THE LATER DEVELOPMENT OF THE FROG . 209 



tinuous strips extends along the entire chorda, while the 

 ventral elements similarly fuse forming a median ventral strip. 



Separate vertebra now become marked out by the appearance, 

 in these continuous cartilages, of metameric rings of fibrous 

 tissue; these are the beginnings of the intervertebral ligaments. 

 They appear opposite the middle of each mesodermal segment, 

 consequently the segments of the vertebral column (vertebrae) 

 alternate with the muscle segments. Cartilage now begins to 

 form across the notochord, between successive vertebrae, so that 

 the notochord becomes completely segmented, remaining only 

 intravertebrally. In each of these transverse partitions appears 

 a curved split, concave anteriorly. The intervertebral carti- 

 lages then fuse with the adjoining vertebrae and thus determine 

 the procoelous character of the vertebral centra. 



The ventral cartilages now grow up around the sides of the 

 chorda, meeting and fusing with the dorsal series. From the 

 former there extend outward short cartilaginous processes 

 which become the transverse processes of the vertebra. Later- 

 ally from these, bits of cartilage are formed later which repre- 

 sent ribs. These fuse with the tips of the transverse processes 

 so that no separate ribs are subsequently distinguishable. 

 In the meantime outgrowths from the dorsal elements have 

 extended inward beneath the nerve cord, as well as laterally 

 and dorsally to it (neural arch). 



Bony tissue appears first, before the beginning of metamor- 

 phosis, in the region between the dorsal and ventral series of 

 cartilages. It soon invades the entire cartilaginous structure, 

 forming a complete shell around the intravertebral notochordal 

 remains, and dorsally around the nerve cord. In addition to 

 the articulation of the procoelous vertebral centra, intervertebral 

 articulations develop on short processies of the neural arches. 



The foregoing description applies only to the nine vertebrae of 

 the body. In the greater part of the tal cartilage is not formed. 

 But in the region of the future urostyle three longitudinal strips 

 of cartilage are formed as in the trunk, but these fuse completely 

 enclosing the chorda in a cylinder of cartilage which is never 

 segmented into vertebrae. 



