430 OSBURN 



A number of arguments in favor of the fin-fold theory yet 

 remain, and may be summarized as follows: 



I. All fins of sharks arise as longitudinal folds of the epi- 

 dermis. 



II. The muscle-buds which give rise to the muscles of the 

 fins originate exactly alike in both kinds of fins. 



III. The nerves which supply the paired fins take their 

 origin in the same way as those of the unpaired. 



IV. The origin of the blood supply is alike in both kinds of 

 fins. 



V. The earliest support of the fins, a plate of thickened 

 mesenchyme, is of the same character in paired and unpaired 

 fins and arises in the same manner in both. 



VI. The time, place, and manner of differentiation of the 

 fin skeleton is similar in all the fins. 



VII. The later growth of the fin fold and the constriction 

 of the fin at its base are similar in paired and unpaired fins. 



VIII. Fusion of rays, basalia, etc., occur sporadically as 

 well as regularly in the fins of both categories. 



IX. Fin spines are known in both kinds of fins. 



X. Ceratotrichia or horny dermal rays are present in all the 

 fins of sharks. 



Examining the above arguments in the order given : 

 I. Whether or not the longitudinal folds which give rise to 

 the fins have once been entirely continuous is a matter of no 

 great consequence, — though it seems entirely possible from the 

 evidence at hand that the fins may have been connected in their 

 early history. Be that as it may there is no disputing the fact 

 that all the fins of sharks, and indeed of practically all fishes 

 (the only known exceptions are very rare and embrace forms 

 of highly specialized development, e. g., Lepidosiren, Gambusia), 

 originate as folds of skin. It must be noted that these folds 

 are always longitudinal. The gill membrane, which may be 

 considered the homolog of the fin according to the gill-arch 

 theory (since it contains the rays of the gill) , is, on the other hand, 

 vertical in origin. If time and place relations have any meaning 

 in embryology we cannot avoid the conclusion that the most 

 primitive ancestral fin was a fin fold. Add to this the evidence 



