SEGMENTS OF TRIGEMINAL NERVE-GROUP 261 
segmental value. As to the segments of the mesoderm in the head, 
the three hindmost or occipital in Petromyzontide remain perma- 
nently, 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-vertebre 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 
vertebrae 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-vertebree 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 segmen- 
tation. 
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-vertebree—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 rudimen- 
tary 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 vertebrae, 
belong to the spinal and not to the cranial region, so that the 
metameric segmentation of the cranial region proper has become 
