G18 Mr. T. P. Buist 071 tlie Development of 



scapula. The scapular process is directed forward as- in the 

 previous stage, but now takes part in the attachment. In 

 this region also ossification has commenced^ but it is as yet 

 only partial and superficial. The proximal radials have now 

 approximately attained their adult shape, but show no traces 

 of osisification, and are continuous with the remainder of the 

 coraco-scapular cartilage. Externally and internally respec- 

 tively the proximal /adials are now overlapped by the cover- 

 plate and the mid lateral plate of the second ring, and are 

 attached to them by connective tissue. Above, the scapula 

 is similarly loosely anchored within a cleft in the clavicle. 



Stage IV., 44 mm. — The post-temporal is now ronghly 

 Y-shaped and is attached in front to the skull by its two 

 branches and behind by its stem to the fore-end of the 

 clavicle. The inner branch of the stem of the clavicle curves 

 toward the mesial plane and is attached at its inferior ex- 

 tremity to the jugnlar plate ; it thus corupletes between the 

 jugular plate and the two branches of the clavicular stem 

 the osseous foramen through which the muscle to the hyoid 

 passes. 



The coracoid (fig. 4, co) is a roundish mass of cartilage 

 lying close to its fellow of the opposite side ; its anterior 

 third is the greatly thickened precoracoid process (pr.p) 

 which connects it with the inner branch of the stem of the 

 clavicle {i.cl). From the upper and posterior part of the 

 coracoid the postcoracoid process {ftp), now comparatively 

 long and slender, passes backward and downward, and its 

 upper and outer surfaces give origin to part of the fin- 

 muscle. The "arm^' of the coracoid arises from the upper 

 part of the external aspect of the coracoid, and passes outward 

 and upward to join the intermediate cartilage. The post- 

 coracoid process is ossified throughout its whole substance, 

 while the remainder of the coracoid, including the precoracoid 

 process and the arm, is ossified superficially. 



The scapula in its form and function is now almost iden- 

 tical with the four proximal radials. In the two uppermost 

 of these ossification has commenced, but there is as yet no 

 sign of the peculiar serrations and " anchoring processes " of 

 the distal end which develop later. 



An important feature of the last two stages is the stiffening 

 of the pectoral skeleton and the changing of the support of 

 the pectoral fin from the coraco-scapular girdle to the dermal 

 plates enclosing the fin-cleft. In the subsequent develop- 

 ment these changes are carried on to their completion. The 

 attachment of the clavicle to the transverse processes of the 



