260 
is cut into segments by the gill-slits, is. l think, nothing 
but this anterior part of the unsegmented lateral r: late. 
Diagrams of Van Wyhe and Ziegler. — Before proceed- 
ing now to the second of the above questions, viz : whether 
the auditory vesicle belongs to this unsegmented head 
region or to the region of the somites, we will first turn 
a moment to the diagrams of the metameric structure of 
the head drawn by the adherents of the views of GE :ENBAUR 
and VAN WYHE. As mentioned before, the scheme lastly 
given by ZIEGLER (1908, 1915), and based on the researches 
of his disciples KLINKHARDT (1905), GUTHKE (1906) and 
BROHMER (1909), is one of the simplest. On the whole 
it agrees with that of VAN WYHE (1882), but the hyoid- 
segment is not considered by him as a double segment, 
and also ZIEGLER's conception of the polymerism of the 
vagus differs somewhat from that of VAN 
Myomerism, neuromerism *) and branchiomerism corres- 
pond; each gill-slit has pierced through the lateral pla:e 
between two somites, thus causing the stalks connecting 
each myotom with the unsegmented lateral plate to appear 
much longer than in the trunk. They each run through a 
branchial bar and communicate at their inferior end with 
the pericard, in he same way as the somites of the trunk 
wit the general coelome. Only in the occipital region 
are myotoms distinctly developed at their superior ends. 
The spiraculum is the first gil:-slit, it originally resembles- 
completely the gill-slits in shrpe and situation and forms 
witt the latter a series of which the mouth seems to be 
again the forward continuation. To each branchial bar 
a segment corresponds; that between the spiraculum 
and the first gill-slit represents the hyoid-segment, that be- 
tween the spiraculum and the mouth the mandibu!ar segment, 
from which the mandibula is form:d and which at its upper 
end dilates into a vesicle which, projecting far forward, 
provides nearly all the m.senchyme of the anterior part of 
the head. Anterior to the mouth we then have a last couple 
of mesoderm segments, situated in front of the anterior end 
