80 



EVOLUTION AND GENETICS 



include a proximal pair, a distal transverse row of five, and two 

 median in position. The five distal bones are followed by five 

 long bones, the metacarpals and metatarsals, and these by five 

 series of shorter phalanges which form the skeleton of the digits. 

 This primitive type is called the pentadactyl appendage (Fig. 52). 

 Specialized Appendages. Modifications are present in such 

 highly specialized structures as the wing of the bird and the foot 



Fig. 55. — Sternum and shoulder girdle of mammals, a, the duck-mole, 

 Ornilhorhynchus, a primitive oviparous mammal; b, human embryo, c, oor- 

 acoid; d, epicoracoid; e, episternum; f, clavicle; g, scapula; h, suprascapula; 

 m, manubrium; stb, sternebrae; x, xiphisternum. (From Wilder 's History 

 of the Human Body, after W. K. Parker; with the permission of Mrs. H. H. 

 Wilder and Henry Holt and Company.) 



of the horse, involving the reduction and fusion of parts, and in 

 the marine mammals the phalanges are multiplied for the support 

 of the flippers; but in a large majority of terrestrial vertebrates 

 the fundamental plan of the pentadactyl appendage is distinctly 

 traceable (Fig. 56). It is well represented in a primitive state in 

 the salamander, Necturus, and is not highly modified in lizards 

 and Crocodilia; in mammals its variations are numerous but often 

 not extreme. Conspicuous examples of modifications in mammals 

 are found in the fore limb of the bat, in which the second, third, 

 fourth and fifth digits are greatly proi nged to support the wing 

 membrane; in the horse, with its single p Tsisting third digit; and 

 in the dolphin, which has some digits prolo. i;ed by the multiplica- 



