MULTICELLULAR ANIMALS 



151 



(Fig. 99). On the other hand, the large plate bone of the forehead, 

 the FRONTAL, is a dermal bone. It is derived from two plates that 

 may be traced to their homologues in the two broad exoskeletal 

 scales of the head of certain fishes. The record shows that these 

 scales, of mesodermal origin, have become associated with the carti- 



GAR 



TURTLE 



MAN 



Fig. 100. — The head of a fish (short nose gar) to show the frontal scales; the 

 head of a turtle to show the frontals, now a part of the endoskeleton; the human 

 skull showing the homologous frontal elements. 



lage bones to form the roof of the skull (Fig. 100) ; the cartilage 

 bones form its floor. 



The human skeleton for convenience in study may be divided 

 into two main divisions, the axial skeleton composed of the skull, 

 vertebral column, ribs, and sternum; and the appendicular skeleton, 

 which consists of the bones of the legs and arms and of the two 

 girdles, shoulder and pelvic. With the exception of the clavicle, 

 which is a dermal bone, all the dermal elements of the skeleton are 

 involved in the skeleton of the head. All the bones are invested 



