344 PROCEEDINGS OF THE ACADEMY OF [1887. 



ON THE HOMOLOGIES AND EARLY HISTORY OF THE 

 LIMBS OF VERTEBRATES. 



BY JOHN A. RYDER. 



I. The imperfect serial homology of the limbs of Vertebrates. 



That any one should seriously question the complete homol- 

 ogy of the anterior and posterior pairs of limbs as found developed 

 in the great classes; Mammals, Birds, Eeptiles, Amphibia and 

 Lyrifera, (Ichthyes, or all fish-like vertebrates, except the Lampreys 

 and Hags,) is, perhaps, at first thought, a somewhat startling prop- 

 osition. The fact that there is an imperfect homology or a Avant of 

 exact morphological equivalency between the parts of the same pairs 

 of limbs in diflferent forms, has been tacitly admitted by such of the 

 transcendental anatomists as Gervais and Gegenbaur, and those 

 anatomical philosophers who have been influenced by their a priori 

 methods, in developing which, certain suppositions had to be made, 

 which at the time, could not be or were not verifiable or refutable from 

 data supplied by general ontogeny or embryology. The case stands 

 differently to day. Since Gervais and Gegenbaur wi'ote on the the- 

 ory of the limbs, owing mainly to the remarkably fruitful labors of 

 Haeckel, Balfour and Dohrn, the great moq^hological problems 

 presented for solution by the organizations of the diverse classes of 

 vertebrata, have j^resented themselves under entirely new aspects. 

 These are not only momentous as affording a key to the interpreta- 

 tion of the anatomy of the adults of the different types, but also as 

 throwing a not inconsiderable amount of light upon the relations 

 and taxonomy of the major groups. 



That the paired limbs have been derived from some common, 

 simple ancestral form of limb, is, I cannot help but believe, proved 

 by the following general truths : 



1. In the most undeveloped condition, the first traces of the 

 paired limbs of all vertebrates, find formal expression as low longi- 

 tudinal, lateral projections of the body, and lie in a plane parallel 

 with that of the axis of the latter. This fact originally observed by 

 K. E. von Baer, has in part afforded J. K. Thacher, F. M. Balfour, 

 and St. G. Mivart, the ba.sis for a theory of the development of the 

 paired limb, but it remained for A. Dohrn to discover that there 

 existed a continuous series of vestigiary structures in certain forms 

 which connected the anterior and posterior limbs together into a 



