. Comparative anatomy. Anatomy, Comparative. 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 in
. Comparative anatomy. Anatomy, Comparative. 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. â POLLEX RADIUS LUNATE ') NAVICULAR' .ââ.. ^ . > CARPALS \MULTANGULUKV MAJUS â/ Fig. 211.âThe 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). Please note that these images are extracted from sca
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