NATURAL SCIENCES OF PHILADELPHIA. 43 



pesad (femur) do not segment. The late Y-shaped ossification 

 (man) is without morphological value. 



Dr. A. endeavored to establish a plan for the construction of the 

 hand and foot, by premising a system of divergent rays project- 

 ing from the distal extremity of the manad and pedad segments 

 respectively. He conceived it to present a better principle of 

 action than does the hypothesis of Gegenbaur, which rests upon 

 marginal chains of ossicles appended distad to the limb. 



In the fish the primal divergence of the rays from the multiple 

 manad are infinite, and do not permit of subsequent divergences. 

 In all other vertebrates the primal divergence is finite (single), 

 and the subsequent divergences vary; they attain their maximum 

 number (four) in the carpus and tarsus of Ichthyosaurus and 

 Plesiosaurus. 



In taking the limbs of the salamander 1 as illustrations, it is found 

 that the humerus gives off at the elbow two rays, the ulna and 

 radius. The ulna at the wrist gives off two rays in the shape of 

 carpal bones, which are serially homologous with the radius and 

 ulna. The lateral (i. e. outer) ray segments three times to pro- 

 duce the outer toe. The median (i. e. inner) ray segments once 

 and then yields divergents to produce by serial segmentation the 

 toes placed median to the axis of the carpal quantity. The radius 

 does not present divergents but segments to project the inner- 

 most toe. In this arrangement the ulna is seen to be the more 

 potential of the divergents of the humerus, since its several diver- 

 gences make up the bulk of the hand and account for the projec- 

 tion of all the toes excepting the innermost. In the foot the fibula 

 is the more potential bone, since the area of its related divergen- 

 ces yields the bulk of the foot, as does the ulna in the hand. Again, 

 of the first ulnar divergence the median is the more potential, 

 since the lateral passes off to the outer toe, exhibiting axial repe- 

 tition of segments only. The tendency for the greater potentiality 

 to be fo\md in the median ray of any given divergence, finds an 

 apparent exception in the penultimate divergence in Ichthyosaurus 

 and Plesiosaurus, where it would appear as though the lateral ray 

 yielding the last divergence is the outer instead of the inner side. 

 Enough is secured, however, to modify the prevalent idea concern- 

 ing the acquisition of toes. Conceding that the above scheme is 

 correct, it will follow, that, inasmuch as in the several diver- 

 gences the median rays are favored, and the toes are the ultimate 

 projections of such rays, they may be initiated from towards 

 the centre and not always from the sides. It was observed, for 

 example, that in the bi-dact}dous foot of the Proteus the ulnar 

 divergent produced an axis composed of five segments to consti- 

 tute the outer toe. The radial divergent repeated segments four 

 times, the terminal three comprising the inner toe. Now, the hand 



1 Gegenbaur. 

 1872.] 



