Now came the investigations of the embryologists, of whom I will take, in the first instance, Balfour, whose observations on the embryology of the Selachians led him to the conclusion that besides the evidence of segmentation to be found in the cranial nerves and in the branchial clefts, further evidence was afforded by the existence of head-cavities, the walls of which formed muscles just as they do in the spinal region. He came to the conclusion that the first head-cavity belonged to one or more pre-oral segments, of which the nerves were the oculomotor, trochlearis, and possibly abducens; while there were seven post-oral segments, each with its head-cavity and its visceral arch, of which the trigeminal, facial, glossopharyngeal, and the four parts of the vagus were the respective nerves.
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, posteriorly. 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, IIIrd, IVth, and Vth nerves, and eight post-oral segments belonging respectively to the VIIth and VIth 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 obliquus 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 ventral 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 corresponding 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.
Ahlborn, in 1884, drew a sharp distinction between the segments of the mesoderm and those of the endoderm. The former segmentation 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 segmental value. As to the segments of the mesoderm in the head, the three hindmost or occipital in Petromyzontidæ remain permanently, and correspond to the three last segments in the selachian head. Of the anterior mesoderm segments, he considered that there were originally six, and that there are six typical eye-muscles in all Craniota, which have been compressed into three segments, as in Selachia.
Froriep (1885) showed in sheep-embryos and in chicks that the hypoglossal nerve belongs to three proto-vertebræ posterior to the vagus region, which were true spinal segments. He therefore modified Gegenbaur's conceptions to this extent: that portion of the skull designated by Gegenbaur as vertebral must be divided into two parts—a hind or occipital region, which is clearly composed of modified vertebræ and is the region of the hypoglossal nerves, and a front region, extending from the oculomotor to the accessorius nerves, which is characterized segmentally by the formation of branchial arches, but in which there is no evidence that proto-vertebræ were ever formed. He therefore divides the head-skeleton into three parts—
1. Gegenbaur's evertebral part—the region of the olfactory and optic nerves—which cannot be referred to any metameric segmentation.
2. The pseudo-vertebral, pre-spinal, or branchial part, clearly shown to be segmented from the consideration of the nerves and branchial arches, but not referable to proto-vertebræ—the region of the trigeminal and vagus nerves.
3. The vertebral spinal part—the region of the hypoglossal nerves.
He further showed that the ganglia of the specially branchial nerves, the facial, glossopharyngeal, and vagus, are at one stage in connection with the epidermis, so that these parts of the epidermis represent sense-organs which do not develop; these organs probably belonged to the lateral line system. As the connection takes place at the dorsal edge of the gill-slits, they may also be called rudimentary branchial sense-organs.
Since this paper of Froriep's, it has been generally recognized, and Gegenbaur has accepted Froriep's view, that the three hindmost metameres, which distinctly show the characteristics of vertebræ, belong to the spinal and not to the cranial region, so that the metameric segmentation of the cranial region proper has become 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.