SEGMENTS OF TRIGEMINAL NERVE-GROUP 26 1 



segmental value. As to the segments of the mesoderm in the head, 

 the three hindmost or occipital in Petroniyzontidfle 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-vertebrre 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 

 vertebras 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 e vertebral 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-vertebraa — 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 vertebras, 

 belong to the spinal and not to the cranial region, so that the 

 metameric segmentation of the cranial region proper has become 



