THE SKELETON 



293 



There now takes place in two of the more primitive groups of 

 Vertebrates— the Elasmobranchii (including the Holocephali) and 

 the Dipnoi— a remarkable process whereby the secondary sheath of 

 the notochord becomes converted into a sheath qf cartilage. Certain 

 of the cartilage cells in the arch rudiment take on an amoeboid 

 character and burrowing their way through the primary sheath, 

 apparently by the help of a digestive ferment, invade the secondary 

 sheath (tig. 145, m.c). Continuing their migration they become 

 distributed equally throughout the whole substance of the secondary 

 sheath, including those 

 portions in the head re- 

 gion which will later on 

 form part of the cranium. 

 The immigrant cells fin- 

 ally settle down in the 

 substance of the second- 

 ary sheath and the latter 

 becomes a cylinder of 

 cartilage. 



It is important, with 

 an eye to the evolution 

 of the vertebral column 

 in Vertebrates higher in 

 the scale, to bear in mind 

 that this invasion of the 

 secondary sheath by im- 

 migrant cartilage cells 

 takes place at four points 

 in the transverse plane, 

 corresponding to the bases 

 of the four arch rudi- 

 ments, and that this arrangement is repeated twice within the 

 limits of one segment owing to the arch rudiments being so 

 repeated. Consequently if we suppose the colonization of the 

 secondary sheath to be restricted to the neighbourhood of the trans- 

 verse plane in which the arch rudiments are situated the result 

 would be the formation of two rings of cartilage within the limits 

 of a single segment. 



In the case of Lung-fishes and Holocephali the chondrified 

 secondary sheath undergoes no further modification but in typical 

 Elasmobranchs it becomes divided up into segments, which form the 

 centra or bodies of the vertebrae, in the manner to be described later 

 on. In this process the originally uniformly flexible notochord with 

 its sheaths becomes replaced physiologically by a series of rigid 

 masses, flexibility being given to the whole by the presence of the 

 intervening joints. As this jointed condition of the vertebral 

 column originated in evolution at a time when the longitudinal 

 muscles of the body were already divided into myotomes, we may 



Fig. 145. — Part of a transverse section through a 

 Lepidosiren of stage 38, traversing one of the neural 

 arch rudiments. 



c.e, notochordal epithelium ; m.c, migrating cartilage cell ; 

 N, notochord ; ti.a, neural arch ; si, primary sheath ; s 2 , 

 secondary sheath. - , 



