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. K. 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, (3) 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 



