752 DR R. BROOM ON 



The Shoulder Girdle in a 10 mm. Intra-uterine Embryo of Trichosurus 



vulpecula (fig. 2). 



In this embryo, though the development of the shoulder girdle is not much in 

 advance of the condition in the previous embryo, there are some interesting changes. 

 The skeletal elements of the chest-wall very nearly meet in front of the heart, and as 

 the figure of this girdle is, like the previous one, a true lateral view, much of the 

 apparent difference in the ventral part of the girdle is due to foreshortening brought 

 about by this further development. 



The scapula is not very much further developed than in the previous stage. Its 

 upper part is, however, broader, and the borders which were there mesenchymatous are 

 now cartilaginous. The broadening out of the upper part is still proceeding ; the 

 cartilaginous borders being continued on for some distance in a semi-cartilaginous con- 

 dition. The lower part of the scapula is considerably better developed, and the scapular 

 part of the glenoid cavity looks almost directly downwards. The acromion very much 

 resembles that in the earlier stage, and is not yet completely chondrified, though the 

 lower part is much more clearly defined. 



The coracoid is only but slightly better developed than in the previous stage, and 

 but little more than the glenoid portion is chondrified. If we assume that the inner 

 half of the glenoid is formed by the coracoid, and also the knob at its anterior end, part 

 of which gives attachment to the long head of the biceps muscle, the cartilaginous part 

 of the coracoid may then be described as an irregular oblong structure attached to the 

 scapula along the inner side of the glenoid cavity, and forming part of the cavity. At 

 its anterior end it is moderately thick, but at its posterior end, after passing the glenoid 

 region, it rapidly tapers away. Though the cartilage here ends, the coracoid is con- 

 tinued as a well marked mesenchymatous structure to the anterior border of the first 

 rib and to the sternum. In the earlier stage this mesenchymatous portion is moderately 

 flat, and its margins ill-defined, but the structure is here fairly thick and much narrower. 

 As the first rib is fairly well chondrified, the posterior border of the mesenchymatous 

 coracoid is readily defined. The limits of the coracoid, where it meets the sternum, can 

 only be guessed at by comparison with the more perfectly developed later stages. In 

 the figure (fig. 2) a dotted line indicates the probable limits of the coracoid. 



Of the thin, imperfectly developed structure seen in front of the coracoid in the pre- 

 vious stage, and which is believed to represent the precoracoid, only the anterior part 

 which forms the coraco-clavicular ligament can be distinctly made out. 



The clavicle is considerably better ossified than in the previous stage, bony spicules 

 being present to the inner end. The inner end is considerably thicker than the outer, 

 and now fits in between the lower end of the coracoid and the anterior end of the 

 sternum. 



The omo-sternum is not yet distinctly differentiated from the sternum proper. 



