THE DEVELOPMENT OF THE MARSUPIAL SHOULDER GIRDLE. 755 



like lower part is of an elongated oval shape, and is undergoing a peculiar sort of 

 degeneration. The axis of this elongated oval portion points in an upward and back- 

 ward direction, and degeneration of the cartilaginous cells can be detected throughout 

 its whole extent except along its outer and/ upper side. The hyaline substance becomes 

 largely absorbed, giving rise to a sort of cellular cartilage, and around the surface of 

 this degenerated cartilage a further degeneration is seen to be taking place in that what 

 remains of the cartilaginous matrix is becoming completely absorbed, the cells appar- 

 ently going to form connective tissue. In the ventral view of the girdle (fig. 11) a 

 dotted line indicates the limits of the cartilages in which degeneration is not taking 

 place. 



The clavicle is well ossified, and, as seen in figs. 4 and 11, the inner part is con- 

 siderably stouter than the outer. The inner third has a well-marked concavity on its 

 anterior side, in which lies the omo-sternum, the cartilage of which, however, is separated 

 from the bone by a thick layer of connective tissue of a peculiar structure. Surround- 

 ing the osseous tissue of the clavicle throughout its whole length, but most developed 

 at the ends, is a fairly thick covering of large connective tissue cells. These cells have 



1 no doubt a slight superficial resemblance to cartilage, but they are certainly not cartilage 

 cells. They are irregularly oval and taper at their apices, and are only separated from 

 each other by a thin layer of connective tissue substance. They are apparently imper- 

 fectly differentiated osteoblasts. Cells scarcely differing in appearance are found 

 surrounding the bony plates of the developing dentary, maxillary, or pre-maxillary 



i elements, but in these bones the surrounding layer of cells is much thinner relatively 

 than in case of the clavicle. In the case of the tympanic bone, however, in many forms 

 the surrounding layer of cells is proportionately very thick, and a condition is presented 

 almost exactly like that found in connection with the marsupial clavicle. 



Later Development of the Shoulder Girdle in Trichosurus. 



The degeneration of the inner end of the coracoid continues, and the coracoid becomes 



detached from the sternum when the embryo attains to about 20 or 21 mm. — greatest 



ength, apparently. In an embryo of 23 mm. the inner end of the coracoid is quite free, 



md is about one-sixth of a millimetre distant from its former point of attachment. To 



he lateral portion of the pre-sternum there is still attached a small portion of degenerat- 



ng cartilage — apparently the inner part of the bulbous portion of the coracoid, whose 



legeneration permits the detachment of the shoulder girdle from the sternum. Though 



his coracoidal fragment apparently completely disappears, its presence is interesting, 



i view of the occurrence in a number of the higher mammals of similarly situated 



lements, and it confirms the correctness of the view that these fragments represent 



le rudimentary sternal end of the coracoid. The coracoid process is pointed, and 



xtends for about "3 of a millimetre behind the point of attachment of the coracoid 



sad of the biceps. This posterior portion is degenerating, and it would seem as if the 



