v EIBS AND STERNUM 305 



complete. The double-headed rib has all the appearance now of 

 simply articulating with a massive projection of the outer side of 

 the neural arch : its original connexion with the haemal arch would 

 never be suspected. 



The question naturally arises whether in other Amphibians in 

 which the transverse process and rib projects from the neural arch, 

 the dorsalward shifting has come about in the same manner as is 

 apparently the case in Urodeles. The probabilities appear to be 

 against this. In the remaining two groups of Amphibia — the 

 Anura and the Gymnophiona — the transverse process, though it 

 springs from the neural arch, lies still ventral to the vertebral 

 artery, which suggests that there has taken place here a simple 

 shifting dorsalward of the whole of the haemal arch carrying the 

 rib, including its basal portion. — 



In the case of the Amniota, Schone (1902) has carefully investi- 

 gated the development of Eeptiles and has failed to find anything 

 corresponding to the rib-bearer of Urodeles. In all probability 

 here as in Anura there has taken place a simple dorsal movement 

 of the rib and transverse process. 



The Amniote rib appears to arise generally in continuity with 

 the anterior half vertebra (a) i.e. from material derived from the 

 posterior half of the sclerotome. In the case of Sphenodon 

 (Schauinsland) the sacral and usually the caudal ribs, on the other 

 hand, appear to contain material derived from both halves of the 

 vertebra, the ribs being in these regions much broader than they 

 are elsewhere and marked by a longitudinal groove indicating their 

 double origin. In the last vertebrae of the tail these may give 

 rise to two separate transverse processes attached to each side of 

 the vertebra tipped each one by a small rib-rudiment. 



The uncinate processes on the ribs of certain Eeptiles and Birds 

 arise as independent centres of chondrification. They may later 

 on ossify and fuse completely with the rib (most Birds) or they 

 may never show complete fusion (Apteryx, Sphenodon). In Sphenodon 

 they become simply calcified without undergoing true ossification 

 (Schauinsland). 



Sternum. — The sternum of the Amniota arises typically by the 

 fusion together of the ventral ends of a number of the anterior rib- 

 rudiments into a continuous plate on each side. The two lateral 

 plates so formed undergo fusion across the mesial plane to form 

 the definitive unpaired sternum, a plate of cartilage still continuous 

 with the ribs. Eventually the sternum becomes segmented off from 

 the ribs and may become calcified by the deposition of limy particles 

 in the intercellular matrix (Eeptiles) or replaced by bone (Birds). 



In Amphibia also the sternum arises by the fusion together of 

 two longitudinal bands of cartilage but no connexion can be traced 

 between these and the ribs. This peculiarity, as compared with the 

 Amniota, is apparently to be correlated with the comparatively short 

 extension of the ribs in a ventral direction which is characteristic 



VOL. II , x 



