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head in Elasmobranchs disappears soon afterwards and the 
mesoderm segments are dissolved and replaced by an 
unsegmented mass of mesenchyme, as is observed in this 
region from the beginning in Amniotes. This mesenchyme, 
however, according to FRORIEP, is not, as one might 
suppose, a product of the dissolution of the somites. 
The latter do not contribute at all to its formation, it is 
exclusively the little area of unsegmented mesoblast lying 
originally in front of the first somite that grows out back- 
wards and pushes below the scattered mesenchyme that has 
resulted from the dissolution of the somites. In this way things 
happen according to FRORIEP (1902, p. 44), but it seems 
to me very difficult to conclude from sections, that this 
view is the right one, and that it is not the dissolution of 
the somites which is the source of the unsegmented mesen- 
chyme mass. FRORIEP himself has been long in doubt, as 
he admits, and 1 cannot feel convinced by his argumentation, 
which is based on a slight difference in density of the 
supposed two kinds of mesenchyme. 
ow the question is, whether the branchiomerism, which 
soon after makes its appearance in this region, is quite 
independent from the original mesomerism or if it corres- 
ponds to it. In studying the work of the above cited authors, 
one cannot avoid the impression that the adherents of the 
first view place themselves on a too restricted stand-point 
and that, to quote once more the words of EISIG (cf. p. 244), 
they are inclined to translate “eine ontogenetische Thatsache 
willkürlich ins Phylogenetische.” Truly, the correspondence 
of branchiomerism to mesomerism is often, especially in 
somewhat further advarced stages of development, far from 
evident. As a rule, especially in higher Chordates, meso- 
dermic segmentation is no longer to be observed in the 
branchial region when the giil-slits have broken through. The 
fact, however, that one gets the impression, that ín ontogeny 
the seriality of the gill-pouches originating from the entoderm 
is the cause of the segmentation of the head-mesoderm, seems 
to me not at all to exclude the possibility, that in phylogeny 
the reverse may have been the case and that also in 
Ontogeny the place where the gill-pouches will arise has 
been determined in an earlier stage by some influence 
from the mesoderm, in Elasmobranchs e.g. by the somites 
observed also by FRORIEP in the branchial region. One 
of the strongest arguments for the latter view seems to 
