v SKELETON OF THE LIMBS 355 



the girdle come to overlap one another in the rnid-ventral line and 

 in the ease of the higher Anura complete fusion takes place. 



Amongst the Eeptilia the first rudiment of the pectoral limb 

 skeleton has been investigated by Mollier (1895) and found to 

 consist of a condensation of mesenchyme in the glenoid region 

 corresponding partly to the glenoid portion of the girdle and partly 

 to the basal portion of the limb skeleton — the two being thus again 

 continuous at first. The girdle portion of the rudiment spreads 

 ventrally to form the coracoid region, then dorsally to form the 

 scapular. The chondrification of the various parts takes place in 

 the order of their appearance. 



In Chelonians the girdle takes on the typical A-shaped form 

 with a more or less pronounced projection from the lower end of 

 the coracoid forwards towards the lower end of the precoracoid which 

 apparently represents the epicoracoid of Amphibians. In Sphenodon 

 and in Lizards on the other hand the ventral portion of the 

 cartilaginous girdle consists of a flattened plate which may become 

 perforated by several foramina. Whether this flattened ventral 

 portion corresponds to coracoid and precoracoid is doubtful. It 

 seems on the whole more probable (Goette) that the precoracoid has 

 disappeared in these forms owing to its functional replacement by 

 the clavicle, a process seen in its incipient form in Anura. This 

 view is supported by the occurrence of a distinct strand of 

 condensed connective tissue in the position where the precoracoid 

 should be though in this case it does not become chondrified but 

 becomes replaced by bone (clavicle) at a later stage. 



In Birds the girdle forms a simple curved rod without any 

 bifurcation into coracoid and precoracoid portions ventrally. 



Each lateral half of the pelvic girdle of quadrupeds is, like the 

 pectoral girdle, typically of a A-shape, the three limbs being known 

 here as ilium (dorsal, more correctly iliac bone or iliac cartilage), 

 pubis (anterior) and ischium (posterior). The frequency with which 

 the pubis and ischium are continuous at their ventral ends suggests 

 that here also they represent the persisting thickened marginal 

 parts of a once flattened plate-like ventral portion of the girdle. 



As in the case of the pectoral girdle the three processes are 

 formed by simple spreading outwards from the original rudiment. 

 In Amphibia chondrification takes place apparently from a single 

 centre on each side (Triton, Bunge, 1880) giving rise to a pair of 

 longitudinal plates of cartilage which meet ventrally. 



In Beptiles each half of the pelvic girdle passes through the 

 typical A-shape. The ventral end of the pubis, like that of the 

 ischium, meets its fellow across the mid-ventral plane forming a 

 symphysis. In some cases, e.g. Sphenodon and certain Chelonia, the 

 pubic symphysis becomes connected up with that of the ischia by a 

 longitudinal bar of cartilage. In the Crocodiles the pubic portion of 

 the girdle becomes eventually segmented off at its dorsal end from the 

 rest of the girdle. 



