750 DR R. BROOM ON 



The Shoulder Girdle in an 8*5 mm. Intra-uterine Embryo of Trichosurus 



vulpecula (fig. 1). 



In this early embryo, which I have elsewhere (Proc. Linn. Soc, N.S.W., 1898) 

 figured and described in some detail, the skeleton is as yet but very imperfectly 

 chondrified. The anterior limbs, though fairly well formed, and having the digits well 

 marked, are still widely apart, owing to the skeletal elements of the chest-wall not 

 having yet met in front of the heart. In the hind limbs there are as yet no indications 

 of digits. 



The shoulder girdle is in a very interesting stage of development, but certain parts 

 of its structure, from their imperfect state of development, can only be clearly under- 

 stood when compared with those in a more advanced stage. 



The scapula is fairly well chondrified and moderately thick. Its long axis points 

 upwards and somewhat forwards, and, as will be seen from the position of the first rib in 

 the figure (fig. 1), the upper border of the scapula lies wholly in the cervical region. 

 The upper half of the scapula is moderately flat, and there is no trace of a spine. From 

 the anterior border of the scapula, near the point of union of the middle with the lower 

 third, there passes outwards and slightly forwards and downwards a moderately thick, 

 rounded, cartilaginous acromion process. After passing outwards for a distance about 

 equal to the width of the scapula at its middle, it curves downwards, slightly back- 

 wards, and inwards, for a distance about equal to the length of the outward passing 

 portion. This downward passing portion, unlike the other, is entirely mesenchymatous. 

 At its lower extremity it meets on its inner side the outer end of the developing clavicle. 



The glenoid cavity is comparatively shallow, and looks outwards and slightly back- 

 wards. Though the cartilage forming the lower part of the cavity is quite continuous 

 with the scapula, there can be very little doubt but that it belongs, if not entirely, at 

 least mainly, to the coracoid. In the figure a dotted line indicates what appears to be 

 the limit of the two elements. As the shoulder girdle is not chondrified much beyond 

 the region of the glenoid cavity, the further tracing of the elements in their mesenchy- 

 matous condition would be a matter of some little difficulty, were we in ignorance of 

 the latter stages of development. As it is, most of the structures can be followed even 

 in their pre-cartilaginous condition with moderate certainty. The cartilaginous portion 

 of the coracoid ends in a rounded process which points almost directly backwards ; but 

 though the cartilage here ends, the element is manifestly continued as a very well 

 marked mesenchymatous structure, which, on passing backwards, spreads out like a fan, 

 and becomes continued into the mesenchymatous anterior portion of the first rib, and 

 also into the less clearly differentiated sternum. When viewed in the light of the more 

 developed condition as found at birth, there seems little doubt but that not only is the 

 portion of cartilage forming the lower half of the glenoid cavity coracoid, but also the 

 whole of its mesenchymatous fan-like continuation, with the exception of so much as 



