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 orgaus 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 Eabl'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 : 



