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mandibular segment, but this was denied by VAN WYHE. 
2, p. 8) and recently by ZIEGLER (1908, p. 659). 
GEGENBAUR (1887, p. 7) suggests that nevertheless in phy- 
logeny the praemandibular segment may have split off 
from the mandibular segment. In Necturus Miss PLATT 
(1897, p. 443) finds that the praemandibular mesoderm in: 
young stages is not divided from the mandibular. In 
Petromyzon recently HATSCHEK (1910, p. 481) states that 
here also the praemandibular and mandibular cavities- 
originally represent one segment, the mandibular segment, 
which accordingly is the first mesoderm segment, to which 
a special significance is attributed by HATSCHEK. It is the 
mesoderm of his “acromerite,” an anterior part of the body 
corresponding to my prostomium + mandibular segment, 
and concerning which HATSCHEK provisionally leaves it 
undecided, as to whether it represents an anterior unseg- 
mented region or an anterior, somewhat diverging, metamere. 
HATSCHEK thinks “dass die Vorgänge bei den Selachiern 
erst durch die in mancher Beziehung einfacheren und wohl’ 
auch primitiveren bei Petromyzon besser verständlich wer- 
den,” and thus seems to be inclined to join GEGENBAUR'S: 
opinion. In Amphioxus also we see the first pair of segments,. 
homologized by VAN WYHE to the mandibular segments 
of Craniates, send out each a forward prolongation into the- 
prostomium. 
That the praemandibular cavities produce muscles (Musc. 
_ rectus superior, internus and inferior, and Musc. obliquus 
inferior, as stated in Selachians), and in this respect resemble- 
myotomes of the trunk, cannot well be adduced as an 
argument for considering them as representing a segment, 
since in the first place it may be doubted if the eye-muscles 
are to be derived directly from segmental muscles. We 
see the latter vanish gradualiy in going from the trunk 
to the head in an embryo. The three occipital segments in 
Pristiurus and Scyllium still have a distinct myotome with 
muscle-formation, the first vagus-somite has only a rudí- 
mentary myomere, while in the segments lying in front of this. 
the glossopharyngeus- and the facialis-segment, no myotomS 
are developed. In the (hyoid-?) mandibular- and praeman- 
dibular segment we then see the eye-muscles appear, which: 
accordingly are considered, e.g. by ZIEGLER (1908, p. 674), 
as relatively young muscles, not directly comparable to- 
segmental trunk muscles. But even if we do not share 
