II THE DEVELOPING VERTEBRAL COLUMN 19 



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

 ultimately become the intervertebral region. This extension of 

 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 (Rana) 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 the 

 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 band 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 ver- 

 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 Pelobates as separately ossified and calcified pieces, 



