DEVELOPMENT OF THE ENDOSKELETON 



687 



Ribs develop in relation to the basidorsal and basiventral elements and 

 extend outward in the myosepta. The dorsal rib appears typically in the 

 position between the epaxial and hypaxial divisions of the primitive skeletal 

 musculature, whereas the pleural rib lies in close relationship to the coelomic 

 cavity (fig. 311B). It is questionable whether or not the hemal arch, when 

 present, is homologous with the ventral or pleural ribs. The shark, Squalus 

 acanthias, has dorsal ribs. This condition is true also of all Tetrapoda. In 

 Amia, the ribs are of the pleural variety, whereas, in most teleosts, pleural 

 ribs are present, supplemented by dorsal or epipleural ribs. 



PRESTERNUM + ANTERIOR STERNUM 

 MANUBR lUM 



COSTAL CONDENSATIONS 



CONDENSATION 

 OF PRESTERNUM 



XIPHISTERNUM' 



Fig. 322. Development of the sternum in the mammal. (A and C redrawn from 

 Hanson. '19, Anat. Rec, 17; B redrawn from Kingsley, '25.) (A) Diagrammatic recon- 

 struction of sternum of '24-mm. pig embryo. The two precartilaginous condensations 

 of the mesosternum are united anteriorly with the presternal condensation. The rib or 

 costal condensations are approaching and uniting with the sternal condensations. (B) 

 Schematic representation of sternal rudiments in the mammal. The mesosternal cartilages 

 have segmented into cartilaginous segments or sternebrae. Bilateral centers of ossification 

 arise in each sternebra which later form the bony sternebra. (C) Sternum of old boar, 

 weight 450 lbs. It is to be observed that the sternebrae have remained distinct, and in 

 two of the sternal segments anterior to the xiphisternum the two centers of ossification 

 produce a dual condition within the sternal segment. In the human and certain other 

 mammals the sternebrae fuse to form the gladiolus or corpus sterni. 



