260 THE ORIGIN OF VERTEBRATES 



Marshall; iu 1882, considered that the cranial segments were all 

 originally respiratory, and that all the segmental nerves are arranged 

 uniformly with respect to a series of gill-clefts which have hecome 

 modified anteriorly and have heen lost, to a certain extent, pos- 

 teriorly. He included the olfactory nerves among the segmental 

 nerves, and looked upon the olfactory pit, the orbito-nasal lacrymal 

 duct, the mouth, and the spiracle as all modified gill-slits, so that he 

 reckoned three pre-oral and oral segments belonging to the 1st, Illrd, 

 IVth, and Yth nerves, and eight post-oral segments belonging respec- 

 tively to the Vllth and Vlth nerves, and to the IXth nerve, and six 

 segments belonging to the Xth nerve. He pointed out that muscles 

 supplied by the oculomotor nerve develop from the outer wall of the 

 first head-cavity ; not, however, the dbliguus superior and rectus 

 cxtcmus, the latter originating probably from the walls of the third 

 cavity. 



In the same year, 1882, came van Wijhe's well-known paper, in 

 which he showed that the mesoderm of the head in the selachian 

 divided into two sets of segments, dorsal and ventral ; that the dorsal 

 segments were continuous with the body-somites, and that the ven- 

 tral segments formed the lateral plates of mesoblast between each of 

 the visceral and branchial pouches. He concluded that the dorsal 

 somites were originally nine in number, that each was supplied with 

 a ventral nerve-root, in the same way as the somites in the trunk, 

 and that to each one a visceral pouch corresponded, whose walls 

 were supplied by the corresponding dorsal nerve-root ; of these nine 

 segments, the ventral nerve-roots of the first three segments were 

 respectively the oculomotor, trochlearis, and abducens nerves. The 

 next three segments possessed no definable ventral root or muscles, 

 and the seventh, eighth, and ninth segments possessed as ventral 

 roots the hypoglossal nerve, with its muscular supply. The corre- 

 sponding dorsal nerve-roots were the trigeminal, facial, auditory, 

 glossopharyngeal and vagus nerves, the difference between cranial 

 and spinal dorsal roots beiug that the former contain motor 

 fibres. 



Ahlborn, in 1884, drew a sharp distinction between the segments 

 of the mesoderm and those of the endoderm. The former segmenta- 

 tion he called mesomeric, the latter branchiomeric. He considered 

 the two segmentations to be independent, and concluded that the 

 branchiomeric was secondary to the mesomeric, and therefore not of 



