April 2, 1874J 



NATURE 



425 



lopment of the vertebrate skull. The plan pursued by 

 Mr. Parker has been to describe first in their adult state, 

 and afterwards in the various stages of their development, 

 jhe skulls of certain prominent vertebrates which should 

 serve as types for the other members of the group, and to 

 deduce from the facts thus established the principles on 

 which the cranium is constructed in the whole sub-king- 

 dom. The types selected were the shark, skate, salmon, 

 axolotl, frog, snake, fowl, and pig. 



I. — Morpliological Elcincnis of the Skull. Nothing 

 can be more hopeless than the attempt to unravel 

 and explain the vertebrate skull by the study of adult 

 forms only. The modification of face and brain-case, in 

 the long line of creatures which begins with the lamprey 

 and ends with man, are so endless that, until the study of 

 embryology put the matter on a new and firm foundation, 

 the best observers failed signally to produce a true 

 " theory of the skull," the most elaborate attempt of the 

 kind — the " vertebral theory " started by Goethe and Oken 

 in Germany, and perfected by Prof. Owen inEngland — 

 having resulted only in a convenient working hypothesis. 



When, however, instead of starting with the highly 

 differentiated skulls of adult animals, the embryos of 

 these animals from their earliest conditions are made the 

 subject of investigation, a new light is shed on the whole 

 question. It is found that the skulls of all the vertebrata 

 which have yet been thoroughly worked out, originate in, 

 practically, a precisely similar manner ; and even in some 

 of the more advanced stages it would be hard to point 

 out very essential differences between the skulls of a fish, 

 a bird, or a mammal. Before entering upon the descrip- 

 tion of the skull of the shark, the first type to be gone 

 into,* it will be advisable to consider the distinct elements 

 of which the primordial cranium of any vertebrate 

 animal is made up. 



a. On either side of the anterior termination of the 

 notochord, or primitive a.xis of the body (Fig. i, Ch.), is 

 developed a cartilaginous plate (I.M), which as a rule 

 unites both above and below the notochord with its 

 fellow of the opposite side. These plates taken together 



were termed by Rathke the investing mass: in the language 

 of Prof. Huxley, they constitute \.\\ft parachordal elements 

 of the primordial skull or chondro-cranimn. 



b. In front of and below the investing mass, cartilagi- 

 nous thickenings with intervening spaces are developed in 

 the side walls of the body, enclosing, rib-like, the 



Fig. I.— Diagram of Vertebrate Skull Ch, Notochord ; Vr", Vr', first 

 and second vertebra: ; I.M, investing mass ; Au, auditory capsule : E, 

 eye: Na, nasal capsu e ; Tr, trabecular; Mn, mandibular: Hy, 

 hyoid; Br.T-5, branchial arches: PI. Pi, pterygo-palatine process: 

 Ty.Eu, tympano- Eustachian pa^sage : Cl.t-=, branchial 

 ' ■- ' "" dllary: V3, mandibular: VJ, pala- 



VH», vidian; VHP, hyo- 

 IX, gloSNOpharyngeal : X, 



mandibula 

 vagus. 



o-nasal : 

 n of the trigeminal 

 divisions of portiodura ; 



pharyngeal cavity. These are the msceral arches (pleural 

 elements, Hu.xley), the spaces between them the visceral 

 clefts. The usual number of these bars is eight, although 

 in certain exceptional cases they may be increased to nine 

 (He.xanchus) or ten (Heptanchus). Taken from before 

 backwards, the pleural aiches are named as follows : — 

 I. trabecular (Tr), 2. mandibular (Mn), 3. hyoid (Hy), 4-8. 

 branchial (Br. 1—5), the clefts separating them being in 

 like manner, the mouth (M), the tympano-Eustachian 

 passage (Ty Eu) and the branchial clefts (CI I — 5). At 

 a very early period, the mandibular arch gives off a for- 

 ward process, the palato-pterygoid arcade (PI Pt) which in 

 certain cases takes on the form of a distinct pleural 

 element. In the branchiate vertebrata (Fishes and 



^■"r V Sp.O Aa. 



C.Hy 



Flc. 2.— Skull or Shark. O.C, occipit.-il condyle ; Pt.O, pterotic. and Sp.O sphenotic process ; S.Or, supra-orbital i 

 13, H 15, labial cartilages ; PI. Tr., palato-trabecular ligament ; M.Pt.. meta-pterygoid ; Mck, Meckel's cartilage 



cerato-hyal : Ph.Br, E.Br, C.Br, H.Br, and B.Br, pharyngo-, epi-, 

 Optic foramen. 



Amphibia) all these arches with the single exception of 

 the trabecular may bear functional gills, the presence of 

 these organs being the chief physiological test of their 

 serial homology. 



Far more important, however, in the determination of 

 these elements of the skull, are the morphological land- 

 marks afforded by the cranial nerves, especially by the 

 5th, 7th, 9th, and icth pairs. It is the constant habit of 

 these nerves to fork above a visceral cleft, one of the 

 branches thus formed supplying the posterior face of the 



♦ The development of the skull in the Marsipobranchii (Lamprey and 

 Hag), the lowest group of craniate Vertebrata, has not yet been studied 

 with sufficient accuracy to allow of the determination of its parts with any 

 degree of certainty. 



B.Tr, b.isi-tr.abe 

 H.M, hyo-mandibula 



r 1>, 1-, 

 C.Hy, 



D-, hypo-, and basi-branchial ; Ex. Br, extra-branchials : Sp, spiracle 



arch in front of the cleft, the other the anterior face of 

 the arch behind and (see Fig. i). The orbito-nasal or 

 ophthalmic (V) and the palatine (V^) divisions of the 

 trigeminal are the special trabecular nerves, the former 

 passing over the optic nerve, the latter below it. The 

 posterior primary subdivision of this nerve passes behind 

 the mouth-cleft, and divides into two branches, both of 

 which are distributed to the anterior (or in the altered 

 position of the three foremost arches, outer) side of the 

 mandibular arch ; the mandibular, or mferior maxillary 

 nerve (V^) passing along the original direction of the 

 arch, the superior maxillary (V=) following the pterygo- 

 palatine process. The seventh nerve, or portiodura, 

 divides above the tympano-Eustachian passage, its anterior 



