754 DR E. BROOM ON 



part of the coracoid lies in a downward, backward, and inward direction from the upper 

 part. It is somewhat round and bulbous. At its lower and inner side its cartilage is 

 continued into that of the sternum, while further back it rests on the first rib. On the 

 outer side of the posterior part of the coracoid near its upper part is the point of attach- 

 ment of the coracoid head of the biceps. The intermediate portion of the coracoid is 

 moderately flat, its outer surface looking downwards and outwards. 



The clavicle is now fairly well developed. The outer part is somewhat slender, while 

 the inner two-thirds is moderately stout. Its posterior part is pointed, and passes into 

 the angle formed by the coracoid and the sternum. This part of the clavicle is hollowed 

 out on its anterior and inner side to receive the well-developed omo-sternum. 



The omo-sternum is a small, irregularly oval cartilage which lies between the sternum 

 and the clavicle, and in a deep depression in the latter. It is quite distinctly differen- 

 tiated from the sternum proper. 



The Shoulder Girdle in a 17 mm. Mammary Foetus of Trichosurus vulpecula 



(figs. 4, 7, 8, and 11). 



The shoulder girdle at this stage, though it resembles the previous stage in being 

 still attached to the sternum, shows a considerable advance in many of its characters. 



The scapula differs from the earlier stages in that the long axis passes upwards and 

 slightly backwards. The posterior angle is developed to a much greater degree, and 

 extends well upwards and backwards. The acromion is of large size. It arises from 

 the outer side of the anterior scapular border, and passes first forwards and slightly 

 downwards and outwards. It then broadens out and passes downwards and slightly 

 forwards and inwards to meet the outer end of the clavicle. From the upper side of the 

 upper part of the acromion the non-cartilaginous part of the spine extends upwards 

 along the anterior part of the outer side of the scapula to about the beginning of the 

 upper third of the blade. This little element — the supposed homologue of the cleithrum 

 — is now a well-developed bony plate. It must be admitted, however, that at this stage 

 it is not an independent bone, as it is continuous, at its attachment with the scapula, 

 with the delicate osseous film which is now developing on the surface of the scapula ; 

 and it is probable that the non-cartilaginous spine is at no time a distinct bone, as ossifi- 

 cation sets in in connection with the perichondrium apparently about the same time as 

 ossification begins in the structure dividing the supra-spinatus and infra-spinatus muscles. 

 The ossification of the scapula is as yet a pure ectostosis, the cartilage being as yet 

 unossified, though the cells are considerably increased in size. 



The coracoid is of much the same absolute size as in the previous stage, and is thus 

 considerably smaller relatively. Its glenoid portion differs but little from that at birth, 

 but there is considerable difference in the portion which articulates with the sternum. 

 At birth this lower and posterior portion is irregularly pear-shaped. Here the kadb 



