in.] DEVELOPMENT. 25 



vertebra by synovial joints, and in other regions a small 

 rudiment, early and firmly united to, and incorporated with, 

 the vertebra itself. 



The ribs in the thoracic region, though primarily formed 

 from a rod of cartilage continuous with that of the vertebra, 

 always become distinct, independent, and movably arti- 

 culated bones; after their original segmentation they can 

 never be properly said to constitute part of the vertebra. 

 But it frequently happens that in certain of the vertebras 

 anterior to the thoracic region, and in certain of those 

 posterior to it, there are bony elements formed at an early 

 period, which, though very different from ribs in the ordinary 



FIG. . Third cervical vertebra of a nearly full-grown Echidna (E. aculeata), the 

 different pieces of which it is composed being slightly separated from one another. 

 ma neural arch ; c centrum ; / transverse process ; v vertebrarterial canal ; IKS 

 neuro-central suture. 



sense of the word, occupy a somewhat similar position 

 in relation to the vertebrae to that which the ribs do in the 

 thoracic region. These have hence been considered as 

 modified conditions of the same part, and have been called 

 pleurapophyses by Professor Owen. 



Perhaps the clearest case of the presence of rib elements 

 in the vertebrae in any Mammal is afforded by the cervical 

 vertebrae of the Monotremata, where the greater part of each 

 transverse process ossifies separately from the rest of the ver- 

 tebra, and remains for a long time only suturally connected 

 with it (Fig. 5). They thus closely correspond to the cervical 



