735 



ON THE OSSIFICATION OF THE VERTEBRA IN THE 

 WOMBAT AND OTHER MARSUPIALS. 



By R. Broom, M.D., B.Sc. 



(Plate xlix.) 



During the course of development there may be recognised 

 many ancestral characters which are either quite lost or greatly 

 obscured in the adult forms. In the adult of the human species 

 only the dorsal vertebra? have distinct ribs, yet during the early 

 stages of development there are clear indications of elements 

 homologous with ribs in the first three sacral vertebrae and in at 

 least the seventh cervical. As in the ancestral reptilian condition 

 well developed pleurapophysial elements were attached to practi- 

 call}' all the vertebrae, it is but natural that further indications 

 should be met with in the development of the vertebrae of the 

 lower mammals. 



On looking into the mode of ossification of the vertebra? in 

 the marsupials, I have come across one or two interesting points 

 in which the condition differs from that ordinarily found in the 

 higher mammals. 



Cervical Vertebrce. — The atlas vertebra in the marsupials, as is 

 well known, differs from that in the higher mammals in the lower 

 piece being in some forms very small and in many others quite 

 absent. In the monotremes the inferior element is well developed 

 as in the higher forms, and in the more primitive marsupials 

 such as the Dasyures, a well developed osseous element unites 

 the two arches below. Though this is the condition in the 

 majority of the Polyprotodonts, the intermediate piece is very 

 small in certain species of Perameles, and even in Thylacinus is 

 quite narrow. In the smaller Diprotodonts {e.g. Petaurus 

 breviceps) a small intermediate piece is present which almost 

 exactly resembles that in certain species of Perameles. In 



