254 



COMPARATIVE ANATOMY 



however, go to form the limbs; the intermediate region atrophies and 

 disappears. 



The Wolffian folds consist of an external covering of ectoderm and a 

 core of mesenchyme, which in man is of uncertain origin. Although in 

 elasmobranchs myotomic buds grow into the fin-folds, no such buds occur 

 in mammalian embryos. So far as can be seen, the mesoderm of the 

 paired appendages in man and mammals is proliferated as loose mesen- 

 chyma from the hypomere and not from the epimere as in the lower 

 vertebrates. However the somatic motor innervation of appendicular 

 muscles throughout the vertebrate series suggests that they are homo- 

 logous. (See p. 289.) 



''--...^PHALANGES 



''^^ METACARRiiLS /HAMATE 



/ /TRIQUETRAl'JCARPALS 



/PISIFORM 



/ULNA 



■ POLLEX 



RADIUS 

 LUNATE ') 



NAVICULAR' .„„.. ^ 

 . > CARPALS 



\MULTANGULUKV 

 MAJUS „/ 



Fig. 211 . — The bones of the human right hand and wrist viewed from above, showing 

 the regions of growth of the long bones. Growth (elongation) continues as long as a 

 disc of cartilage separates the diaphysis from the epiphysis. 



In their early development, arms and legs take the form of shovel- 

 shaped outgrowths. At this stage two surfaces may be distinguished, 

 of which the dorsal forms the extensor surface of the appendage while 

 the ventral side becomes the flexor surface. During development, how- 

 ever, each appendage is rotated on its long axis. The rotation of the arm 

 is, however, the reverse of that of the leg, with the result that the flexor 

 muscles of the arm are in front and the elbow bends back, but the extensor 

 side of the thigh is in front and the knee bends forward. In the mean- 

 while, the elongation of the appendages is correlated with division into 

 proximal and distal portions. Fingers and toes appear as early as the 

 second month. Up to this point there has been little difference in the 

 skeletons of the two appendages. (Fig. 210) 



