ORIGIN AND STRUCTURE OF THE HEAD 67 



of a number of vertebrae, since ontogeny as well as com- 

 parative anatomy of lower Vertebrates teaches us that the 

 cranium is older than the vertebral column, the former 

 being present in Cyclostomes, Chondrostei, Holocephali 

 and lower Elasmobranchs, where vertebrae are still absent. 

 Thus the contrast between FRORlEP's spinal and praespinal 

 region of the head is fully recognized by GegENBAUR 

 (1887, p. 94) and expressed by him in the names primary 

 or palingenetic and secondary or caenogenetic part, but 

 considered as of less importance than that between chordal 

 and praechordal region. On the other hand, FRORIEP( 1887, 

 p. 834) recognizes that in his praespinal region a chordal 

 and a praechordal part may be distinguished, but considers 

 Ihis distinction as of less importance, since the latter is 

 taken to be merely an outgrowth from the former. In 

 reality, as FRORIEP (1887, p. 833) remarks, GEGENBAUR 

 and he are dealing with two distinct problems. FRORIEP 

 starts from the praespinal or palingenetic region of the 

 head as datum and tries to trace the further history of the 

 cranium, while GEGENBAUR tries to analyze the past history 

 of the branchial or praespinal region itself. The latter problem, 

 however, is yet the main point of divergence between them. 

 Mesomerism irt branchial region of Elasmobranchs. — Now 

 FRORIEP (1902), turning to the study of early stages of 

 Elasmobranchs, viz. of Torpedo, coukl only confirm that 

 here indeed the series of somites originally reaches very far 

 forward, nearly as far as the anterior end of the notochord and 

 in front of the first (spiracular) gill-slit. By far the greatest 

 part of the branchial region is occupied by the somites, 

 and, though FRORIEP keeps to his original view that in 

 front of the foremost somite there is to be found an un- 

 segmented mass of "head-mesoderm", it cannot be denied 

 that the latter is reduced here to a very trifling and insig- 

 nificant remnant. Nothing remains of the chordal part of 

 the praespinal region but the little anterior extremity of 

 the notochord, which atrophies soon after its differentiation 

 from the archenteron-roof. FRORIEP (l.c.p. 43) accordingly 

 calls this stage an Acraniate stage, the whole animal being 

 vertebral column. In studying his clear drawings one would 

 conclude that the series of somites here indeed reaches to 

 in front of the auditory vesicle or, in the youngest stages, 

 of the auditory area, the latter being situated over the 

 second somite (cf. fig. 16 on the next page). 



