76 THE ANCESTRY OF VERTEBRATES 



of ganglia fused to form the vagus-ganglion appears not 

 to correspond to the number of gill-arches innervated by 

 the ventral branches of the vagus, being four in pentanch 

 Elasmobranchs. According to GUTHKE (1906), the complex 

 vagus ganglion sends three roots into the medulla and also 

 three branches into the last three i nter branchial gill-archs. 

 Thus ZlEGLER (1908) and his disciples consider the vagus 

 ganglion as formed by the fusion of three ganglia, the ganglion 

 of the fourth branch, which runs behind the last gill-slit, 

 (cf. fig. 17) or of the last branches in hexanch and heptanch 

 forms, having been lost and these branches themselves having 

 united with the fore-going, so that the third or last branch coming 

 from the ganglion sends nerves to two, three or four gill-slits. 

 Van Wyhe, who originally (1882) considered the vagus as a 

 quadruple nerve, afterwards (1889, p. 562), on the discovery of 

 rudimentary dorsal ganglia to the last two head-myotomes 

 (Van Wyhe, 1886, OSTROUMOFF, 1889), recognizes only two 

 vagus-roots. The results of embryological and anatomical 

 researches on Petromyzon point to the conception, advocated 

 by HATSCHEK (1892, p. 157), that the primary vagus is here a 

 single nerve only, which in Gnathostom.es probably fuses with 

 one spinal nerve and its ganglion, which are still independent 

 in Petromyzon, and further "collects" the ventral part {rami 

 prae- and posttrematici) of the dorsal roots of as many 

 subsequent spinal nerves as there are gill-slits supplied 

 by it ("partial polymerism" of the vagus, HATSCHEK, 1892, 

 p. 152). Thus the ramus branchio-intestinalis of the vagus 

 would be a collector in the same way as the ramus lateralis 

 of that nerve according to EISIG and others. 



The main ganglion of the trigeminus lies over the man- 

 dibular arch, and the first ganglion of the trigeminus, 

 the ganglion ciliare with the ramus ophthalmicus profundus, 

 is considered to belong to the praemandibular segment. 

 Thus the trigeminus is considered by VAN WYHE and 

 ZlEGLER to represent a double segmental nerve. 



Neural crest of head and trunk. — Another question 

 which has occasioned controversy is, whether the neural 

 crest, which gives rise to the centrogenetic part of the 

 head ganglia, is to be considered as the direct conti- 

 nuation of the neural crest of the trunk. In fact there is 

 this difference between the spinal and the head ganglia, 

 besides the mixed character of the latter, that the former 

 are found on the inner side of the somites, the latter on the 



