THE STERNUM : ITS EARLY DEVELOPMENT AND OSSIFICATION. 23 



connected with the sternum ; but there is still an obvious 

 distinction between the character of the tissues composing 

 the sternum on the one hand and the costal cartilages on the 

 other (fig. 16). 



(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 pre-sternum in the upper lateral angles, along 

 with the formation of the clavicle (fig. 11). In the meso- 

 sternum, cartilage cells first appear at the periphery of the 

 band, in the intervals between the attachments of the costal 

 cartilages (fig. 16). The parts of the sternum opposite the costal 

 attachments I'emain longest in a cellular condition. In the 

 meta-sternum, cartilage formation begins at the anterior end. 



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 costal cartilages on each side, 

 and presenting absolutely no differentiation of its component 

 parts. 



Three human sterna of the third month, measuring respec- 

 tively 8, 9, and 10 mm., show clearly this condition (figs. 17, 18, 

 19). The demarcation of sternal elements and of costal carti- 

 lages is shown in the special arrangements of the cartilage cells in 

 lines, along which, later on, the joint cavities will be formed. In 

 all three sterna the sterno-clavicular joint is completely formed, 

 with its inter-articular fibro-cartilage. But in only one case is 

 there a definite costo-sternal joint cavity (between the meso- 

 sternum and the right second costal cartilage) (fig. 18).^ 



(d) The further changes in the cartilaginous sternum preced- 

 ing ossification occur in rodents (rat, mouse) without cleavage 

 of the parts of the sternum or separation of costal cartilages. 

 Opposite to the costal attachments the cartilage is distinctly 

 hyaline, poorer in cells, and stained less deeply than in the 

 intervals between the costal cartilages. In these intervals the 

 cells are massed together, stain deeply, and the process of 



' Figures 19 (A-J) show the existence of a sternal foramen. In several 

 sections it presented the appearance of fissura sterni. The perforation contained 

 vessels, one of which was of considerable size. — 



