THE APPENDICULAR SKELETON. 213 



phosis, into a dorsal blade-like part, the suprascapula, ss, which 

 remains in great part cartilaginous throughout life ; and a 

 ventral, more slender, and shaft-like part, the scapula, sc, round 

 which a ring of bone soon forms. 



The coracoid portion is, from the first, split into two diver- 

 ging processes ; an anterior or pre-coracoid portion, and a pos- 

 terior or coracoid proper. The ventral ends of the coracoid and 

 pre-coracoid of each side grow towards each other and meet, 

 forming a longitudinal band of cartilage, the epi-coracoid ; the 

 two epi-coracoids lie in close contact with each other in the 

 median plane, but do not fuse. Along the anterior border of 

 the pre-coracoid cartilage, a bony rod, the pre-coracoid bone or 

 clavicle, is formed ; and around the coracoid cartilage a tubular 

 sheath of bone, the coracoid bone, is developed. There is thus 

 at first no trace of a sternum, either as a median or paired 

 structure. 



During and after the metamorphosis further changes occur. 

 The bones increase in size, especially the scapula and coracoid. 

 The two epi-coracoid cartilages, in place of merely meeting in 

 the median plane, overlap each other to a certain extent, the 

 left epi-coracoid lying dorsal to the right one. From the anterior 

 ends of the epi-coracoid cartilages a pair of small processes grow 

 forwards ; these soon fuse to form the omosternum, which rapidly 

 increases in size. Behind the epi-coracoids, and in close contact 

 with them, a pair of cartilaginous bands appear, which fuse 

 together to form a flat median plate of cartilage ; this gives rise 

 in front to the sternum, round which a ring of bone soon forms, 

 and behind to the xiphi-sternum, which remains permanently 

 cartilaginous. 



b. The pelvic girdle also consists at first of two half-rings of 

 cartilage, encircling the hinder part of the trunk. The ventral 

 ends of the half-rings, which are flattened and expanded, are in 

 contact in the median plane, and very early fuse firmly together 

 to form the pelvic symphysis. The dorsal ends are more slender : 

 at first they lie free in the muscles of the body wall ; later on 

 they become connected with the transverse processes of the 

 ninth or sacral vertebra. 



Each half-ring has on its outer surface, close to its ventral 

 end, a depression, the cotyloid or acetabular cavity, for articula- 

 tion with the head of the femur. The part above the acetabulum, 



