STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 787 



then (2) the external layer of the sheath (sh.), which is, however, 

 less thick than before, and exhibits a very faint form of radial 

 striation ; and (3) internally, a fairly thick and concentrically 

 striated layer. The whole thickness is, on an average, O'l8 

 millim. 



In the intervertebral regions the membrana elastica externa 

 is still present in most parts, but has become absorbed at the 

 posterior border of each vertebra, as shewn in longitudinal section 

 in Plate 42, fig. 77. It is considerably puckered transversely. 

 The sheath of the notochord within the membrana elastica 

 externa is formed of a concentrically striated layer, continuous 

 with the innermost layer of the sheath in the vertebral regions. 

 It is puckered longitudinally. Thus, curiously enough, the 

 membrana elastica externa and the sheath of the notochord 

 in the intervertebral regions are folded in different directions, 

 the folds of the one being only visible in transverse sections 

 (Plate 42, fig. 79), and those of the other in longitudinal sections 

 (Plate 42, fig. 77). 



The osseous and cartilaginous structures investing the noto- 

 chord may conveniently be dealt with in the same order as 

 before, viz. : the neural arches, the haemal arches, and the 

 intervertebral cartilages. 



The cartilaginous portions of the neural arches are still 

 unossified, and form (Plate 42, fig. 78, n.a.) small wedge-shaped 

 masses resting on the sheath of the notochord. They are in- 

 vested by a thick layer of bone prolonged upwards to meet 

 the dorsal processes (n.a'.}, which are still formed of cartilage 

 invested by bone. 



It will be remembered that in the last stage there was no 

 key-stone closing in the neural arch above. This deficiency is 

 now however supplied, and consists of (i) two bars of cartilage 

 repeated for each vertebra, but intervertebral ly placed, which are 

 directly differentiated from the ligamentum longitudinale supe- 

 rius, into which they merge above ; and (2) two osseous plates 

 placed on the outer sides of these cartilages, which are continuous 

 with the lateral osseous bars of the neural arch. The former 

 of these elements gives rise to the cartilaginous elements above 

 the osseous bridge of the neural arch in the adult. The two 

 osseous plates supporting these cartilages clearly form what we 



