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MESSRS. F. M. BALFOUR ANT) W. N. PARKER OX THE 
vertebral region (Plate 29, fig. 78) and the other through that of an intervertebral 
region (Plate 29, fig. 79), and by a horizontal section (Plate 29, fig. 77). 
In the last stage the notochord was only constricted vertebrally. Now, however, by 
the great growth of intervertebral cartilage there have appeared (Plate 29, fig. 77) very 
well-marked intervertebral constrictions, by the completion of which the vertebrae of 
Lepidosteus acquire their unique character amongst Fishes. 
These constrictions still, however, coexist with the earlier, though at this stage 
relatively less conspicuous, vertebral constrictions. 
The gelatinous body of the notochord, retains its earlier condition. The sheath has, 
however, undergone some changes. In the vertebral regions there is present in any 
section of the sheath—(1) externally, the mernbrana elastica externa (m.el.) ; 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, 0T8 millim. 
In the intervertebral regions the mernbrana elastica externa is still present in most 
parts, but has become absorbed at the posterior border of each vertebra, as shown in 
longitudinal section in Plate 29, fig. 77. It is considerably puckered transversely. 
The sheath of the notochord within the mernbrana 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 mem- 
brana 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 29, fig. 79), and those of the other in longitudinal sections (Plate 29, 
fig. 77). 
The osseous and cartilaginous structures investing the notochord may conveniently 
be dealt with in the same order as before, viz.: the neural arches, the lisemal arches, 
and the intervertebral cartilages. 
The cartilaginous portions of the neural arches are still unossified, and form (Plate 29, 
fig. 78, n.a.) small wedge-shaped masses resting on the sheath of the notochord. They 
are invested 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 howmver supplied, and consists of (l) two 
bars of cartilage repeated for each vertebra, but intervertebrally placed, which are 
directly differentiated from the ligamentum longitudinale superius, 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 w T e have called in our description of the adult the osseous roof of the spinal 
canal. 
