DEVELOPMENT OF THE ENDOSKELETON 693 



dorsad from the sternal area on either side, the clavicle, procoracoid, and 

 coracoid to which is attached dorsally the scapula. Often a suprascapula is 



attached to the scapula. The pelvic girdle of the Tetrapoda, on the other 

 hand (fig. 323F), is composed of two lateral columns on either side. The 

 anterior column is called the pubis, and the posterior column is the ischium. 

 An ilium is attached to the dorsal ends of the pubis and ischium on either 

 side. Epipubic and hypoischial midpieces are sometimes present at the mid- 

 ventral ends of the pubic and ischial columns in some species. 



As in the development of the skeleton of the free appendage, all the rudi- 

 ments of these structures are laid down in cartilage and later ossify, with the 

 exception of the clavicle which may be of intramembranous origin (Hanson, 

 '20a and '20b). The clavicles are more strongly developed in man, whereas 

 the coracoidal elements are vestigial (fig. 323E). In the cat, the coracoidal 

 and clavicular elements are reduced. However, in the chick and frog, the 

 coracoidal elements are dominant (fig. 323C, D). In the pelvic girdle, the 

 iliac, pubic, and ischial elements are constant features in most Tetrapoda. 

 In the shark, a single coracoid-scapula unit is present in the pectoral girdle 

 and the pelvic girdle is reduced to a small transverse bar of cartilage (fig. 

 323B, G). 



e. Growth of Bone 



Bone once formed is not a static affair, for it is constantly being remodeled 

 and enlarged during the growth period of the animal. In this process, bone 

 is destroyed and resorbed by the action of multinucleate giant cells, called 

 osteoclasts, or specialized, bone-destroying cells and is rebuilt simultaneously 

 in peripheral areas by osteoblasts from the surrounding periosteal tissue. 



To understand the processes involved in bone growth, let us start with 

 the conditions found in the primitive shaft of a long bone (fig. 314F). Within 

 the bony portion of the shaft, there is a network of cancellous bone, and, 

 peripherally, there are lamellae of compact bone. The following transforma- 

 tive activities are involved in the growth of this bone: 



( 1 ) Within the bone, the cancellous columns of bony substance are de- 

 stroyed by osteoclasts, the bony substance is resorbed, the marrow 

 spaces are enlarged, while, peripherally, circumferential lamellae are 

 deposited around the bones beneath the periosteum. 



(2) Distally, cartilage is converted into cancellous bone while outer cir- 

 cumferential lamellae are fabricated beneath the periosteum. The bony 

 substance thus creeps distally, lengthening the shaft of the bone. 



(3) As the bone increases in length, some of the bony substance, forming 

 the wall of the shaft or diaphysis is destroyed. This alteration is ef- 

 fected to a degree by vascular buds which grow into the bony sub- 

 stance from the periosteum around the outer surface of the bone and 

 from the endosteum which lines the marrow cavities. These vascular 



