434 THE DEVELOPMENT OF THE CHICK 
cipital appears as a pair of ossifications in the tectum synoticum 
on each side of the dorsal middle line, subsequently fusing 
together. 
A detailed history of the mode of ossification of all the various 
bones of the skull would be out of place in this book. The figures 
illustrate some points not described in the text. The reader is 
referred to W. Ix. Parker (1869) and to Gaupp (1905). 
V. APPENDICULAR SKELETON 
The appendicular skeleton includes the skeleton of the limbs 
and of the girdles that unite the limbs to the axial skeleton. The 
fore and hind-limbs, being essentially homonymous structures, 
exhibit many resemblances in their development. 
The Fore-limb. The pectoral girdle and skeleton of the 
wing develop from the mesenchyme that occupies the axis and 
base of the wing-bud, as it exists on the fourth day of incuba- 
tion. It is probably of sclerotomic origin, but it is not known 
exactly how many somites are concerned in the chick, nor which 
ones. After the wing has gained considerable length (fifth day) 
it can be seen from the innervation that three somites are prin- 
cipally involved in the wing proper, viz., the fourteenth, fifteenth, 
and sixteenth of the trunk. But it is probable that the mesen- 
chyme of the base of the wing-bud, from which the pectoral 
girdle is formed, is derived from a larger number of somites. 
It is important, then, to note first of all that the scapula, 
coracoid, clavicle, humerus, and distal skeletal elements of the 
wing are represented on the fourth day by a single condensation 
of mesenchyme, which corresponds essentially to the glenoid 
region of the definitive skeleton. From this common mass a 
projection grows out distally in the axis of the wing-bud, and 
three projections proximally in different directions in the body- 
wall. These projections are (1) the primordium of the wing- 
skeleton, (2) of the scapula, (8) of the coracoid, (4) of the 
clavicle. 
The Pectoral Girdle. The elements of the pectoral girdle are 
thus outgrowths of a common mass of mesenchyme. The scapula 
process grows backward dorsal to the ribs; the coracoid process 
grows ventralward and slightly posterior towards the primordium 
of the sternum, thus forming an angle slightly less than a right 
angle with the scapular process; and the clavicular process grows 
