350 PROF. oWBN ON EMBRYOLOGiCAL [June 5, 



This homological geueralization implied and inferred that the 

 embryonal basis of such diverging appendages should be a continu- 

 ous fold of blastema on each side of the body, projecting some way 

 between the neural, or upper, and the hsemai, or lower, primitive folds, 

 in which the unpaired fins, dorsal and anal, are developed in Fishes. 



To raise the foregoing generalization ' 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 oi the development of the " Pectoral and Pelvic 

 Girdles," Prof. Balfour proceeds to that of the " Limbs." 



" The first rudiments of limbs appear as shght longitudinal ridge- 

 like thickenings of the epiblast, which closely resemble the first 

 rudiments of the unpaired fins"^. The anterior portion of the 

 lateral ridge is " immediately behind the last visceral fold " ; the 

 posterior portion is "on the level of the cloaca"^. "In some 

 Elasmobranch embryos, more especially in Torpedo, they are con- 

 nected together at their first development by a line of columnar 

 epiblast-eells " * ; but " this connecting line of columnar epiblast is 

 a very transitory structure, and after its disappearance the rudimen- 

 tary fins become more prominent "\ "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 liiis"''. 



Whether the first recognizable trace of the locomotive fin be in 

 the form of a single ray, or of " a median axis and two rows of rays," 

 would be, ou 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 deve- 

 loped "- 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 back- 

 wards and being parallel to the surface of the body"^. 



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 (ii, a) of the succeeding arches is immistakable. If, then, the 

 diverging rays of the thoracic and abdominal vertebrx 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.' " (' On the Nature of 

 Limbs,' 8vo, 18-19, p. 65.) 



2 Treatise on Comparative Embryology, Bvo, 1881, vol. ii. p. 500. 



3 Ibid. * Ibid. ^ Ibid. " Tom. cit. p. 501. '• Tom. cit. p. 508. 



