SKULL 67 



by a spreading inwards, as it were, of the tendency to form bone 

 within the connective tissue system, remains a doubtful point ; but 

 the main conclusion, that the phylogenetic development of the bony 

 skeleton has, on the whole, followed this course, is now generally 

 accepted. 



Glancing briefly at the parts played by bone and cartilage in 

 the general development of the vertebrate skeleton, we find that 

 the endoskeleton appeared first, and in the form of cartilage 

 (Cyclostomes). Subsequently, it would seem, the skin became 

 covered with tooth-like scales, the primitive exoskeleton of fish. 

 The first bone may then have been developed in relation to these, 

 and it is to such superficial bones that the term ' primary ' should 

 be applied. Owing to the necessity for further consolidation, it 

 became advantageous either for the cartilages to be strengthened 

 by an abundant deposit of lime salts (Chondrichthyes, p. 122), or 

 for the process of ossification to extend inwards in the connective 

 tissue to the deeper regions. Moreover, bony plates, superficial 

 in origin, may, in the course of evolution, sink so as to come into 

 close connection with the underlying skull and pectoral girdle 

 (Vrolik [476], Walther [480], Osteichthyes, p. 212). The develop- 

 ment of the denticles, and their relation to the bones of the skeleton, 

 are dealt with further on (p. 214), 



The skull becomes more developed in the Gnathostomes. The 

 nostrils and nasal sacs are paired. The two capsules usually 

 develop in continuity with the trabeculae, which fuse in front into 

 an ethmoid cartilage forming a median nasal septum (Fig. 6). The 

 brain -case is more complete, though membranous fontanelles 

 generally remain above and below in the chondrocranium. An 

 occipital region is always continuous with the parachordals and 

 auditory capsules, and includes several ' scleromeres ' behind ; so 

 that not only are the glossopharnygeal and vagus nerve -roots 

 invariably enclosed in the skull, but also one or more segmental 

 elements of the hypoglossal (p. 10). 



The characteristic mouth is bounded above and below by the 

 biting jaws, developed from the first pair of visceral arches. This 

 arch bends over the angle of the mouth, the upper limb becoming 

 separated as the palato-quadrate bar, or primary upper jaw; the 

 lower limb forming Meckel's cartilage, or primary lower jaw 

 (Fig. 5). There are never more than eight pairs of gill -slits. 

 The first, or spiracle, is always different from the others, is 

 generally not pierced, and is often vestigial. It is followed by 

 the hyoid arch behind, and the remaining slits by corresponding 

 branchial arches. These visceral arches develop as paired cartilages 

 next to the wall of the pharnyx, internal to the muscles, nerves,' 

 blood-vessels, and coelom. Below they join a series of median 



