THE PIGEON. 



207 



two slightly concave articular surfaces for the condyles of the 

 femur, and is produced in front (dorsally) into a prominent 

 cnemial crest divided into two diverging plates. The 

 distal extremity is pulley- shaped, and its transverse axis is 

 inclined at an angle to that of the proximal extremity : the 

 two condyles of which the distal pulley is formed are 

 more prominent on the anterior (dorsal) than on the 

 posterior face of the bone, thus differing from the very 

 similar condyles of the femur. 



mtl 2 



FIG. 51. Columba livia. Left tarso-metatarsus of an unhatched 

 embryo, with the distal end of the tibio-tarsus, viewed from the 

 anterior (dorsal) face ( x 5). 



The tarsal cartilages (tl.i, tl.2) are represented as separated from 

 the tibia and metatarsus respectively. 



The cartilaginous parts are dotted. 



m'.Z.2, second, mtl. 3, third, and mtl.^, fourth metatarsal : ti, tibia : 

 //.I, proximal tarsal cartilage, afterwards fusing with tibia : tl.2, distal 

 tarsal cartilage, afterwards fusing with metatarsus. 



114. In a pigeon embryo towards the end of incubation, 

 the distal end of the tibio-tarsus is a separate cartilage, the 

 proximal tarsal cartilage (Fig. 51, //.i) : its separateness 

 in the cartilaginous state distinguishes it from an epiphysis, 

 which is a distinct ossification in a continuous cartilage. 



