52 



VERTEBRATE SKELETON 



statement is that the Amniote sternum arises from the fusion of 

 cartilages derived from the ventral ends of the ribs. In Amphibia 

 the ribs never approach the sternum. 



To understand the questions involved a slight review of the development in 

 mammals where it is best known, is necessary. In this group there is a condensa- 

 tion of mesenchyme in the mid-ventral line, between the two halves of the 

 secondary pectoral girdle which have replaced the coracoidal structures of the 

 lower groups (p. 259). This condensation becomes precartilage and eventually 

 chondrifies. At about the same time a pair of precartilage bars extend back on 

 either side,, at first entirely independent of the ribs (fig. 55, .4). Later these 

 unite with the ribs (B) and with the anterior procartilage, and still later with 

 each other, the union progressing backwards. The posterior part of each bar 

 remains unconnected with the ribs. The median anterior procartilage is the 



A 



Fig. 55. — Scheme of development of mammalian sternum. A, early stage; B, 

 cartilage, the halves beginning to unite; C, beginning ossification, c. Pcoracoid pre- 

 cartilage; cl. clavicle; co, centres of ossification; m, mesosternal parts; mfi, manubrium; 

 p, presternum; r, ribs; st, sternebras; x, xiphisternum. 



presternum (it may contain some coracoid elements) ; the part with the ribs is the 

 mesostemum, while the posterior rib-free portion is the xiphisternum (metaster- 

 num, ensiform process). Centres of ossification in the continuous cartilage are 

 irregular in number and arrangement, but what evidence there is points to a 

 typical arrangement of a single centre in the presternum and a series of pairs 

 of centres in the mesosternum, the pairs alternating with the attachment of the 

 ribs (fig. 55, O- In the adult of many mammals the most anterior mesosternal 

 centres fuse with the presternal, giving rise to a single bone, the manubrium. 

 The other centres unite in pairs to a series of bones, the stemebrae, which alter- 

 nate with the ribs. In the xiphisternum the ossification is more or less incom- 

 plete. In many mammals the sternebra? remain separate through life, in others 

 they may fuse to a single mesosternal bone, the corpus stemi (gladiolus). 



