255 
is provided by the craniovertebral limit, the correct deter- 
mination of which in succeeding stages he guarantees, 
though with the somewhat restricting addition “(lrrtum 
vorbehalten)”. 
Thus FRORIEP reaches the conclusion that a consider- 
able part, nay, by far the greatest part, of the branchial 
region originally indeed exhibits segmentation, though 
secondarily this is destroyed and subsequently replaced by 
the branchiomerism, of which only the latter would have been 
observed by VAN WYHE and ZIEGLER, who studied stages 
much too far advanced in development. We get the impres- 
sion, that it is no easy task for FRORIEP to demonstrate 
that the auditory vesicle still belongs to the unsegmented 
region in front of the first somite, since in his own drawings 
we see the series of somites clearly reaching to in front of 
the auditory vesicle. One would be inclined to ask if, 
in such Vertebrates, where no prootic somites are to be 
observed, the reduction demonstrated by FRORIEP in Elas- 
mobranchs could not have already set in in the earliest stages 
observed and even before the somites become evident ? 
Main points on which opinions differ. — At any rate we 
can state that the main differences between the two concep- 
tions prevailing until the present day is reduced to a diver- 
gence of opinion on the following questions : 
. Is there a part of the head that is primarily unseg- 
mented and is there a primarily unsegmented head mesoderm ? 
Does the auditory vesicle belong to this unsegmented 
head-region or to the region of the somites? 
3. Does the branchiomerism correspond to the meso- 
merism and do the gill-slits belong to the first or to the 
second region mentioned under 2? 
_Unsegmented region of the head, — As regards the first 
question, both parties recognize the existence of an anterior 
part in which no segmentation can be traced. It is the ever- 
tebral region of GEGENBAUR — corresponding, together with 
the mandibular segment, to the acromerite of HATSCHEK 
(1909, 1910) — resp. the praespinal or cerebral region of 
FRORIEP, which reaches to the foramen of the vagus. The 
praechordal part, by the adherents to the first view, how- 
ever, is generally considered as a secondary outgrowth from 
the first segment, their starting point being accordingly 
a body uniformly segmented from the tip of the nose 
to the end of the tail; the foremost segments having 
