92 report— 1877. 



•wise describes the same condition, a primarily epiblastic superficial process, replaced, 

 as development progresses, by a mesoblaslic outgrowth ; and in the bird-embryo 

 Foster and Balfour, Kolliker, and others describe similar stages. Here we have 

 two very remarkable conditions — first, an indication that the primitive vertebrate 

 form possessed an epiblastic system of appendages ; and, secondly, that the limbs 

 are the remnants of a continuous lateral fin. To the first hypothesis, that the pri- 

 mary appendicular system was epiblastic, there are no ontological objections ; for 

 in such generalized forms among Vermes as Sagitta we have lateral epiblastic fin- 

 processes, which, like the hooks of Peripatus or the bristles of Polychceta, may well 

 indicate allied epiblastic forms of archaic protovertebral limbs. As the surviving 

 forms of protovertebrates, such as the Tunicates and Amphioxus, as well as the 

 lowest craniotes, like the Lampreys, are limbless, they give us no information as to 

 the primary condition, as this deficiency may in them be due to obsolescence*. 

 Thus, as from the now classic researches of Ivleinenbergt, we have learned that, 

 in the primitive Metazoa there was a primary muscular system derived from the 

 epiblast, which has vanished in higher forms of animals, being replaced by a secon- 

 dary musculature of mesoblastic origin, and with the mesoblast derived, at least 

 in Invertebrates and Ichthyopsids, from the hypoblast, so we have now learned 

 that there was probably iu the protovertebrates an epiblastic appendage system, 

 which, having no inherent muscular apparatus, and being incapable of sufficient 

 specialization, vanished or became rudimental, and was replaced by the more per- 

 manent and more plastic mesoblastic limbs. 



The second point is of still greater interest, namely, that Embryology supplies 

 us with demonstrative evidence that the limbs are the remains of a continuous 

 lateral fin. This Mr. Balfour has found to be the case in Elasmobranchs, and there 

 are traces of similar though shadowy thickenings along the Wolffian ridge in frogs 

 and in birds. But while the epiblastic ridges are thus continuous, the mesoblastic 

 enlargements are much more indistinctly so, even from the first, exhibiting the two- 

 fold arrangement of fore and hinder linibs. 



I cannot forbear in this connexion referring to a closely related point in the 

 general subject of the morphology of limbs. 



The vertebrate animal is primarily composed of a chain of similar segments, and 

 there is no a priori reason in morphology why any one metamere should not bear 

 limbs as well as any other. Nay, from the analogy of Chsetopod worms, we might 

 expect that, as in these, each zonite usually bears two pairs of parapodia or stumpy 

 foot-processes, so in similarly derived and similarly segmented forms there might 

 be at least traces of a similar multiplication of appendages. 



In effect, we really do find a somewhat parallel series in the metameres of fishes ; 

 for, as Mr. Balfour has shown, the mediodorsal fin comes into existence precisely 

 in the same manner as the lateral fin-ridge ; and being a double structure, as we 

 learn both in its specialized form and even in the structure of the cartilaginous 

 precursor of the interspinous boues, it may reasonably be supposed to represent 

 structures homologous with the system of notopodia in a laterally compressed 

 worm, fused together, while the paired fins may be regarded as the neuropodia, 

 separated by the visceral cavity, and which in the degraded and compressed meta- 

 meres behind the visceral cavity also coalesce, forming another primary ridge, that 

 of the anal and caudal fin. 



In relation to the primary source of origin and method of derivation of limbs, 

 we have to account for two separate factors, the limb-girdle and the limb-rays ; 

 with regard to the former, I can now only refer to the hypothesis of Gegenbaur 

 and Dohrn, that the limb-girdles represent modifications of the visceral arches, and 

 I pass this by with two comments : — 1st, that the visceral arches are themselves, to 

 a certain extent, specialized, and, consequently, it would be better to state the 

 hypothesis^ thus — that the limb-girdles and visceral arches are specializations of 

 corresponding paraxial structures in different metameres ; 2nd, in the light of the 

 evident fundamental complexity of the limb-girdles, it seems a simpler explanation 

 of phenomena to regard each girdle as made up of the arches of several (probably 

 three or more metameres fused) rather than as subdivisions of a single arch. 



* Cf. Dohrn, Ursprung d. Wirbellhiere n. Functionsmcch. t Hydra. 



