II THE DEVKLOl'ING VERTEBRAL COLUMN 19 



caual, their bases grow head- aiul tail-wards into what will 

 ultimately become the inter vertel)ral regidn. This extension of 

 cartilage leads to a fusion with that of the next following ])air 

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

 a right and left longitudinal ridge of cartilage which sends elf 

 dorsal processes, neural arches, in metanieric succession. Xe.xl, 

 the intervertebral cartilage increases in such a way as t«i 

 constrict the chorda either laterally {Rana) or obliquely I'roni 

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

 cartilage the interdorsalia. Ventral arcualia are late and nnidi 

 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). ]\Iuch 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 l)and, equivalent to the basi- 

 ventralia, become semilunar in a transverse view, their liorns 

 tending upwards towards the basidorsal cartilages, but there is 

 no actual meeting. Both dorsal and ventral elements arc, 

 however, joined together and form the chief portion of the V( 1- 

 tebrae, owing to the rapidly proceeding calcification and lab r 

 ossification of the all-surrounding " niembrana 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 Discogiossidae, 

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

 or opistho-coelous character has been much exaggerated, ^^'e 

 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 PeJohatrs as separately ossified and calcified pieces. 



