v SKELETON OF THE LIMBS 357 



From the plate-like rudiment of carpus or tarsus there spread out 

 r;uli;itiii- extensions normally five in number to form the skeleton of 

 the digits. In the Amniota these appear practically synchronously 

 although in Amphibians there is a tendency for them to develop in 

 n 'IT i ilar sequence according to the number of the digit (Rabl, 1901). 

 In the substance of these the phalanges make their appearance as 

 discrete chondrifications. 



In the Birds the loss of individuality of the digits involved in 

 the conversion of the tip of the pectoral limb into a rigid support for 

 the flight feathers has been accompanied by processes of reduction 

 and fusion of the orginal elements. In the prochondral stage five 

 digits are laid down but only II, III and IV proceed with their 

 development. Of these metacarpal II 

 becomes reduced to a small stump project- j[ IH. ]y 



ing from III : metacarpals III and IV , o o o y 



become fused with one another at both Q o" o 



ends : and the three distal carpals become Q 



fused with the metacarpals to form the o o ^ 



carpo-metacarpus characteristic of the Bird. O O o...Q:l ce 



In the hind limb of Birds there are c&-35 Q , ..<> . ^ 

 also laid down prochondral rudiments of ,--0 -.... 



the five digits and again I and V become Q ^ 



reduced although not so completely as in 

 the fore limb. V reaches the stage of a 



small metacarpal nodule of cartilage which FIG. 171. Cartilaginous eie- 

 however soon disappears. Metacarpals II, lneuts which develop in the 

 III and IV fuse with one another and J^I^ESftSR 

 with a cartilage which represents the distal (After Mehnert, 1897.) 

 row of tarsals to form the characteristic W| ce ntraie ; ci-5, distal canis; 

 tarso-metatarsus. Metatarsal I disappears , intermedium; //, hum.-] us: //. 

 except in its distal portion. And finally ;j;^ ; ' n 

 the two proximal carpals which are visible 

 for a time fuse with the end of the tibia to form the tibio-tarsus. 



BONY SKELETON OF THE LIMBS. PECTOEAL GIRDLE. In the 

 Sturgeons the original cartilaginous pectoral girdle persists, lying 

 close under the skin of the posterior branchial region. Plates of 

 bone corresponding exactly with those on the rest of the skin develop 

 superficial to the girdle and serve to reinforce it. Of these bony 

 plates there are two principal ones on each side, one in the region of 

 the glenoid surface the cleithrum (Gegenbaur) and one extending 

 ventrally to meet its fellow the clavicle. In existing Crosso- 

 pterygians where the evolution of the bony skeleton has reached a 

 higher level than in the Sturgeons the same two bony elements 

 develop but here the original shoulder girdle its function being to 

 a great extent taken over by the cleithrum becomes relatively 

 reduced in size. It lies on the inner surface of the cleithrum and 

 its cartilage gives place in part to two replacement bones the 

 scapula dorsal, and the coracoid ventral. It is to be noted also 



