THE ENDOSKELETON 



163 



iceds of Am$hiaxus, and not transmitted to any of its de- 

 scendants. 



Judging from the final results as we see them in the great 

 Class of fishes, it may be supposed that the inequalities in the 

 development of this fin-fold increased through localized func- 



FIG. 42. Diagrams illustrating the fin-fold theory. 



(a) and (b) [after WIEDERSHEIM] represent the unmodified theory. A continuous 

 fin-fold, stiffened by skeletal rays, extends along the median dorsal line, around the 

 tail, and along the mid-ventral line as far as the cloaca, where it divides into two 

 lateral folds that extend along the sides of the trunk. The retention of portions 

 of this fold and the loss of the intermediate portions results in the formation of both 

 median and paired fins. In (c) [after RABL] is shown RABL'S modification of this 

 theory. The two lateral folds are from the beginning distinct from the median one, 

 and are hence subjected to external influences, especially at their free anterior and 

 posterior ends, thus modifying the first and last rays the most, and the others in 

 a progressively decreasing series towards the middle of the fold. When, later, the 

 first and the last portions become set off (along the dotted lines) and the interme- 

 diate portion suppressed, they form fins, of which the anterior one shows greater 

 modifications along its anterior, the other along its posterior border, precisely as 

 is the actual case among fishes. 



tional activity, until certain portions became especially well 

 developed, while the intermediate portions were entirely lost. 

 (Fig. 42, b). The significance of this is seen if this figure be 

 now compared with any good, typical fish, which shows the 

 perfected type resulting from this process. Here the fins are 

 all alike in structure, proving their derivation from a common 

 origin, but are divided into two groups in accordance with 

 their position, median and paired. Of the median fins the 



