70 



THE ELASMOBRANCH FISHES 



b.a." V 



'■•ip^ ■ 



A. Heterodontus francisci. 



In Elasmobranchs the column is particularly instructive because of its rela- 

 tion to the still more primitive notochord found in the prechordata and in the 

 embryo of all vertebrates. This new cartilaginous column arises around and in 

 the sheath of the notochord as a secondary and more effective support. Its 

 mode of development may be noticed briefly. 



In the embryo, cells proliferate from the sclerotome or inner part of the 

 somite (see p. 96, fig. 97, scl.) and migrate inward to a position around, above, 

 and below the notochord. Those which collect at the upper and lower levels 

 form four cogs with the notochordal sheath as the center. These cells lay down 

 cartilage for the neural and haemal arches and around the notochord. 



Many of the cells around the notochord, however, may perforate its external 

 frd UA2 li.^ ,Lj wall (elastica externa) and deposit car- 



tilage within the notochordal sheath. 

 A sagittal section through a vertebra of 

 Scylliuni canicula is shown in figure 75. 

 The sheath between the outer (e.e.) and 

 inner (e.i.) layers in which cartilage is 

 deposited may be divided into outer 

 (oz.), median (mz.), and inner (iz.) 

 zones. The median zone is the one in 

 which calcium is usually deposited. 



The central column, in 

 a simple type like Hep- 

 tanchiis maculatus, is es- 

 sentially a thin tube of 

 cartilage {oz., fig. 52) 

 deposited in the sheath of 

 the notochord. The mid- 

 dle zone here is composed 

 of transverse fibers in the 

 sheath, which in some re- 

 spects appear to be like 

 the "chordafaserscheide" 

 described for Amphioxits by Von Ebner ( 1895) . As to the layer which is desig- 

 nated "iz." in figure 52 I am not sure whether this represents the inner zone of 

 more specialized Elasmobranchs or is a part of the elastica interna. 



In types only a little more specialized than Heptanchus maculatus (Chlam- 

 ydoselachus and Heptanchus cinerens), a slightly greater amount of calci- 

 fication may be present in the column; and in still more specialized forms, 

 where the column becomes a stronger support than that just described, calci- 

 fication is a pronounced feature. But different regions vary greatly in the 

 amount of calcification. For convenience of description the column may be 

 divided into three regions. The first of these, the anterior section, joins the 

 head; the second is in the area of the trunk; and the third we may designate 

 simply as the caudal segment of the column. 



B. Ehinobatis productus. (C'liester tStock, orig.) 



Fig. 7(5. Cervical vertebrae. 



hd., dorsal basal plate (basidorsal) ; hv., ventral basal 



plate; c, centrum; f.d., foramen for dorsal nerve; id., 



dorsal interealarv (interdorsal plate) ; r., rib; shd., neural 



si^ine. 



