LIMBS. 609 



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 praepubic 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 Gotte 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 

 development 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 

 longitudinal 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 

 immediately behind the last visceral fold, and a posterior on the 

 level of the cloaca. In most Fishes they are in no way con- 

 nected, but in some Elasmobranch embryos, more especially in 

 Torpedo, they are connected together at their first development 

 B. in. 39 



