STERNUM 



909 



cartilage on either side. Thereupon the lateral portion of the first 

 2 or 3 Ribs is absorbed, so that the anterior portion of each band 

 loses its connexion with the vertebral column, and is transformed 

 into a ribless process, the future processus lateralis anterior of the 

 Sternum, the dorsal part of these reduced Ribs remaining as cervico- 

 dorsal Ribs (p. 788). A similar reduction or withdrawal of 2 

 or 3 Ribs takes place at the posterior end of each band, trans- 

 forming it into the processus lateralis posterior. In the mean- 

 while both bands have met in the middle line, and fuse together, 

 from the anterior end backward, thus forming the sternal plate or 

 body of the Breastbone. The inner margins of the bands, how- 

 ever, do not unite smoothly, but turn downward, producing two 



P.Lp: 



P. I. a. 



^P.obL 



R.io 



Early and Later Stages of the Development of the Chick's Sternum. 



C.B. sternal bands ; Ut. Metasternum ; V.l.a. P.l.j), and P.oil. Processus lateralis anterior 

 posterior and obliqnus ; E. Rostrum ; R.1-10, Ribs. 



median ridges which are the foundation of the future Keel (cf. 

 Carinat^, p. 76). The sternal plate now develops considerably 

 posteriorly, forming the 3Ietasternum, which, not being directly caused 

 by withdrawing Ribs, is not homologous to the Xiphosternwn of other 

 Vertebrates, whose equivalent is the two posterior lateral processes. 

 This Metasternum grows to a great length in many Birds, so that, 

 as in the Galliiuv, it may form the most conspicuous part of the 

 whole, and the same remark applies to the posterior lateral processes 

 from the lateral margin of which grows out in many Birds a processus 

 ohliquus. The anterior end of the Sternum receives in facets the 

 distal end of the Coi"acoids, between which grows out a median 

 apophysis, the rostrum or spina sternalis, which serves chiefly for the 

 attachment of the ligaments which connect it with the Clavicles, 

 and also close the whole space between them and the Coracoids, 



