| OCR Text |
Show 1883.] TESTIMONY TO GENERAL HOMOLOGY. 351 first a ray diverging from, or near to, the articular interspace between the scapula and coracoid, or the ilium and pubis. This primitive condition of limb he terms, in Fishes, the " basipterygial bar " \ aud represents it as such in his figure 346, under the letters mpt, of a section of the embryonal pectoral fin in Scyllium stellare. So, in Fishes, " In both fins the skeleton in its earliest stage consists of a bar springing from the posterior side of the pectoral or pelvic girdle, and running backwards parallel to the long axis of the body. The outer side of this bar is continued into a plate which extends into the fin, and which becomes very early segmented into a series of parallel rays at right angles to the longitudinal bar2. In other words, tbe primitive skeleton of both the fins consists of a longitudinal bar running along the base of the fin, and giving off at right angles a series of rays which pass into tbe fin. The longitudinal bar, which may be called " basipterygium," is moreover continuous in front with the pectoral or pelvic girdle as the case may be" 3. Gegenbaur and his followers believed the "bar" and " rays" to be contemporaneous in appearance ; and truly they come early into view and follow quickly. Balfour, however, derived, from apparently closer or earlier observation, the conviction that they showed two stages, and that the " rays " were consecutive in appearance to the "bar"4. If this view, as is probable, be preferably accepted, the ° diverging appendages " of the haemal arches or so-called " girdles," intervening between the scapular and pelvic ones, m a y be viewed as " embryonal limbs " arrested at the " bar-stage." It may be objected that such "costal appendages," as a rule, are lamellar, or in form of a "plate" rather than a " bar;" but such is the shape assumed by the pri- ^§ilii!^\.. Ohondropterygian embryonal fin (after Balfour). mordial fin, when the " basipterygium " (annexed figure, bp) becomes the " metapterygium " (ib. mp), or sustainer of the " meso-pterygium," or rudiment of the consecutive or future "pro- and mesopterygia " (a, b). 1 Tom. cit. p. 504. 2 Tom. cit. p. 502. 3 Tom. cit. p. 502. 4 Tom. cit. p. 501. 24* |
