SKELETAL SYSTEM— SKULL. 427 



The muscles of the limbs arise in Elasmobranchs as buds from the 

 primitive segments ; buds from several contiguous segments grow into 

 each fin. In most other Vertebrates the formation of the Umb muscles 

 is more complicated ; they seem in some cases to arise independently of 

 the primitive segments. 



Most of the visceral muscles consist of unstriped fibres, but those of 

 the trunk, head, and limbs, as well as of the heart, show the usual striped 

 structure. 



Skeletal System. 



Apart from the exoskeleton of skin-teeth, scutes, shields, 

 &€., the skeleton consists of the following parts : — 



(The skull and its associated " arches." 

 The backbone and associated ribs. 

 (The notochord is transitory except in 

 the simplest Vertebrates). 



(b) AppendicularJFore limbs, and pectoral girdle. 

 Skeleton (Hind limbs, and pelvic girdle. 



Skull. 



The notochord grows forward anteriorly as far as that 

 region of the brain known as the optic thalami. Around 

 notochord and brain the mesenchyme forms a continuous 

 sheath which is the foundation of the skull. 



As in the case of the notochordal sheath of the trunk 

 region, so also here cartilage is formed in the primitive 

 membranous cranium. The first cartilages to appear are 

 the two parachordals, which lie on the lower surface of the 

 head at the sides of the notochord, and the two trabecule 

 lying in front. The parachordals grow round and above 

 the notochord, producing the basilar plate, while the 

 trabecule unite in front to form the ethmoid plate. 

 The continuance of the process of cartilage formation, 

 together with the addition of cartilaginous nasal capsules in 

 front and auditory capsules behind, completes the forma- 

 tion of the primitive cartilaginous brain box or chondro- 

 cranium of the lower Vertebrates. 



Also connected with the head region, and of great import- 

 ance are the visceral or gill arches which loop around the 

 pharynx on either side, and separate the primitive gill clefts. 

 At the time when cartilage begins to be formed in the mem- 



