ONTOGENY AND PHYLOGENY OF THE STERNUM 63 



So far as I am aware, this will constitute the only explanation 

 in the literature of the rise of the sternebrae, other than the 

 statement that they represent the original costal contributions, 

 which is, as we have shown, absolutely untenable. 



4. Stages in the ontogeny of the mammalian sternum 



1. Appearance of two laterally situated sternal bands, inde- 

 pendent of ribs. 



2. Appearance of a single median anterior rudiment, inti- 

 mately associated with the shoulder-girdle. 



3. Gradual approximation and union of sternal bands with the 

 anterior sternal rudiment on the one hand, and with the ventrally 

 growing tips of the ribs upon the other. 



4. Gradual approach and fusion of sternal bands in midline of 

 body to form a sternum. 



5. Division of sternum into a number of sternebrae. Lines 

 of division (sutures) always appearing opposite the ends of each 

 pair of ribs. 



6. Ossification of the intercostal sternebrae by the appearance 

 of one or more centers for each segment. 



7. Fusion of the sternebrae in Primates into three parts: 

 manubrium, gladiolus, and xiphisternum. 



5. Conclusions 



1. That the sternal bands arise and remain as two unseg- 

 mented structures until the relatively late process of ossification 



begins. 



2. That the sternebrae are invariably intercostal; arise by 

 reason of functional demands for greater freedom of movement, 

 and play no part in the origin of the sternum. 



3. That in the mouse and rat embryos a mesenchymatous 

 horseshoe-shaped girdle extends across the ventral midline; 

 from this material are derived presternum, coracoids, and 

 scapulae; this girdle being the homologue of the adult cartilag- 

 inous pectoral girdle of Hexanchus. 



THE AMERICAN JOURNAL OF ANATOMJf, VOL. 26, NO 1 



