Wyraan.] 264 [June 5, 



under other circumstances, or in other worlds, rise to the dignity of 

 perfect limbs. The possibilities of the vertebrate archetype may not 

 as yet have been exhausted.* It does not appear, however, that the 

 processes in question ever take on in the actual vertebrates an ap- 

 proach to a form which might be considered a limb, and it might be 

 in'ge.l as an objection to Prof. Owen's view, that in fislies they seem to 

 be au integral part of the muscular system, and that in neither fishes, 

 reptiles or birds, do they occupy a position homologous with that of 

 limbs, viz., at the junction of the rib with its cartilage, or of the 

 pleurapojihysis with its hasmapophysis. 



It is unfortunate that in the attempts to determine the general hom- 

 ology of limbs so little attention has been given to their development, 

 which is indispensable to tlie complete solution of the question. On 

 studying their transitional phases in the embryo, we are, first of all, 

 struck with the fact that in their primary conditions limbs have so 

 strong a resemblance to the median fins of fishes and the flukes of ceta- 

 ceans. The fins on the median line in fishes all agree in this, that they 

 appear as an outgrowth from the integuments, in the form of a ridge 

 extending continuously along the back, ai'ound and under the tail. 

 This ridge is a mass of embryonic cells, all alike, but which subsequently 

 become differentiated into fin-rays and other structures. The fin-rays 

 are secondary structures, and cannot therefore be said, as has some- 

 times been asserted, to push out from beneath and carry the integu- 

 ments with them. The adipose fin of the Salmonidaj permanently 

 retains an early embryonic condition, and no fin-rays are formed at 

 any period. The fore and liind limbs, in like manner, are outgrowths 

 of the tegumentary cells, and for a time the cells undergo no differentia- 

 tion into bones or other tissues. These are, at length, developed in the 

 limbs, and subsequently grow pari passu with them. The bones do not, 

 therefore, force the integuments out by their protrusion, but the integu- 

 ments themselves have already grown out, and the limb is formed before 

 the bones are developed. Limbs in their primary condition do not ap- 

 pear to be dependences of the scapular and pelvic arches, any more than 

 the median fins of fishes, or the flukes of cetaceans, which last have some- 

 times been compared to limbs, are dependences of the vertebral column, 

 or teeth are dependences of the jaws with which, notwithstanding 

 their totally different origin, they become so intimately united at last. 

 In view of the above considerations, and in view of the fact, also, that 

 in fishes the ventral fins never pass beyond the condition of appendages 

 to the integuments, we believe there is ground for the hypothesis that 

 limbs belong to the category of tegumentary organs, and that their 



* On the Archetype and Homologies of the Vertebrate Skeleton. London, 1848. 

 p. 102. 



