74 Vertebrate Embryology 



cartilaginous sheath occur, which grow inwards so as to 

 constrict and ultimately obliterate the notochord. Each 

 of these vertebral rings becomes, after the metamorpho- 

 sis, divided into an anterior and a posterior portion, 

 which fuse with the bony centra of the adjacent verte- 

 brae, and ossify to form their articular ends. 



" From the circumference, and from the articular ends 

 of each vertebra, ossification gradually spreads inwards ; 

 but a small portion of notochord persists in the middle 

 of each centrum for a long time, or even throughout 

 life. 



" The vertebras are not placed opposite the myotomes, 

 but alternate with these ; so that each vertebra is acted 

 on by two myotomes on each side, one pulling it for- 

 wards, and the other backwards. 



" The transverse processes are at first independent of 

 the corresponding vertebrae, but very early fuse with 

 them. They extend into the septa between the myo- 

 tomes, and probably correspond to the ribs of other 

 vertebrates. 



" The urostyle is the part of the axial skeleton behind 

 the vertebrae ; it is not divided into vertebrae at any 

 stage in development. The anterior end of the noto- 

 chord, imbedded in the base of the skull, is gradually 

 encroached on by the cartilage and bone around it, and 

 ultimately completely absorbed. 



" The skull. The skull of the tadpole consists almost 

 entirely of cartilage ; none of the bones of the skull, 

 with the exception of the parasphenoid, appearing until 

 nearly the time of metamorphosis. In the adult frog 

 this cartilaginous skull is replaced to a considerable ex- 

 tent by cartilage bone ; while other bones, primitively 



