673 



Anat. Anz., VI, 1891, p. 338. — Zool. Jahrb., Abt. Morphol., B. V, 



1893, p. 387—428. 

 Eabl, Carl, Theorie des Mesoderms. Morphol. Jahrb., B. XIX, Oct. 1892, 



p. 100—115. 

 Seelet, H. G., Further Observations on the Shoulder Girdle and clavicular 



arch in the Ichthyosauria and Sauropterygia. Proc. Roy. Soc, Vol. 54, 



1893, p. 153. 

 ZiTTEL, Kael A., Handbuch der Palaeontologie. I. Abt. Palaeozoologie. 



III. Bd., 1888, 2. Lief., p. 344—412. 



Nachdruck verboten. 



The Fill -fold Origin of the Paired Limbs, in the Light of the 

 Ptychopterygia of Palaeozoic Sharks. 



By Bashfobd Dean, Columbia College, New York City. 



With 8 Figures. 



The theory that the paired extremities of the jaw -bearing 

 Vertebrates origiuated as continuous dermal outgrowths of the body 

 wall receives at present a very general support. Since the time 

 of its conception and development by Thacher, Balfour, Dohrn, 

 WiEDERSHEiM and others, the arguments in its behalf seem to have 

 become continually stronger, in spite of the repeated and indeed re- 

 cent attacks of Prof. Gegenbaur and his followers. In the majority 

 of instances the stumbling blocks which have retarded the accep- 

 tance of this theory, appear to have been based upon develop- 

 mental chaiacters interpretable as caenogenetic. But to attempt 

 to determine in all or even many cases the limits of these caeno- 

 geneses, or even to establish definitely that they are caenogeneses, is 

 at least puzzling and unsatisfactory. On all sides one is confronted 

 with lack of definite knowledge as to the degree to which develop- 

 mental growth processes^) may be accelerated, retarded, masked. 

 And if we must now believe that embryonic characters are to be ac- 

 cepted as tests of homology only after the most careful scrutiny, 

 we are certainly to be cautious in basing our faith upon ontogenetic 

 phylogeny, upon embryonic fins, fin-supports, muscle buds, nerves, — 

 as aids in settling disputes upon archipterygia and fin folds. 



1) Indeed it is only at the present moment that they have even 

 been in any way satisfactorily enumerated (cf. Davenpobt. Bull. Mus. 

 Comp. Zool. Harvard, Vol. XXVII, No. 6). 



44 



