234 THE FIFTH DAY. [CHAP. 



even curved edge of which they do not project. On the 

 sixth or seventh day the three digits of the wing (the 

 median being the longest) and the four (or in some 

 fowls five) digits of the foot may be distinguished, and 

 on the eighth or ninth day these begin to project from 

 the edge of the expanded foot and wing, the substance 

 of which, thin and more or less transparent, remains for 

 some time as a kind of web between them. By the 

 tenth day the fore and hind extremities, save for the 

 absence of feathers and nails, are already veritable 

 wings and feet. 



Within the mesoblast of the limbs a continuous 

 blastema becomes formed, which constitutes the first 

 trace of the skeleton of the limb. The corresponding 

 elements of the two limbs, viz. the humerus and femur, 

 radius and tibia, ulna and fibula, carpal and tarsal 

 bones, metacarpals and metatarsals, and phalanges, be- 

 come differentiated within this, by the conversion of defi- 

 nite regions into cartilage, which probably are at first 

 united. These cartilaginous elements subsequently ossify. 



The pectoral girdle. The scapulo-coracoid elements of the 

 shoulder girdle are formed as a pair of cartilaginous plates, 

 one on each side of the body. The dorsal half of each plate 

 ossifies as the scapula, the ventral as the coracoid. The clavicles 

 are probably membrane bones. 



The pelvic girdle is derived from a pair of cartilaginous 

 plates, one on each side. Each of them is developed in con- 

 tinuity with the femur of its side. The dorsal half of each plate 

 ossifies as the ilium ; the ventral half becomes prolonged into 

 two processes, the anterior of which ossifies as the pubis, the 

 posterior at the ischium. 



Ribs and sternum. The ribs appear to arise as 

 cartilaginous bars in the connective tissue of the body 



