KIBS AND STERN I'M 



253 



same is true, according to some authorities (0. Schultze), for the cartilaginous 

 stage. The processes remain attached to and become parts of the vertebrae in 

 the cervical, lumbar, and sacral regions, but in the thoracic region they grow 

 round the body-wall to form the free ribs. The costo-central joints are 

 produced by absorption in the matrix between ribs and vertebrae, the surrounding 

 mesenchyme giving origin to the costo-vertebral ligaments. The rib-process 

 and growing transverse process are at first united by a continuous blastema. 

 This is absorbed as anastomoses are established between the segmental 

 arteries (Bardeen), but between the end of the process and the rib a joint-cavity 

 is formed and the surrounding mesenchyme gives rise to costo- transverse 

 ligaments (see fig. 268, p. 214). 



The ventral ends of the ribs become continuous with two longitudinal cellular 

 bands (sternal bands), which are laid down from before backwards. The cellular 

 strands are derived according to some from the rib -ends, but according to others 

 they are independent formations ; opposite the first seven pairs of ribs they fuse by 

 differentiation of the intervening tissue to form the sternum (fig. 306). This 

 remains cellular for some time after the ribs are converted into cartilage. The 



FIG. 307. MEDIAN SAGITTAL, SECTION OP THE HEAD IN EABLY EMBRYOS OF THE RABBIT. 

 Magnified. (From Mihalkovics.) 



A, from an embryo 5 mm. long. B, From an embryo 6 mm. long. 



In A, the faucial opening is still closed ; in B, the septum is perforated at /; c, anterior cerebral 

 vesicle; me, mesencephalon ; mo, medulla oblongata ; m, medullary epiblast; if (in B), infundibulum ; 

 sp.e, spheno-ethmoidal ; be, sphenoidal ; and sp.o, spheiio-occipital parts of the basis cranii ; i, fore- gut ; 

 cJi, notochord ; py, buccal pituitary involution ; am, amnion ; 7i, heart. 



process of chondrification begins in the upper lateral angles of the presternum and 

 in the mesosternum between the ribs (Paterson) ; it proceeds from the margin 

 inwards, repeating the process by which the chondroblast sternum is formed. 

 The nietasternum is developed separately, but also from independent lateral 

 rudiments in all probability. 



The usual view adopted regarding the development of the sternum is that of Ruge, who 

 derives the sternal bands from the ventral ends of the ribs, which unite with one another, as it 

 were, from before backwards to form the bands. According to Paterson, they are at first quite 

 separate and independent of the ribs, and are only united with them secondarily. He describes 

 an early stage in which the primitive sternum is separate from the ribs, but connected with the 

 rudiments of the shoulder-girdles, and believes, therefore, that ontogenetically the sternum is 

 an independent structure connected with the limb-girdles. It is not yet clear whether, or 

 in what degree, the limb-girdles contribute to the formation of the manubrium, and the relations 

 of the chondroblastic areas to the cartilaginous skeleton are not yet sufficiently elucidated 

 to warrant a more definite statement than that contained in the text. 



