other classes of vertebrates except 

 Reptiles. Whether or not I have come nearer a solution of the 

 head problem than have many of my predecessors, depends, I 

 am convinced, on whether or not I have adhered with greater 

 fidelity than they to the principle above enunciated. I regard 

 my results as in great part a confirmation of those of van 

 Wijhe ('82) and valuable as such. 



First, as regards the nature of cephalic metameres, I con- 

 clude with the majority of investigators that they are serially 

 homologous with trunk metameres, although the homology is 

 today but partial. To my mind, the differences which have 

 been considered as objections to this view by certain morpholo- 

 gists, such for example as the fact that [a) visceral elefts and 

 arches are confined to the head region (Gegenbaur) ; that {U) 

 excretory organs are confined to the trunk region ; that {c) there 

 are no somites in the head, at least in its pre-otic portion, 

 (Kastschenko, Rabl, Froriep) ; that cephalic nerves and spinal 

 nerves cannot be compared by reason of the fact that (^) ceph- 

 alic dorsal nerves receive cellular material from the skin, while 

 spinal dorsal nerves do not ; that (r) cephalic dorsal nerves are 

 mixed, while spinal dorsal nerves are sensor in function ; that 

 (/) cephalic dorsal nerves extend lateral, and spinal dorsal 

 nerves median, to the somites; that (^") — at least some — ceph- 

 alic dorsal nerves have component sensor fibers which innervate 

 lateral line organs, while in spinal nerves these are wanting ; 

 that (/^) in one and the same occipital metamere there can be 

 found (i) a cephalic dorsal nerve, (2) a spinal dorsal nerve, and 

 (3) a spinal ventral nerve and that therefore spinal and cephalic 



' Neal, H. v., '98. The .Segmentation of the Nervous System in Squalus 

 acanthias — A contribution to the Morphology of the Vertebrate Head. Bui). 

 Mils. Comp. Zool. Harvard Univ., Vol, 31, No. 7, pp. I45-J94, with nine 

 plates. 



