262 THE ORIGIN OF VERTEBRATES 
more and more associated with the branchial segmentation. Froriep’s 
discovery of the rudimentary branchial sense-organs as a factor in 
the segmentation question has led Beard to the conclusion that the 
olfactory and auditory organs represent in a permanent form two 
of these rudimentary branchial sense-organs. He therefore includes 
both the olfactory and auditory nerves in his list of cranial segmental 
nerves, and makes eleven cranial branchial segments in front of the 
spinal segments represented by the hypoglossal. 
A still larger number of cranial segments is supposed to exist, 
according to the researches of Dohrn and: Killian, in the embryos 
of Torpedo ocellata. The former, holding to the view that vertebrates 
arose from annelids, considered that the head was formed-of a series 
of metameres, to each one of which a mesoderm-segment, a gill-arch, 
a gill-cleft, a segmental nerve and vessel belonged. He found in the 
front head-region of a Torpedo embryo, corresponding to van Wijhe’s 
first four somites, no less than twelve to fifteen mesoderm segments, 
and concluded, therefore, that the eye-muscle nerves, especially the 
oculomotor, represented many segmental nerves, and were not the 
nerves of single segments ; so, also, that the inferior maxillary part of 
the trigeminal and the hyoid nerve of the facial are probably not 
single nerves, but a fusion of several. Killian comes to much the 
same conclusion as Dohrn, for he finds seventeen to eighteen separate 
mesoderm segments in the head, of which twelve belong to the tri- 
geminal and facial region. 
Since Rabl’s paper, a number of papers have appeared, especially 
from America, dealing with yet another criterion of the original 
segmentation of the head, viz. a series of divisions of the central 
nervous system itself, which are seen at a very early stage of 
development, and are called neuromeres; the divisions in the cranial 
region being known as encephalomeres, and those of the spinal region 
as myomeres. Locy’s paper has especially brought these divisions 
into prominence as a factor in the question of segmentation. They 
are essentially segments of the epiblast and not of the mesoblast ; 
they are conspicuous in very early stages, and appear to be in 
relation with the cranial nerves, according to Locy. He recognizes 
in Squalus acanthias, in front of the spino-occipital region, fourteen 
pairs of such encephalomeres and a median unsegmented termination, 
which may represent one more pair fused in the middle line, making 
at least fifteen. He distributes these fifteen segments as follows: 
