1887.] NATURAL SCIENCES OF PHILADELPHIA. 345 



continuous chain of serially homologous or homodynamous elements 

 These lateral elements of the limb, are therefore, to be regarded as 

 metameric structures, in that they correspond with the metamerism 

 of the body. The fore and hind limbs are, therefore, and presuma- 

 bly in all forms, to be regarded as directly or indirectly differentiated 

 from a single pair of lateral folds, it matters not in what way the 

 process may be obscured by extreme secondary modifications or 

 specializations of development. 



2. The subsidiary doctrines which confirm the preceding, may 

 be stated as follows : 



a. The lowest truly limb-bearing vertebrates are the only ones 

 in which the lateral folds, from which both pairs of limbs are derived, 

 are continuous or which show a continuous series of limb-buds un- 

 der the integument along either side of the embryo. 



h. This lowest limb-bearing group is also the only one in which 

 the branches of the paired spinal nerves, which pass out to the met- 

 americally repeated limb-buds, ever form a continuous series, or in 

 which the paired limbs maintain their primordial horizontal posi- 

 tion. 



e. The group here referred to, the Elasmobranchii, is, moreover, 

 primitive in many other ways, especially as respects : (1) or histo- 

 logical development, in that no parosteal or membrane bones are 

 developed, as in the higher types, and, (2) or morphological devel- 

 opment, in that the branchiae are at first naked, with no operculse 

 in the adult ; skeleton principally cartilaginous ; distal part of ver- 

 tical fins and paired limbs, supported and stiffened by actinotrichia 

 above, v/ith shagreen or dermal spines covering them ; teeth gener- 

 ally successional from a thecal fold, and transitional to the spines or 

 denticles found iii the common external tegumentary covering of 

 the body ; no air bladder or pneumatic apparatus ; shoulder and 

 pelvic girdles simple ; jaws and mandibular arch simple, suspended 

 directly to the skull ; a wide, spiracular, branchiferous cleft ; gener- 

 ative and renal apparatus of a primitive type ; muscular buds, which 

 are thrust into the n^dian and paired limb-folds, with traces of a 

 lumen or cavity, which has been derived from the cavity found in 

 the myotomes, from which the muscular buds have been derived as 

 outgrowths or diverticula. 



This evidence is quite sufficient for us at present to build upon, 

 for the Ascidians, Leptocardians and Enteropneusta have no claim, 

 as based either upon their morphology or upon their ontogeny, to 



23 



