Neal, Problem of the Vertebrate Head. 1 5 5 



conflicting evidence and conclusions, both as to composition 

 and number of cephalic segments, of those who have advocated 

 the prevalent morphological opinion. Of the differences stated 

 by them I shall speak later. The confusion in, and, as must be 

 admitted, generally unsatisfactory condition of the literature 

 bearing on the head problem, is in my judgment attributable in 

 great part to the fact that the observations of investigators have 

 been confined often to a single species, often to a single organ 

 system, while their conclusions deduced from such limited ob- 

 servations have applied to the phylogenesis of the entire vertebrate 

 head ! That such methods are inadequate for the solution of 

 such a difficult problem seems in view of the many divergent 

 opinions too obvious to need insistence here, and I venture to 

 predict that some time, if not now, it will seem strange that a 

 morphologist should assume, or seek to demonstrate that the 

 serial parts of any single organ system, whether neuromeres, 

 or nerves, or somites or visceral arches, or epibranchial " sense 

 organs," or what not, are the essential criteria of head seg- 

 ments. In my opinion, pJiylogenetic eonclusions concerning the 

 metainetism of the head based upon the study of a 'single animal 

 or a single organ system need to be "controlled'' and confirmed 

 by the study of other organ systems in the same animal. The 

 solution of no problem requires a broader knowledge of compar- 

 ative embryological and anatomical facts. 



Fig. I. Diagrammatic representation of the cephalic metameres in Selachii, 

 showing the component organ systems and their relations to one another. 



I-XI, cephalic neuromeres (segments of the central nervous system) ; a, 

 Miss Piatt's "anterior" somite; 1-12, van Wijhe's first to twelfth somites ; 

 /'-5', first to eighth visceral clefts ; abd., abducens ; ad^-'*, aortic arches, first to 

 eighth; ch., chorda; dors, nv., dorsal nerve; cp, vag., epibranchial portion of 

 vagus nerve; fac, facialis nerve ; glossuph. (gts.), glossophuryngeus nerve ; /typ., 

 hypophysis; ;«., mouth ; f;ied. hit. /., mediolateral line ; neur, (n ), neuromere ; 

 ocm., oculomotorius ; ot/., olfactorius ; ophth.prof. (//), ophthalmicus profundus 

 nerve; (?/., otic capsule (ear) ; posttrein., posttrematic branch ; practrcm, praetre- 

 matic branch; r. hit. vag., ramus lateralis vagi ; r. int. vag., ramus intestinalis 

 vagi ; soiii., somites (van Wijhe's) : sp}-^, spinal ganglia first to third; troch., 

 trochlearis ; vent, ao , ventral aorta; vent, nv., ventral nerve; vise, clefts, viscer- 

 al clefts ; vag.^-^, vagus ganglia first to third (dorso-lateral series); z/jr.* third 

 visceral arch. The arrow marks the posterior termination of the cranium in 

 Squalus. All neuromeres anterior to this point are included in the cranium. 



