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of the notochord, close to the eye-balls, They are distinguish- 
ed by their minute size and the absence of ventral parts. 
These are BALFOUR's preemandibular cavities, which VAN 
WYHE and ZIEGLER consider to be the first pair of meso- 
derm segments. According t> this view the mouth is enclosed 
on both sides between two mesoderm segments in the same 
way as the gill-slits. 
pr. mand. | | | 
Fig. 17 Diagram of the head of a Selachian. The heavy 
line indicates the limit between the cerebral 
and the spinal part of the head after FRORIEP. 
Se cranio-vertebral limit in Acanthias and 
Scyllium, au auditory vesicle, m mouth, oc eye, 
Pr. mand. prae-mandibular cavity. 
With the arrangement of the mesoderm segments that 
of the head ganglia corresponds; after VAN WYHE's (1882) 
example the four cranial nerves of mixed functicn are 
compared to the dorsal roots of the spinal nerves of the 
trunk Above the hyoid arch lies the ganglion of the 
facialis-acusticus, above the first interbranchial gill-arch 
the ganglion of the glossopharyngeus, above the next 
gill-arches that of the vagus. e vagus ganglion after 
GEGENBAUR'’s (1872, p. 269) example has been originally 
considered as a complex ganglion corresponding to as 
many segments as gill-slits are innervated by the vagus. 
“Wenn wir für den ersten bis dritten Kiemenbogen je einen 
diskreten Kopfnerven bestimmt sehen,’ GEGENBAUR (1887, 
Pp. 103) says, “für die letzten Kiemenbogen aber einen 
gemeinsamen Stamm, so liegt es nahe genug, diesen aus 
einer Summe von Nerven entstanden zu betrachten, aus 
derselben Anzahl, welche jener der von ihm versorgten 
Kiemenbogen entspricht” Yet, in ontogeny, the number 
