ORIGIN AND STRUCTURE OF THE HEAD 



75 



of the notochord, close to the eye-balls. They are distinguish- 

 ed by their minute size and the absence of ventral paits. 

 These are BALFOUR'S praemandibular cavities, which VAN 

 Wyhe and ZlEGLER consider to be the first pair of meso- 

 derm segments. According to this view the mouth is enclosed 

 on both sides between two mesoderm segments in the same 

 way as the gill-slits. 



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. 

 Ac, Sc 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 function 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. The 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 fiir den ersten bis dritten Kiemenbogen je einen 

 diskreten Kopfnerven bestimmt sehen," GeGENBAUR (1887, 

 p. 103) says, "fur 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 



