Cope.] -^"^ I Oct. 7, 



Oil Some Points in the Kiiieto genesis of the Limbs of Vertebrates. 



By E. D. Cope. 



{Bead before the American PhilosopJdcal Society, October 7, 1S92.) 



The following paper Is designed to supplement some omissions from my 

 previous discussion of this subject in the memoir "On the Mechanical 

 Causes of the Origin of the Hard Parts of the Mammalia."* 



I. The Segmentation op the Chiropterygium. 



The segmentation of the limbs in the Vertehrata is a simple mechanical 

 problem. Paleontology and embryology concur in proving that the 

 limbs originated in primitive folds in the external integument, and that 

 their connection with the internal skeleton was of later accomplish- 

 ment. At first free, they sought points of support on the skeleton, but 

 did not lose their free mobility when this contact was attained. Appro- 

 priately to the meclianical conditions of rigidity and flexibility necessary to 

 their use in a fluid medium, they were originally composed of slender rods 

 which were segmented by interruptions at suitable points. The articula- 

 tions of the fin rays of fishes have been made the subject of an interesting 

 research by Ryder, who finds them to be fractures, due to flexures during 

 motion in the water medium. f The limb of land vertebrates (the chirop- 

 terygium) Avas derived from one of the forms of fins (rhipidopterygium) 

 of water vertebrates. This is the simple type of primitive fin displayed 

 by the Paleozoic Teleostomi of the superorder Rhipidopterygia. Whether 

 the subdivisions of the chiropterygium, the propodial, metapodial and 

 phalangeal bones, etc., were divided from the primitive branches of the 

 archiplerygium, as held by Gegenbaur; or whether they liave developed 

 by sprouting from a simple axial series of segments, as held by Baur ; or 

 whether, as I have suggested, it is a derivation from the rhipidoptery- 

 gian type of paired fin (Fig. 1, p. 280), is not yet decided. In either case, 

 the limbs of tlie first land animals were segmented and flexible at the 

 joints between the segments. The necessities of such limbs are twofold : 

 first, to serve as supports when at rest or in progression ; second, to be 

 applied to the body in protection from enemies, or in aiding the functions 

 of feeding, reproduction, etc. The first function requires principally 

 mobility at the point of connection with the body. The second, flexi- 

 bility at some point on the shaft of the limb. The two kinds of move- 

 ments in question would conserve two principal points of flexure, and 

 these would be for the fore limb, just what we find, the shoulder and 

 elbow joints ; and for the hind limbs, the hip and knee joints. The two 

 median joints are directed in opposite ways, the elbow backwards and 



* American Journal of Morpholo{,'j', iii, 1889, p. 137. 



t Proceedings of the American Philosopliical Society, 1889, p. 547. 



