v I'WTOIIAL LIMB SKELETON 351 



the course of further development these, doubtless to give greater 

 mobility to the tin, disappear. In the details of its development 

 each ray repeats that of the main axis. 



In addition to the normal rays, which are attached to the axis at 

 the level of the intersegmental spaces, occasional rays make their 

 appearance opposite the segments themselves. According to Semon 

 these sprout out from the thin superficial layer of the axial tissue 

 which like that between the segments persists for long in an 

 unchondrih'ed condition. These extra rays are most frequent on the 

 postaxial side of the limb which in the pectoral limb becomes ventral, 

 in the case of the pelvic limb dorsal (see Chap. VII.). 



Growth of the fin and of its enclosed skeleton continues for a 

 long period even after the adult condition is attained. As regards 

 the skeleton this continued process of growth takes place by two 

 methods (1) by a simple continuation of the extension at the apex, 

 and (2) by the already formed elements of the cartilaginous skeleton, 

 axial or radial, continuing their individual growth in size. 



As the definitive condition of the skeleton is reached, the inter- 

 segmental tissue chondrifies, towards the apex forming soft hyaline 

 cartilage with a sparse matrix and towards the. body taking the form 

 of fibro-cartilage. Towards the base of the limb this fibro-cartilage 

 develops many fluid-filled cavities so as to assume an almost spongy 

 character and in this way give greater mobility. This is specially 

 marked at the junction with the shoulder girdle and between the 

 first and second segments of the limb-axis and in these two cases the 

 apposed surfaces of hyaline cartilage are curved and concentric so 

 as to afford a distinct development in the direction of a true ball 

 and socket joint. 



The details of development of the pelvic limb skeleton apparently 

 agree with those of the pectoral. In this case also the girdle arises 

 in the form of two originally separate halves. 



ELASMOBRANCHII. The earliest stage in the development of the 

 pectoral limb and its girdle in Elasmobranchs is that described by 

 Huge and by Braus (1904) in Spinax where there exists a condensa- 

 tion of connective tissue in the form of a curved rod on each side 

 of the body close under the skin, in the position shown in Fig. 159, 

 p.g (p. 321). This forms the rudiment of the pectoral girdle. 

 From it there grows outwards a short projection into the limb-rudi- 

 ment which as it is clearly homologous with the axial cartilage of 

 Ceratodus we may call by the same name. The girdle rudiment 

 increases in length both dorsally and ventrally, and ventrally the 

 two rudiments come to be in apposition. On each side a tract of 

 cartilage* now develops in the prochondral rudiment : the t wo 

 cartilaginous rudiments show a similar dorsal and ventral extension 

 and presently they also come into apposition ventrally and form a 

 continuous structure across the mid-ventral line. 



In other Elasmobranchs (see Mollier, 1894) the conditions appear 

 to be similar on the whole to those described in Spinax. 



