358 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



that the cleithrum sinks more deeply into the tissues while the 

 clavicle remains superficial. 



In the higher bony fishes — Ganoids and Teleosts — the conditions 

 are very similar to those of Polypterus — the primitive shoulder girdle 

 being small and usually becoming replaced in great part by bone 

 (scapula and coracoid) and the main supporting function being 

 exercised by the independently developed cleithrum. 



In the Dipnoi more nearly primitive conditions are retained as 

 the original cartilaginous girdle remains well developed throughout 

 life and retains its continuity with its fellow ventrally. Cleithrum 

 and clavicle however are also developed and they show a higher 

 condition in that they are developed in intimate contact with the 

 surface of the cartilaginous girdle, the clavicle ensheathing the 

 anterior face of the coracoid portion. 



In Amphibians the scapula becomes replaced incompletely or 

 completely by bone which spreads dorsalwards from the region of 

 the glenoid articulation. The coracoid may remain cartilaginous 

 (most Urodeles) or become replaced by bone. The precoracoid also 

 tends to be strengthened by the formation of bone. In the common 

 frog (Bana) and Toad (Bufo) the bone (" clavicle ") is in the form of 

 a splint lying along the anterior side of the precoracoid and originat- 

 ing in the connective tissue some little distance from the cartilage. 

 In other cases the bony tissue completely surrounds and to a great 

 extent invades and replaces the cartilage. We may infer with con- 

 siderable probability that the bone in question was originally in 

 phylogeny a " membrane " bone and that becoming more and more 

 intimately related to the precoracoid cartilage it has in the latter form 

 become more or less completely a " cartilage " bone — a good example 

 of the type of evidence which has led morphologists to minimize 

 the importance of the distinction between these two types of bone. 



In the Amniota scapula and coracoid are replaced nearly or 

 quite completely by bone. A clavicle like that of Amphibians 

 develops in relation to the precoracoid in Reptiles except Crocodiles. 

 In Birds what appears to be the same element (furcula) is widely 

 separated from the coracoid, probably for mechanical reasons con- 

 nected with flight, while a separate centre of ossification appears at 

 the apposed ventral ends of the two bones. In Reptiles a somewhat 

 similar element — the episternum — makes its appearance and is con- 

 tinued tailwards along the mid-ventral surface of the sternum and it 

 is possible that in Birds the ossification lying between the ventral 

 ends of the clavicles represents the anterior segmented off portion of 

 this and the keel of the sternum the rest. 



Pelvic Girdle. — The cartilaginous pelvic girdle becomes replaced 

 by bone less or more completely without receiving any reinforcement 

 from investing bones. The iliac, pubic and ischial portions ossify 

 each from its own centre except in Amphibia where the pubic 

 region remains cartilaginous. 



In bony Teleostomatous fishes each half of the pelvic girdle 



