ORIGIN OF THE LIMBS. 307 



vol. i. p. 362 ; /, fore-leg, b, hind-leg.) In all, the first rudi- 

 ment of each limb in the embryo is a simple wart, or small 

 knob, which grows from the side of the body between the 

 dorsal and ventral sides (Figs. 119 and 120, vol. i. pp. 357, 359 ; 

 136 and 137, pp. 381, 382). The cells composing these knobs 

 belong to the skin-fibrous layer. The outer surface is coated 

 by the horn-plate, which is rather thicker at the apex of 

 the protuberance (Plate IV. Fig. 5, x). The two anterior 

 protuberances appear at a rather earlier period than the 

 two posterior. By differentiation of the cells, these simple 

 rudiments develop immediately, in Fishes and in the 

 Dipneusta, into fins. In the higher vertebrate classes, on 

 the contrary, each of the four protuberances, in the course 

 of its development, assumes the form of a stalked plate, the 

 inner portion of which being narrower and thicker, the 

 outer broader and thinner. The inner portion, or the handle 

 of the plate, then divides into two sections : the upper and 

 lower legs (or arms). Four notches then appear in the free 

 edge of the plate, and these gradually become deeper ; these 

 are the divisions between the five digits (Plate VIII. Fig. 1). 

 The latter soon become more prominent. At first, however, 

 all the five digits, both on the fore and on the hind limbs, 

 are joined by a thin connecting web-like membrane; this 

 recalls the original adaptation of the foot as a swimming-fin. 

 The further development of the limbs from this most simple 

 rudiment takes place in the same way in all Vertebrates ; 

 that is, by the modification of certain groups of the cells of 

 the skin-fibrous layer into cartilage, of other groups into 

 muscles, yet others into blood-vessels, nerves, etc. Probably 

 the differentiation of all these various tissues occurs actually 

 in the limbs. Like the vertebral column and the skull, the 



