390 
MESSRS. F. M. BALFOUR AND W. N. PARKER ON THE 
tion. In the first place the attention of the observer is at once struck by the fact that, 
m the vertebral regions, the sheath is much thicker (’014 millim.) than in the inter¬ 
vertebral ('005 millim.), and a careful examination of the sheath in longitudinal sections 
shows that the thickening is due to the special differentiation of a superficial part 
(Plate 28, fig. 69, sh.) of the sheath in each vertebral region. This part is somewhat 
granular as compared to the remainder, especially in longitudinal sections. It forms 
a cylinder (the wall of which is about *01 millim. thick) in each vertebral region, imme¬ 
diately within the membrana elastica externa. Between it and the gelatinous tissue 
of the notochord within there is a very thin unmodified portion of the sheath, which is 
continuous with the thinner intervertebral parts of the sheath. This part of the sheath 
is faintly, but at the same time distinctly, concentrically striated—a probable indication 
of concentric fibres. The inner unmodified layer of the sheath has the appearance in 
transverse sections through the vertebral regions of an inner membrane, and mav 
perhaps be Kolliker’s u membrana elastica interna.” 
We are not aware that any similar modification of the sheath has been described in 
other forms. 
The whole sheath is still invested by a very distinct membrana elastica externa ( m.el ). 
The changes which have taken place in the parts which form the permanent vertebrae 
will be best understood from Plate 28, figs. 69-71. From the transverse section (fig. 
70) it will be seen that there are still neural and haemal arches resting upon the 
membrana elastica externa; but longitudinal sections (fig. 69) show that laterally these 
arches join a cartilaginous tube, embracing the intervertebral regions of the notochord, 
and continuous from one vertebra to the next. 
It will be convenient to treat separately the neural arches, the haemal arches with 
their appendages, and the intervertebral cartilaginous rings. 
The neural arches, except in the fact of embracing a relatively smaller part of the 
neural tube than in the earlier stage, do not at first sight appear to have undergone any 
changes. Viewed from the side, however, in dissected specimens, they are seen to be 
prolonged upwards so as to unite above with bars of cartilage directed obliquely back¬ 
wards. An explanation of this appearance is easily found in the sections. The carti¬ 
laginous neural arches are invested by a delicate layer of homogeneous bone, developed 
in the perichondrium, and this bone is prolonged beyond the cartilage and joins a 
similar osseous investment of the dorsal bars above mentioned. The whole of these parts 
may, it appears to us, be certainly reckoned as parts of the neural arches, so that at 
this stage each neural arch consists of: (l) a pair of basal portions resting on the noto¬ 
chord consisting of cartilage invested by bone, (2) of a pair of dorsal cartilaginous bars 
invested in bone ( n.a .), and (3) of osseous bars connecting (1) and (2), 
Though, in the absence of the immediately preceding stages, it is not perfectly 
certain that the dorsal pieces of cartilage are developed independently of the ventral, 
there appears to us every probability that this is so ; and thus the cartilage of each 
neural arch is developed discontinuously, while the permanent bony neural arch, 
