298 EMBRYOLOGY OF THE LOWER VERTEBRATES < 11. 



methods by which vertebral centra are produced (1) by the 

 segmentation of the cartilaginous secondary sheath (sheath centra ; 

 chorda centra Gadow) and (2) by the enlargement of the bases of 

 the arch-elements which grow round the notochord and give rise 

 to centra outside the primary sheath (perichordal centra ; arch 

 centra Gadow). 



SHEATH CENTRA are seen in Elasmobranchs. In the region 

 which will develop into a centrum the chondrified secondary sheath 

 becomes thickened so as to bulge inwards and constrict the noto- 

 chord (Fig. 150). A more deeply staining " middle zone " soon 

 becomes distinguishable in this thickened part of the secondary 

 sheath (Fig. 150, m.z) having a shape something like that of a 

 dice-box, its central part lying much nearer to the axis of the 



FIG. 150. Part of sagittal section through the secondary sheath of a Scyllium of 61 mm. 

 total length showing an early stage in the development of a centrum. (After C. Rabl, 

 1893.) 



i.z, inner zone ; 7/1.2, middle zone ; 0.2, outer zone ; N, notochord ; si, primary sheath. 



notochord than do its two extremities. This middle zone becomes 

 the main part of the wall of the amphicoelous centrum, its substance 

 becoming usually strengthened by the calcification of its intercellular 

 matrix. 



The inner zone (Fig. 150, i.z) may grow in thickness so as to 

 cause greater and greater constriction of the notochord. This process 

 attains to its maximum in the Skates where it extends inwards 

 to the axis and causes the formation of thick septa \\hirh divide 

 the notochord into isolated intervertebral fragments. More usually 

 however the inner zone does not UH<ITM-O this increase, it tends to 

 become absorbed at each end and forms simply a ring of cartilage 

 in the centre of the vertebra. 



The outer zone in many RUsmobranchfl becomes caleilied in 

 parts: the calcified regions often showing a regular arrangement 

 Micrnlric cylindrical shells or radial iirj septa. 



I'KI:I< IIUKDAL CENTRA. As a matter of fact tin; pmvly chorda- 

 centrous condition is merely a temporary ( "i<- in tin- Klasmobranch, 

 as in later stages of development i he sheath centra become sur- 



