478 STRUCTURE OF VERTEBRATA. 
gives rise to the myotomes forming all the segmented muscles, to out- 
growths into the limbs, to the cutis or dermis, to a sheath round the 
notochord, etc. The ventral part gives rise to the splanchnic or visceral 
muscles (usually unstriped), to the coelomic epithelium, etc. 
Skeletal system.—Apart from the exoskeleton of skin- 
teeth, scutes, shields, etc., the skeleton consists of the 
following parts :— 
The skull and its associated “ arches.” 
(2) Axial The backbone and associated ribs. 
Skeleton. (The notochord is transitory except 
in the simplest Vertebrates.) 
(4) Appendicular {Fore 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 trabeculee 
lying in front. The parachordals grow round and above 
the notochord, producing the basilar plate, while the trabe- 
cule 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 formation of 
the primitive cartilaginous brain-box or chondrocranium 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 
membranous cranium, the arches also become chondrified, 
and at the same time divided into segments. 
Of these arches there are never more than nine. The 
most anterior is the mandibular arch which bounds the 
mouth, the second the Ayord; these two are of great 
importance in the development of the skull. The others, 
in Fishes and at least young Amphibians, bound open gill- 
