212 THE MESODERMAL DERIVATIVES 



hindlimbs, although the hindlimbs are the first to emerge during the 

 process of metamorphosis. 



The sclerotome arises at loose cells which are proliferated off from 

 the ventro-medial portion of the myotome of the somite. They will 

 give rise to the entire axial skeleton, as described below. By this time 

 (5 mm. stage) the somites are entirely separate from the lateral plate 

 mesoderm. 



The Vertebral Column. 



The development of the notochord has been described already. Its 

 origin cannot be attributed exclusively to any of the three germ layers. 

 Its cells remain few in number, become flattened antero-posteriorly, 

 and are highly vacuolated. The notochord then becomes surrounded 

 by (1) the primary or elastic sheath derived from the notochordal cells, 

 (2) the secondary or fibrous sheath, and (3) eventually an outer skele- 

 togenous sheath which is of mesodermal origin. This latter connective 

 tissue sheath encapsulates the nerve cord and also extends laterally 

 between the myotomes by the 15 mm. stage. The notochord finally 

 disappears in the frog. It is partially replaced by and partially con- 

 verted into material of the centrum of each vertebra. Neural arches 

 and transverse processes develop outwardly from the cartilaginized 

 skeletogenous sheath. 



The vertebral column of the frog consists of ten vertebrae of which 

 the last is modified into an elongated rod-like bone known as the uro- 

 style, mentioned above. The first vertebra is the cervical atlas, fol- 

 lowed by seven abdominal vertebrae and finally the ninth or sacral 

 vertebrae. The urostyle takes the place of the caudal vertebra. The 

 skeletogenous sheath from the sclerotome encloses the spinal cord 

 and notochord by the time of hatching, and by the 15 mm. stage 

 some of this sheath has given rise to cartilage. It is within this cartilage 

 that the vertebrae develop. The formation of this cartilage, due to 

 the accumulation of the sclerotomal cells between the myotomes, is 

 segmented in a manner which alternates with the developing muscle 

 segments. However, at all levels the spinal cord and notochord will 

 be enclosed in this double ring of connective tissue which is being 

 transformed progressively into cartilage and finally into bone. Around 

 the notochord it is known as the perichondrium and around the 

 spinal cord as the vertebral cartilaginous arch. 



Ossification begins, and the cartilage gradually is displaced by an 



