12 THE HUMAN STERNUM 



sterno-clavicular joints, and the inner ends of the clavicles. The cellular 

 mesosternum (Figs. 8, 9) consists of two definitely separate strands of 

 mesoblastic cells, which are separated in the middle line by a layer of sparser 

 and more loosely arranged cells. The cellular sternum is now joined by 

 the first six costal cartilages (the seventh is still separate on each side), but 

 there is an obvious distinction in the character of the cells composing 

 sternum and costal cartilages (Pis. I, II, Figs. 11, 12). The eighth costal 

 cartilages are not in any way related to the sternum, and there is no indica- 

 tion of a metasternum. 



In rat embryos, 13 mm. in length, the process of development has 

 perceptibly advanced. Though the sternum is still cellular, and not yet 

 converted into cartilage, the median separation between the two halves of 

 the mesosternum has disappeared, and the mesosternum consists of a 

 single median band of cellular tissue (PI. II, Fig. 14). The metasternum 

 (Fig. 15) has appeared ; cellular in character and tapering off in the angle 

 between the chest wall and the diaphragm. The clavicle in its outer part 

 is undergoing ossification, and its inner end is cartilaginous. It is clearly 

 demarcated from the presternum, although no joint cavity has yet appeared 

 in the sterno-clavicular articulation (Fig. 13). All the seven costal cartilages 

 are now connected with the sternum ; but there is still an obvious distinc- 

 tion between the character of the tissues composing the sternum on the one 

 hand and the costal cartilages on the other (Fig. 14). 



(c) The third stage in the development of the sternum is associated 

 with the conversion of its cellular tissue into cartilage — at first cellular, 

 afterwards hyaline. The process begins in the presternum in the upper 

 lateral angles, along with the formation of the clavicle (PI. I, Fig. 11). 

 In the mesosternum, cartilage cells first appear at the periphery of the 

 band, in the intervals between the attachment of the costal cartilages 

 (PI. II, Fig. 12). The parts of the sternum opposite the costal attach- 

 ments remain longest in a cellular condition. In the metasternum, 

 cartilage formation begins at the anterior end (PI. II, Fig. 1 5). 



Ultimately the sternum is laid down — a model of the future bone — 

 as a simple median band of hyaline cartilage, separated at this stage from 

 the clavicles by cellular connective tissue, but in complete fusion with the 



