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. +Fig. 49, C), the 
points at which this is effected becoming the shoulder and hip-joints. 
bs. | 
| 
Fic. 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 (6s.) ; 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 + 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/., cloacal 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 further 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. 
