PECTORAL ARCH. 87 



of tlie cerebro-spinal axis, these two folds fused together in the median dorsal 

 line. Thus all the four longitudinal epiblastic folds (of Thatcher, Mivart, 

 and Balfour) are possibly to be considered as arising originally from separate 

 metameric outgrowths of the body-segments, that is, from parapodia, which 

 have thus become gradually transformed into the fins existing at the present 

 time. 



Paul Meyer supports this hypothesis by finding parapodia-like outgrowths 

 arranged in'i'our rows along the caudal region in embryos of Pristiurus and 

 Scyllium. 



Later investigators in this subject no longer even accept the 

 hoinodynamy (i.e. the serial homology) of the pectoral and pelvic 

 arches and limbs, but suppose that even ontogenetically the 

 two arches can in no way be compared with one another, for, 

 arising in an entirely different manner, they can only be 

 regarded as "apparently similar" structures. Which of these 

 attempts at an explanation of the problem comes nearer to the 

 truth cannot yet be definitely stated, and the relative merits of the 

 views just put forward cannot be discussed here. 



Pectoral Arch. 



Fishes. Owing to the absence of paired fins in Amphioxus 

 and Cyclostomi, pectoral and pelvic arches are also wanting. In 

 Elasmobranchs the pectoral arch consists of an extremely simple 

 cartilaginous bar, the two halves of which are united ventrally 

 by cartilage or fibrous tissue (Fig. 69, SB} ; and it has at first a 

 similar structure in embryos of Ganoids and Teleostei. 



Later, however, in both the last-named groups, a row of bony 

 structures arising in the perichondrium becomes developed in this 

 region ; so that one can distinguish between a secondary or bony 

 pectoral arch, and aprimaryor cartilaginous one. 



The free extremity, or fin, is connected with the hinder and 

 outer circumference of the (primary) arch, and its point of 

 attachment may be taken as separating an upper dorsal and a 

 lower ventral section. The former, which is often connected with 

 the vertebral column, corresponds to a scapula, and the latter to a 

 coracoid plus precoracoid (clavicle) of the higher Vertebrata. 1 



In Teleosteans and bony Ganoids, the bony (secondary) arch 

 forms the principal support of the fin in the adult, the main 

 element being a large clavicle (Fig. 70, Z>). The primitive rela- 

 tions are thus much altered. The arch becomes secondarily 

 connected with the skull (Cm}. For further details, compare 

 Fig. 70. 



Amphibia and Reptilia. In these types, the secondary bony 

 apparatus is of less importance, for the primary arch is more 



1 The pectoral arch of Dipnoi is intermediate in character between that of Elasmo- 

 branchs and Ganoids. It shows so many special peculiarities as regards form and 

 POM don, that it cannot be fully described here. 



