351 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



giuiii " and " metapterygium " (Fig. 169, F). The main portion of the 

 plate is becoming split up towards its margin by a series of slits into 

 a number of radiating pieces which represent the separate radii of 

 the fully developed fin (Fig. 169, G). The close correspondence 

 between the fin skeleton at this stage of its development and the fin 

 skeleton of a shark is obvious from Fig. 169. 



In Actinopterygians the pectoral limb skeleton is in the pro- 

 chondral stage a continuous mass, of which the anterior and mesial 

 part separates off to form the pectoral girdle, while the distal portion 

 spreads outwards to form the rays. 



The pelvic limb skeleton shows a fundamentally similar origin 

 from a continuous prochondral rudiment, but here the skeletal rudi- 

 ment appears first in the projecting limb and only secondarily spreads 

 within the body wall into the region of the pelvic girdle. It appears 

 to the present writer that no special weight need be attached to 

 such cases where the skeleton develops earlier in ,the limb than in 

 the body wall : they are probably to be regarded simply as special 

 cases of the frequent tendency for highly specialized organs to be 

 laid down precociously in development. 



II. LIMBS OF THE TETRAPODA. In the Amphibia also the pectoral 

 girdle and the skeleton of the limb itself are foreshadowed by a single 

 condensation of rnesenchyine which extends in a dorsal and ventral 

 direction to form the girdle and out into the limb to form its skeleton. 

 No general rule can be given as to the relative time of development 

 of the various parts. In Bombinator according to Goette the girdle 

 rudiment appears first and the limb skeleton sprouts from it : in 

 Proteus according to Wiedersheim the limb skeleton appears first 

 and the girdle later. In the girdle rudiment the dorsal or 

 scapular portion becomes apparent first. Chondrification takes 

 place separately in the girdle and the limb, the joint remaining 

 unchondrified. 



The cartilaginous pectoral girdle of the Amphibian, as of other 

 quadrupeds, takes on the form of a A upon each side of the body 

 the three branches of the A being known as scapular, coracoid and 

 precoracoid portions respectively and the glenoid articulation for 

 the limb being situated at the meeting point of the three portions. 

 As the two ventral 1 tranches of , the A are in some cases continuous 

 with one another at their tips through a strong membrane, il sinus 

 not improbable that they had originally the form of a continuous 

 flattened plate of cartilage, of which the central portion has now 

 di>,ippeared, IcaMii'j the thickened marginal parts as precoracoid 

 (anterior) and coracoid (posterior) respectively. On this view the 

 epicoracoid when present would represent tin- persisting thickened 

 ventral margin of the primitive girdle. 



In actual ontogeny 1 lie 1 hrec I. ranches spread gradually out \\ards 



from I he original rudiment, while the epicoraeoid when present, is 



formed by the coracoid spreading forwards at itn \entral end and 



;lh the end of I he ] H'GCOraCOld. The two lateral halves of 



