138 



COMPARATIVE ANATOMY 



m. ~ 



doubtful how far this comparison is justifiable. As is usually the 

 case in the median fins, certain parts of these lateral folds have 

 undergone reduction, only the anterior and posterior portions 

 remaining to form respectively the pectoral arid pelvic fins, which 



must therefore be looked upon as 

 the localised remains of a con- 

 tinuous lateral fin-fold on either 

 side of the body. 1 



Into these paired folds extend 

 metameric processes of the myo- 

 meres, which undergo further 

 development in those regions 

 which will give rise to the pectoral 

 and pelvic fins, and disappear in 

 the intermediate region. More or 

 fewer spinal nerves pass into the 

 fins, and finally also cartilaginous 

 supports (pterygiophores), as in 

 the case of the median fins. These 

 radii appear first of all at the base 

 of the fin, gradually extending 

 centrifugally into the latter, and 

 also, becoming fused, centripetally 

 into the body- wall (Fig. 98). 2 An 

 articulation is then formed second- 

 arily between the fused basal part 

 of the skeleton situated in the free 

 portion of the fin (basipterygium) 

 and that which extends into the 

 lateral body-wall and serves as a 

 support for the limb proper : this 

 constitutes the limb-arch or girdle, 

 The arch may remain compara- 

 tively small and not extend far 



dorsally ; but when the extremity is destined to perform more im- 

 portant movements in locomotion or to give a more definite 



1 The essential part of this conception as to the origin of the paired 

 extremities is due to Thacher, Mivart, Balfour, Haswell, and Dohrn, and a 

 somewhat similar idea was put forward by Goodsir as early as 1856. The 

 Palffiozoic Cladoselache is very suggestive in this respect. 



' Thus phylogenetically both anterior and posterior extremities can be traced 

 to a metameric ground-plan. At the same time it must be borne in mind that the 

 above account is not altogether borne out oiitogenetically. The muscle-buds are 

 not strictly metameric, as they fuse together before coming into connection with 

 the skeletal parts, with which they do not always correspond numerically and 

 which appear to consist at first of a single unsegmented basipterygium : in other 

 words, the radii arise secondarily. Moreover, it is held by some embryologists 

 that oiitogenetically the girdle is the primary part of the extremity from which 

 the free portion grows out secondarily, and a similar axifugal growth can be 

 recognised in the median tins. All this, however, may only mean that recapitula- 

 tion is incomplete, and the arguments against the lateral fin-theory are still not 

 conclusive. 



FIG. 97. TRANSVERSE SECTION 

 THROUGH THE EMBRYO OF A 

 SHARK (Pristiurus melanosto- 

 mns), 9 MM. LONG, SHOWING 

 THE MODE OF ORIGIN OF THE 

 PECTORAL LIMB-BUDS. 



ap, limb-buds ; ch, notochord ; co, 

 ccelome ; m, myomeres, which 

 are extending ventrally ; my, 

 spinal cord ; re', re", rudiment 

 of kidney tubule and duct. 



