THE DEVELOPMENT OF THE SKULLS OF FISHES. 187 



quadrate arch ; the metapterygoid in the same relation to its 

 posterior eras ; the quadrate bone, in its inferior process. The 

 symplectic is a cortical ossification of the styliform part of the 

 hyoinandibular cartilage, the ossification of the rest of the latter 

 giving rise to the hyoinandibular bone itself (Fig. 72). 



In many osseous fishes, such as the Carp, the cartilaginous 

 cranium disappears, with age, as completely as it does in Man ; 

 but, in the Pike, it not only persists, but grows and enlarges 

 with age, so that the relations of the cranial bones to cartilage, 

 or to membrane, can be investigated at any period of life. 



If the skull of an adult Pike be macerated, or, better, steeped 

 for a short time in boiling water, a number of the cranial bones 

 will separate with great ease from a sort of model of the skull 

 chiefly composed of cartilage. 



This " cartilaginous skull " forms a complete roof over the 

 cranial cavity (Fig. 73, A), whence it is continued, without in- 

 terruption, to the anterior end of the cranium, forming the narrow 

 inter-orbital septum {I. Or.) and the broad internasal rostrum 

 (ML), and giving rise to two antorbital processes (Prf.), which 

 separate the orbits from the nasal chambers, and are perforated 

 by the olfactory nerve, and by the nasal division of the fifth. 



The inter-orbital cartilage is interrupted by an oval space 

 filled with membrane, just in front of the basi-sphenoid, so that 

 it is continued to the lower end of that bone only by a slender 

 cartilaginous rod, which passes into the stem of the Y-shaped 

 basi-sphenoid (Fig. 73, C). 



The cartilaginous basis of the skull, therefore, is not con- 

 tinued back along the floor of the canal for the orbital muscles. 

 The roof of the orbital canal contains cartilage in the middle 

 line, which is almost completely hidden in front by the ex- 

 tension towards one another of the horizontal laminae of the 

 pro-otic bones. The under-surface of the inter-orbital septum 

 and of the greater part of the cartilaginous rostrum is marked 

 by a deep groove (a, Fig. 73, B), into which a median ridge of 

 the parasphenoid is received. 



The bones which, being developed in perichondrium, are 

 easily removed from the macerated skull, are the parietals, the 

 frontals, the bones 1.1. and 2.2. (Fig. 69), the squamosals (when 



