THE SKELETON 69 



nerves. Of the cartilaginous arch thus formed, the middle portion becomes 

 in the fore-limb the pectoral, and in the hind the pelvic girdle, and both of 

 these must therefore be regarded as products of the skeletogenous blastema 

 of the free limbs. The segmentation into a central girdle and lateral 

 limb supports is effected by a process of resorption (cf. fFig. 49, C), the 

 points at which this is effected becoming the shoulder and hip-joints. 



FIG. 49. A, B, C, DIAGRAMMATIC REPRESENTATION OF THREE SUCCESSIVE STAGES 



IN THE DEVELOPMENT OF THE PELVIC FINS OF A SHARK. 



rd., primitive skeletogenous rays ; in A these are already commencing to grow together to 

 form a basal plate (bs.) ; in B this fusion has taken place on both sides, and at * 

 the proximal ends of the basal plates are approximating to form the limb girdle ; 

 in C the process is completed, and at t the free limb skeleton is being constricted 

 off. The formation of secondary rays at the periphery is delineated to the left of 

 C ; figure fo., foramen obturatorium ; c/., cloaca! aperture. 



It would appear from the foregoing that not only the girdles, but also 

 the basal limb supports which articulate with them (the later femur or 

 humerus), were primarily the products of fusion of parallel .rays. Inasmuch 

 as this consideration, as will appear later, is of profound importance in 

 dealing with the morphological significance of the limbs, this brief digression 

 into Embryology has been unavoidable. Fig. 50 farther illustrates the same 

 subject, showing the probable manner in which the number of skeletal rays 

 which unite to form the limbs of terrestrial Vertebrates is reduced. 



