218 THE MESODERMAL DERIVATIVES 



The Appendicular Skeleton. 



The pectoral girdle consists of the scapuia, coracoid, and precora- 

 coid, the last two to be replaced by the clavicle. Except for the clavicle, 

 which is dermal, this girdle arises from ossified cartilage that is calci- 

 fied only at the base, and articulates with the elongated scapula. The 

 epicoracoid (or precoracoid) and the xiphisternum remain as cartilage 

 while the single sternum, paired coracoids, and single episternuni be- 

 come ossified. The sternum arises from the fusion of a longitudinal 

 pair of cartilages which never join the ribs but remain both anterior 

 and posterior to the pectoral girdle. The humerus, or upper portion of 

 the forelimb, articulates with the glenoid cavity of the pectoral 

 girdle. 



The pelvic girdle is a V-shaped fused mass of bone which supports 

 the hindlimbs and is associated with the urostyle. The paired anterior 

 ends of the pelvic girdle (ilium) are united with the enlarged trans- 

 verse process of the ninth or sacral vertebra. Other bones of this girdle, 

 arising from cartilage, are the ischium and pubis. The pubis alone 

 does not ossify. The ilium forms a cup, the acetabulum, which re- 

 ceives the head of the femur of the hindlimb. The bones of both 

 anterior and posterior girdles and limbs arise by the ossification of 

 preformed cartilage (i.e., endochondral in origin). There is evidence 

 that appendicular muscles of some amphibia may develop in situ 

 from somatic mesoderm. 



The Mesomere (Intermediate Cell Mass) 



Early in embryonic development the upper level of the lateral 

 plate mesoderm becomes separated off from the ventral sheets of 

 mesoderm, to give rise to the excretory and parts of the reproductive 

 systems. This is known as the intermediate cell mass or mesomere 

 because of its position relative to the other masses of mesoderm. It 

 is also known as the nephrotome because it gives rise to excretory 

 units, both the larval pronephros and the functional mesonephros of 

 the frog. The derivatives of this mesoderm will now be described; 

 the description of the development of the hypomere and its deriva- 

 tives, the coelom and circulatory system, will be deferred until later 

 in the book. 



