DEVELOPMENT OF THE ENDOSKELETON 691 



tween the pelvic girdle and the axial skeleton and is most highly developed 

 in those species which use the hind limbs vigorously in support and propul- 

 sion of the body (fig. 3231, L). 



Two main types of bilateral appendages are found in the vertebrate group: 



( 1 ) the ichthyopterygium of Pisces and 



(2) the cheiropterygium of Tetrapoda. 



The former is flattened dorso-ventrally, and assumes the typical flipper or 

 fin shape, while the latter is an elongated, cylindrical affair. 



2) Development of the Skeleton of the Free Appendage. The paired ap- 

 pendages arise either as a dorso-ventrally flattened fold of the epidermal 

 portion of the skin, or as a cylindrical outgrowth of the epidermis. (See 

 Chap. 10.) Within the epidermal protrusion, is a mass of mesenchyme (figs. 

 262D, E; 324A). As development proceeds, condensations of mesenchyme, 

 centrally placed, begin to foreshadow the outlines of the future skeletal struc- 

 tures of the limb (fig. 324A, C, D). This mesenchyme gradually becomes 

 more compact to form a pre-cartilage stage, to be followed by a cartilaginous 

 condition. 



The pattern, which these cartilages of the limb assume, varies greatly 

 in the two types of limbs mentioned above. In the ichthyopterygium (fig. 

 323B, G), they assume a radially arranged pattern, extending out from the 

 point of attachment to the girdle, whereas, in the cheiropterygium (fig. 323A), 

 they assume the appearance characteristic of the typical limb of the Tetrapoda. 



In the tetrapod limb, such as that of the hog, chick, or human, elongated, 

 cylindrically shaped bones begin to make their appearance in mesenchyme 

 (fig. 324A-E). Following the cartilaginous condition, a center of ossification 

 arises in the shaft or diaphysis of each developing bone, transforming the 

 cartilage into bone (figs. 314E, F; 324E). Cancellous or spongy bone is 

 formed centrally within the shaft, while compact bone is deposited around 

 the periphery of the shaft (fig. 314E, F). Later, the cancellous bone of the 

 shaft is resorbed, and a compact bony cylinder, containing a relatively large 

 marrow cavity, is formed. Separate centers of ossification, the epiphyses, 

 arise in the distal ends of the bones (fig. 3241). Each epiphysis is separated 

 from the bone of the shaft by means of a cartilaginous disc, the epiphyseal 

 cartilage (fig. 3241). At maturity, however, the bony epiphysis at each end 

 of the bone becomes firmly united with the shaft or diaphysis by the appear- 

 ance of an ossification center in the epiphyseal cartilage (fig. 324J). Inter- 

 nally, the ends of the long bones tend to remain in the cancellous or spongy 

 condition, whereas the shaft is composed of compact bone with an enlarged 

 central marrow cavity (fig. 324J). For later changes of the bony substance 

 involved in the growth of bone, see growth of bone, p. 693. 



3) Formation of the Girdles. The typical tetrapod pectoral girdle (fig. 

 323 A) is composed of a sternal midpiece, three lateral columns, extending 



