192 AYERS AND JACKSON. [Vol. XVII. 



anterior region (Fig. 3, j/^„ sh^, sh^. The fibers of the inner 

 and outer of these three layers have the same general direc- 

 tion, and are usually parallel, while those of the middle layer 

 are often interlaced in the most complex fashion (see Fig. 3, 

 sh^. The relative and actual thickness of these layers varies 

 considerably in different regions, but the inner layer is always 

 the thinnest. In some places all three layers are fused into 

 one, the boundaries being indistinguishable. 



The external boundary of the notochordal sheath is always 

 formed by a thin, dense membrane, which stains deeply. This 

 is the elastica externa (Fig. 3, nile). Though usually homo- 

 geneous in appearance, in some places it shows a distinctly 

 fibrous structure. Whether the elastica externa is really a 

 part of the notochordal sheath proper, or a derivative of the 

 surrounding skeletogenous layer, is a matter of doubt. The 

 structure and appearance of the layer indicate the former. 

 It may be remarked that although both the central core and 

 the notochordal sheath are doubtless derived from the chordal 

 tissue (the latter from the cellular sheath), it is evident that 

 neither is a cuticiilar product, in the ordinary sense of the 

 term, although it is always so described. 



An interesting histological variation is found in the anterior 

 region, where the notochord lies imbedded between the para- 

 chordal cartilages. The elastica externcu, which separates the 

 notochord from the surrounding cartilage, becomes irregular in 

 outline, and the different sheath layers fuse into one. The 

 cellular sheath of the chordal tissue becomes irregular, while 

 the chordal tissue itself is very gradually replaced by carti- 

 laginous tissue. Thus near the anterior end of the notochord 

 we find the space inside the notochordal sheath entirely filli d by 

 cartilage (Fig. 4, nt) Just behind the tip of the notochord, 

 which projects in the median line in front of the parachordal 

 cartilages (PL XXIII, Fig. 7), the notochordal sheath disappears, 

 and we find that the tip of the notochord is a conical carti- 

 laginous structure, surrounded only by the skeletogenous layer. 

 The cartilage replacing the chordal tissue has an appearance 

 somewhat different from that of the surrounding parts. Near 

 the posterior end of the notochord we find in places a similar 



