340 THE ANIMAL KINGDOM. 



inclose the spinal cord, and in Elasmobranchii form a complete carti- 

 laginous investment, which is segmented into neural arches proper, placed 

 vertebrally, and intercalary pieces placed intervertebrally. The inter- 

 vertebral regions are occupied by a fibrous membrane in other Vertebrata, 

 and it is only in a few instances that intercalary pieces are rudimentarily 

 present. Haemal arches are well-developed in Pisces, where they originate 

 like the neural arches from a continuous right and left ridge. Haemal 

 intercalary pieces are present in the tail of Elasmobranchii, but not in 

 the dorsal region. Haemal arches are developed also in the tail of 

 Urodelous Amphibia. How far the transverse processes ( = di- and par- 

 apophyses) of the vertebrae in other Vertebrata can be said to represent 

 them is doubtful. The processes in question are continuous both with 

 the centrum and the neural arch, or with one of the two in the adult. 

 They are said to arise independently and then to fuse with the vertebrae 

 in Urodelous Amphibia ; in other cases they appear to be out-growths 

 from them. Vertebral centra are formed by the growth of the cartilaginous 

 or cellular sheath of the notochord, together with additions from the bases 

 of the arches in Elasmobranchii, or from the surrounding mesoblast in 

 Teleostei. The centrum is primitively due to a growth of cartilage in the 

 vertebral region, and so it remains in Pisces and Mammalia, with isolated 

 examples, living or extinct, in other classes. Large intervertebral remnants 

 of notochord persist in Pisces ; very slight in Mammalia. Traces of the 

 vertebral growth may be visible in the development of Amphibia and 

 Sauropsida, but they are masked by a great growth of cartilage in the 

 intervertebral region. The vertebrally placed remnants of notochord are 

 then converted into cartilage and very commonly eventually into bone. 

 The vertebral centrum may be biconcave (amphicoelous), biconvex, concave 

 in front or behind ( = pro- and opistho-coelous), or flat. It may remain 

 independent of the arches or fuse with them. It is always ossified, and 

 ossification may spread from it to the arches ; or the latter may ossify 

 separately. The centra alone may be connected together (most Pisces, 

 a few Reptilid) or articulating processes ( = zygapophyses) may be developed 

 from the neural arches both from their anterior and posterior aspects, 

 as in Amphibia and higher Vertebrata. The neural arches are generally 

 provided with a dorsal neural spine (neurapophysis), formed independently 

 or in continuity with them ; and the centra may have a ventral outgrowth 

 or spine (=hypapophysis). Each vertebra, consisting of centrum and neural 

 arches, corresponds to a myocomma ; the intervertebral region to the 

 centre of a myomere. However in the tail of some Elasmobranchii the 

 centra are twice as numerous as the myomeres. 



The ribs support the body walls. They coincide with the myocommata 1 



1 The ribs extend outwards horizontally in the fibrous septum between the dorso-lateral and 

 ventro-lateral muscles in Elasmobranchii. It is possible that this position is secondarily acquired. 

 In other Vertebrata they lie close to the peritoneum. 



