ONTOGENY AND PHYLOGENY OF THE STERNUM 61 



the latest defender of the Gegenbaurian hypothesis, and, going 

 a step further even than Gegenbaur, declares that in the case of 

 the frog's girdle there is a " preparatory action on the part of the 

 coracoid cartilage directed toward the reception and assimila- 

 tion of the corresponding dermal accession of the clavicle." 



In later stages of the mouse and human embryos, after the 

 mesenchymatous girdle has become broken up into its component 

 parts, the coracoid process is relatively much larger than in the 

 adult and has a medial and ventral extension. I have observed 

 this repeatedly in embryos of pigs, mice, cats, and man. In one 

 mouse embryo, 7.75 mm., there seemed to be a distinct thicken- 

 ing of the mesenchyme between the very large coracoid process 

 and the yet partly mesenchymatous clavicle. This was a strik- 

 ing spectacle in the pig, because of the well-known fact that in 

 the adult no coracoid process is present, but only the subcoracoid, 

 glenoid-sharing portion. 



3. Sternebrae 



The segmentation of the sternum into sternebrae occurs late 

 both in phylogeny and ontogeny. A glance through the figures 

 in the literature assures one that sternebrae are unknown in 

 four of the five classes of vertebrates, being found only in the 

 mammals. Hence they play little or no part in the origin or 

 development of the sternum phyletically. 



Likewise, they are a secondary and- acquired character in 

 ontogeny. It is contrary to all expectation, if sternal bands 

 are derived from ribs, to find that the sternebrae are invariably 

 intercostal, and not at the point opposite the ends of the ribs. 

 Figures 7 to 10 show what is the true condition in all fetal mam- 

 mals as regards the formation and ossification of its sternebral 

 elements. The center of ossification always occurs at a point 

 midway between two ribs, while the line of transverse division 

 crosses the sternum exactly in the center of the area of union 

 of ribs and sternum. According to Ruge's theory, this should 

 be just the reverse. The sutures between the sternebrae should 

 be intercostal, the sternebrae themselves opposite the costal 

 cartilages. 



