THE SKULL.
Three distinct sets of elements may enter into the composition of the skull. These are (1) the cranium proper, composed of true endoskeletal elements originally formed in cartilage, to which are usually added exoskeletal osseous elements, formed in the manner already described p. [542], and known in the higher types as membrane bones. (2) The visceral arches formed primitively as cartilaginous bars, but in the higher types largely supplemented or even replaced by exoskeletal elements. (3) The labial cartilages.
These parts present themselves in the most various forms, and their study constitutes one of the most important departments of vertebrate morphology, and one which has always been a favourite subject of study with anatomists. At the end of the last century and during the first half of the present century the morphology of the skull was handled from the point of view of the adult anatomy by Goethe, Oken, Cuvier, Owen, and many other anatomists, while Dugés and, nearer to our own time, Rathke, laid the foundation of an embryological study of its morphology. A new era in the study of the skull was inaugurated by Huxley in his Croonian lecture in 1858, and in his lectures on Comparative Anatomy subsequently delivered before the Royal College of Surgeons. In these lectures Huxley disproved the then widely accepted view that the skull was composed of four vertebræ; and laid the foundation of a more satisfactory method of dealing with the homologies of its constituent parts. Since then the knowledge of the development of the skull has made great progress. In this country a number of very interesting memoirs have been published on the subject by Parker, which together constitute a most striking contribution to our knowledge of the ontogeny of the skull in a series of types; and in Germany Gegenbaur’s monograph on the cephalic skeleton of Elasmobranchii has greatly promoted a scientific appreciation of the nature of the skull.
In the present chapter only the most important features in the development of the skull will be touched on.
It will be convenient to describe, in the first instance, the development of the cartilaginous elements of the skull.
Fig. 323. head of embryo Dogfish, second stage; basal view of cranium from above, the contents having been removed. (From Parker.)
ol. olfactory sacs; au. auditory capsule; nc. notochord; py. pituitary body; pa.ch. parachordal cartilage; tr. trabecula; inf. infundibulum; C.tr. cornua trabeculæ; pn. prenasal element; sp. spiracular cleft; br. external branchiæ; Cl. 2, 4. visceral clefts.
The Cranium. The brain is at first enveloped in a continuous layer of mesoblast known as the membranous cranium, into the base of which the anterior part of the notochord is prolonged for some distance. The primitive cartilaginous cranium is formed by a differentiation within the membranous cranium, and is always composed of the following parts ([fig. 323]):
(1) A pair of cartilaginous plates on each side of the cephalic section of the notochord, known as the parachordals (pa.ch). These plates together with the notochord (nc) enclosed between them form a floor for the hind- and mid-brain. The continuous plate, formed by them and the notochord, is known as the basilar plate.