260 THE ORIGIN OF VERTEBRATES 
Marshall, in 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 become 
modified anteriorly and have been 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 Ist, I1Ird, 
I1Vth,and Vth nerves, and eight post-oral segments belonging respec- 
tively to the VIIth and VIth nerves, and to the [Xth 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 obliqguus superior and rectus 
externus, 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 being that the former contain motor 
fibres. 
Ablborn, 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 
