THE DEVELOPMENT OF THE MARSUPIAL SHOULDER GIRDLE. 751 



may be taken to represent the first rib and the sternum. In the figure a dotted line 

 indicates what is probably the limit of the mesenchymatous portion of the coracoid. 



In front of the coracoid, and between it and the developing clavicle, is a very thin 

 and feebly developed continuous sheet of mesenchymatous cells. It is best developed 

 at its anterior end, but it can be traced throughout the whole space between the 

 clavicle and the coracoid, with the exception of the extreme anterior part of the 

 coracoid. Near the anterior border of this sheet, and about its middle, is a small 

 foramen through which passes a minute blood-vessel, and apparently a nerve. The 

 degree of development of this structure is so slight that it is probably well not to place 

 too much weight on its importance, but as it represents almost exactly the area 

 oceupied by the precoracoid (epicoracoid of most authors) in the Monotremes, it seems 

 to me probable that it represents that structure. Its anterior part gives rise to the 

 coraco-clavicular ligament, and if its morphology be here correctly interpreted, it of 

 course follows that this ligament in the higher mammals and man is the homologue, 

 and apparently the sole representative of the precoracoid of the Anomodonts. 



The clavicle can be very distinctly traced, though as yet it is but very imperfectly 

 ossified. Where ossification has not commenced, as in almost the whole lower half, the 

 structure of the developing clavicle differs but little from that of mesenchyme, which is 

 about to give rise to cartilage. There is, however, no trace of cartilage at this stage, 

 nor for long after, in the clavicle. Whether any distinct histological difference can be 

 made out between the mesenchymatous cells which give rise to cartilage and the very 

 similar cells which give rise to bone, I am not aware, but, at any rate, in their further 

 evolution, the cells become quite distinct. In the case of developing cartilage, the nuclei 

 become more rounded, slightly enlarged, and more translucent, and become fairly 

 uniformly surrounded by the hyaline substance. In the development of the clavicle, 

 on the other hand, the nuclei do not increase in size, and instead of becoming rounder 

 they become more oval, and develop angular processes. The little osteoblasts then 

 arrange themselves in little groups of twos or threes, and the little bony spicules form 

 along the side of the little groups. So far, then, as the Marsupial is concerned, it may 

 he definitely stated that the clavicle is a pure membrane bone in whose early develop- 

 ment cartilage plays no part. At the part where the clavicle meets the acromion, the 

 mesenchyme of the acromion may be said to merge into the mesenchyme of the clavicle, 

 but to assume that therefore the clavicle has a similar basis to the acromion, as has been 

 done by Hoffmann, seems to me unjustifiable. 



