500 PECTORAL AND PELVIC GIRDLES. 



Comparison of Pectoral and Pelvic girdles. 



Throughout the Vertebrata a more or less complete serial homo- 

 logy may be observed between the pectoral and pelvic girdles. 



In the cartilaginous Fishes each girdle consists of a continuous 

 band, a dorsal and ventral part being indicated by the articulation 

 of the fin ; the former being relatively undeveloped in the pelvic 

 girdle, while in the pectoral it may articulate with the vertebral 

 column. In the case of the pectoral girdle secondary membrane 

 bones become added to the primitive cartilage in most Fishes, which 

 are not developed in the case of the pelvic girdle. 



In the Amphibia and Amniota the ventral section of each girdle 

 becomes divided into an anterior and a posterior part, the former 

 constituting the praecoracoid and pubis, and the latter the coracoid 

 and ischium ; these parts are however very imperfectly differentiated 

 in the pelvic girdle of the Urodela. The ventral portions of the 

 pelvic girdle usually unite below in a symphysis. They also meet 

 each other ventrally in the case of the pectoral girdle in Amphibia, 

 but in most other types are separated by the sternum, which has no 

 homologue in the pelvic region, unless the prsepubic cartilage is to 

 be regarded as such. The dorsal or scapular section of the pectoral 

 girdle remains free ; but that of the pelvic girdle acquires a firm 

 articulation with the vertebral column. 



If the clavicle of the higher types is derived from the mem- 

 brane bones of the pectoral girdle of Fishes, it has no homologue in 

 the pelvic girdle ; but if, as Gb'tte and Hoffmann suppose, it is a part 

 of the primitive cartilaginous girdle, the ordinary view as to the 

 serial homologies of the ventral sections of the two girdles in the 

 higher types will need to be reconsidered. 



Limbs. 



It will be convenient to describe in this place not only the de- 

 velopment of the skeleton of the limbs but also that of the limbs 

 themselves. The limbs of Fishes are moreover so different from those 

 of the Amphibia and Amniota that the development of the two types 

 of limb may advantageously be treated separately. 



In Fishes the first rudiments of the limbs appear as slight longi- 

 tudinal ridge-like thickenings of the epiblast, which closely resemble 

 the first rudiments of the unpaired fins. 



These ridges are two in number on each side, an anterior imme- 

 diately behind the last visceral fold, and a posterior on the level 

 of the cloaca. In most Fishes they are in no way connected, but 

 in some Elasmobranch embryos, more especially in Torpedo, they 

 are connected together at their first development by a line of 

 columnar epiblast cells 1 . This connecting line of columnar epiblast 

 is a very transitory structure, and after its disappeai'ance the rudi- 

 mentary fins become more prominent, consisting (fig. 343, b) of a pro- 

 jecting ridge both of epiblast and mesoblast, at the outer edge of 

 1 F. M. Balfour. Monograph on Elasmobranch Fishes, pp. 101 2. 



