Proceedings of the Scientific Meetings of the Zoological Society of London 1883

Page 448

350 PROF. OWEN ON EMBRYOLOGICAL [June 5, This homological generalization implied and inferred that the embryonal basis of such diverging appendages should be a continu­ous fold of blastema on each side of the body, projecting some way between the neural, or upper, and the haemal, or lower, primitive folds, in which the unpaired fins, dorsal and anal, are developed in Fishes. To raise the foregoing generalization1 from the hypothetical level required the evidence of the competent embryologist, and such, by common consent, was the late lamented Biological Lecturer of Trinity College, Cambridge. After treating of the development of the " Pectoral and Pelvic Girdles," Prof. Balfour proceeds to that of the '* Limbs." " The first rudiments of limbs appear as slight longitudinal ridge­like thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins"2. The anterior portion of the lateral ridge is " immediately behind the last visceral fold"; the posterior portion is " o n the level of the cloaca"3. "In some Elasmobranch embryos, more especially in Torpedo, they are con­nected together at their first development by a line of columnar epiblast-cells " 4 ; but " this connecting line of columnar epiblast is a very transitory structure, and after its disappearance the rudimen­tary fins become more prominent"5. " The connexion of the two rudimentary fins [of one and the same side] by a continuous epithelial line suggests the hypothesis that they are remnants of two contiunous lateral fins" 6. Whether the first recognizable trace of the locomotive fin be in the form of a single ray, or of " a median axis aud two rows of rays," would be, on proof and acceptance, a test of the hypothesis of the rays or plates diverging or continued from the arches homologous serially with the pectoral and pelvic supporters of their more deve­loped " diverging appendages." Prof. Gegenbaur, who maintains the embryological evidence of the "primitive type of fin, consisting of a central multisegmented axis with numerous rays," confers on this alleged incipient form the term " archipterygium." Professor Balfour, accepting the term as applied to the limbs of Fishes, calls the embryonal limb of Amniota the " cheiropterygium." After repeating that " the limbs arise as simple outgrowths of the sides of the body formed both of epiblast and mesoblast," and that " in the ' Amniota' they are processes of a special longitudinal ridge, known as the Wolffian ridge," he notes that " both limbs have at first a precisely similar position, both being directed back­wards and being parallel to the surface of the body"7. The parts of the limb or fin as they successively appear are 1 " The serial homology of the pectoral and pelvic limbs with the shorter appendages (a, a) of the succeeding arches is unmistakable. If, then, the diverging rays of the thoracic and abdominal vertebra? of Fishes, of Reptiles, and of Birds be the serial repetitions of the more developed appendage of the scapulo-coracoid arch, they must be ' rudimental limbs.' " (' O n the Nature of Limbs,' 8vo, 1849, p. 65.) 2 Treatise on Comparative Embryology, 8vo, 1881, vol. ii. p. 500. 3 Ibid. i Ibid. 6 Ibid. e Tom. cit. p. 501. 7 Tom. cit. p. 508.