249 
the latter probably is to be considered as a secondary 
outgrowth. from the former. The vertebral part only is 
traversed by the notochord, and the number of vertebrae 
the former contains is indicated by the number of visceral 
archs — seven in the ca e of pentanch Elasmobranchs —, 
to which originally two arcnes in front were added by 
‘GEGENBAUR, represented by the labial cartilages. 
Ontogenetic researches. — As might be expected, since 
GEGENBAUR a great number“of investigators have tried to 
rediscover ín ontogeny the metameric structure so muc 
obscured in the adult Vertebrate head. The resu:ts of their 
researches, however, have not yet led to unanimity «f opi- 
nion, neither in details nor even in fundamental questions. In 
regard to the latter we may distinguish two schools of thought 
viz: those who in the main adhere to GEGENBAUR's opinion, 
that the arrangement of the gill-clefts corresvonds to the 
number of head segments and that the segmental structure 
reaches as far as the fore-end of the notochord, and those 
who reject this view and consider only the occipital region 
the neurocranium to be derivable from segments like 
those of the trunk. According to the former the situation 
of the gill-slits is intersegmental, each having broken 
through between two mesoderm segments; according to 
the latter the mesoderm in front of the vagus shows no 
metameric segmentation at all, but is unsegmented, and 
only the piercing of the gill-slits makes it appear segmen- 
ted, causing a kind of pseudo-segmentation, the branchio- 
merism, which has nothing to do with the mesomerism 
of the trunk. To the adherents of the first view belong 
those investigators especially who studied the development 
of Elasmobranchs, like BALFOUR (1878, p. 211), MILNES 
MARSHALL (1879, 1882), VAN WYHE (1882, 1889), BEARD 
(1885), DOHRN (1881-1902), SEWERTZOFF (1899), ZIEGLER 
(1908) and his disciples, while those of the second view, 
put forward especially by FRORIEP (1882, 1887), were led to 
their opinion mainly by the study of other Vertebrates, such 
as Amphibians and Amniotes. 
Mesomerism, neuromerism and branchiomerism. — The first 
group of investigators agrees with GEGENBAUR in that, in gene- 
ral, the number and arrangement of the gill-slits corresponds 
to that of the somites, and that there is a coincidence of branch- 
iomerism, myomerism and neuromerism. Truly, there is no 
