142 



HISTORY OF THE HUMAN BODY 



Among the Mammalia the lowest Order, the Monotremata, 

 possess in this region a large T-shaped bone, the stem of 

 which, very broad and flat, articulates with the true sternum, 

 forming its anterior extension, while the lateral arms are ap- 

 plied along the sides of the clavicles. This piece has been 

 called by some an episternum and by others- an interclavicle, 

 but its precise homologies are not definitely determined. In 

 all other mammals the clavicles apparently articulate directly 

 with the most anterior of the sternal pieces, the manubrium; 



FIG. 37. Sternum and shoulder-girdle of mammals. [After W. K. 

 PARKER.] 



(a) Ornithorhynchus. (b) Human embryo. 



c, coracoid; d, epicoracoid; e, episternum; f, clavicle; g, scapula; h, suprascapula; 

 m, manubrium; stb, sternebrae; x, xiphisternum. 



but in the embryo there are found definite disc-shaped 

 skeletal elements, interposed between the two, which develop 

 later into thin, interarticular discs. These, usually designated 

 omosternum, have been likened to the lateral arms of the 

 T-shaped bone of the Monotremata. 

 X"The ontogenetic history of the skull, a complex of skeletal 



/ elements developed at the anterior end of the notochord, is 

 singularly constant in all classes, and we may thus feel con- 



\ fident that we have in this a repetition of stages once passed 



