n THE DEVELOPING VERTEBRAL COLUMN 19 



canal, their bases grow head- and tail-wards into what will 



ultimately become the intervertebral region. This extension (jf 



cartilage leads to a fusion with that of the next following pair 



of arches, so that the axial column at this early stage consists of 



a right and left longitudinal ridge of cartilage which sends off ' 



dorsal processes, neural arches, in metameric succession. Next, 



the intervertebral cartilage increases in such a way as to 



constrict the chorda either laterally (liana) or obliquely from 



above downwards and inwards {Bufo, Hyla). "We recognise in this 



cartilage the interdorsalia. Ventral arcualia are late and much 



obscured. There is scarcely any cartilage which could represent 



the interventralia, the intervertebral cartilage being almost 



entirely made up of the interdorsalia. These fuse together and 



form a disc or nodule, which later fuses either with the 



vertebra in front, and in this case fits into a cup carried by tlie 



vertebra next behind (procoelous vertebrae), or the knob is added 



to the front end of the vertebra, fitting into a cup formed by 



the tail end of the vertebra next in front (opisthocoelous 



vertebrae). Much later than the two longitudinal dorsal bands 



there appears on the ventral side an unpaired Ijand in which 



appear metamerically repeated swellings of cartilage, likewise 



unpaired. These swellings become confluent, in a way similar 



to that which- produced the dorsal bands, and form the unpaired 



ventral band of cartilage, the hypochordal cartilage of some 



authors. The swellings in this band, equivalent to the basi- 



ventralia, become semilunar in a transverse view, their horns 



tending upwards towards the basidorsal cartilages, but there is 



no actual meeting. Both dorsal and ventral elements are, 



however, joined together and form the chief portion of the vi r- 



tebrae, owing to the rapidly proceeding calcification and later 



ossification of the all-surrounding " membrana reuniens " or 



skeletogenous layer so far as that is not cartilaginous. 



Procoelous vertebrae exist in the overwhelming majority of 

 Anura ; opisthocoelous are those of the Aglossa, the Discoglossidae, 

 and of some Pelobatidae. The systematic value of this pro- 

 or opistho-coelous character has been much exaggerated. We 

 have seen that the centra of the vertebrae of the Anura are 

 formed entirely by the interdorsal elements, hence the term 

 "notocentrous," and these centra sometimes remain in adult 

 specimens of Pelolafes as separately ossified and calcified pieces, 



