COMPOSITION OF THE SKULL. 37? 
extreme modification in form. At first all limbs arise as 
little pads, in which the skeletons subsequently develop, and 
in early life the limbs of all Vertebrates above the fishes are 
much alike, the mod- 
ifications taking place 
shortly before birth. Ac- 
cording to Gegenbaur 
and others, the limbs of 
Vertebrates have been 
probably derived from 
the pectoral and ventral 
fins of fishes in which 
the fin-rays are irrela- 
tively repeated.* 
In the fins of fishes 
walling on land, a com- one (ap tee ia), lend 
pound system of lever- a pair of ankle-bones (tarsals); p,q, foot 
age (Wyman). 
- The head of all Vertebrates above the lancelet is supported 
by a more or less perfect cartilaginous or bone framework, 
the skull (cranium), or brain-box (Fig. 381). It isa contin- 
uation of the vertebral column, and protects the brain, 
besides forming the support of the jaws, tongue-bone 
(hyoid bone), and branchial arches. The series of lateral 
(visceral or branchial) arches varies, but there may be nine ; 
the most anterior (if it be counted as the first one, Fig. 
382, a, 6, c) is formed by what are called the labial carti- 
lages; next comes the mandibular arch (0, 2), which is suc- 
ceeded by the hyoid arch (II.) and the six branchial arches. 
In the embryos of all Vertebrates these visceral arches are 
* A modified form of this theory is advocated by Balfour and J. K. 
Thatcher, who attempt to show that the limbs with their girdles were 
derived from a series of similar simple parallel rays, and that they 
were originally a specialization of the continuous lateral folds or fins of 
embryo fishes, and probably homologous with the lateral folds of the 
adult lancelet (Amphioxus), : 
