253 
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 vertesral column, the former 
being present in Cyclostomes, Chondrostei, Holocephali 
and lower Elasmobranchs, where vertebrae are still absent. 
Thus the contrast between FRORIEP's spinal and praespinal 
ee of the head is fully recognized by GEGENBAUR 
(1887, p. 94) and expressed by him ín the names primary 
or Sulisgendio 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 
this 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 cf 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 in branchial region of Elasmobran_hs. — Now 
FRORIEP (1902), turning to the study of early stages of 
Elasmobranchs, viz. of Torpedo, could only c_nfirm that 
here indeed the series of somites originally reaches very far 
forward, nearly as far as the anterior end ot 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). 
