Cuap. IL. HISTOLOGY. 605 
own combination; and, again, it would seem that at the free surface of the cells 
(a) the wall is very thin, like that of the interior ones (fig. 5b, 2). The interior 
cells are nearly globular in shape, and do not appear to press against each other. 
The walls are excessively thin, and their contents, as well as the contents of the 
superficial cells, are perfectly homogeneous, and hyaline. At no time, as far as we 
have investigated the cells of the chorda, could the least trace of a mesoblast 
be detected in them. No reagents were used in this investigation. 
The Vertebre. At the time the vertebrae have appeared along nearly the 
whole length of the embryo, and the branchial fissures have begun to develop, 
(see Pl. 12, fig. 5, 8, 9, 9a, 11, and p. 548,) each vertebra is composed of a 
single layer of cylindrical, wedge-shaped cells, (Pl. 19, fig. 3,) having their broader 
ends outward, and the narrower ends pointing towards the square, hollow, clear 
space in the interior. (See also p. 591, and Pl. 19, fig. 5.) Their contents are 
perfectly homogeneous and hyaline, without any trace of a mesoblast. At the 
time the Turtle is born, the peripheric portion (w-c. 12, a, p. 591) of the ver- 
tebre is ossified to a certain depth. The outer, very elastic layer (Pl. 22, fig. 
3, a) can hardly be distinguished from the unossified, fibrous lining of the shield; 
in fact, not at all, except by its hardness. These hardened, calcified fibres lie 
close to each other, but leave here and there numerous elongate-oval, clear spaces, 
which trend, with the fibres, in the same direction as the axis of the vertebra. 
They consist of rows of minute granular bodies, contaiming, or in fact made up, 
of lime (fig. 3a). Transverse to the fibres of the outer layer, those of another 
layer (fig. 3, 6) may be found lying closely pressed upon them, and similar in the 
arrangement of the fibres and of the clear spaces. A third fibrous layer (¢) has clear 
spaces, the longer diameter of which trends in the same direction as in the third 
layer. The clear spaces of the second and third layer are more or less broad 
and irregular, resembling the very large and jagged, broad, oval, or round open- 
ings in the spongiform, innermost mass (d@) of the bone. The fibres of the third 
layer (c) cannot be made out at all, and those of the second layer are very 
faint, whilst those of the first (a) are easily recognized. The meshes or inter- 
spaces of the spongiform, calcareous, Imnermost layer are very clear, and appear 
perfectly amorphous when seen in this position. From the manner in which the 
fibres of the torn edge of the first (w) and second (4) layers bristle out, each 
one supporting itself, it is made clear that the granules composing the fibres (fig. 
‘ 3a) are the recipients of a calcareous deposit. This deposit gradually fills up the 
minute interspaces between the fibres, and thus forms a uniform, apparently homo- 
geneous layer, (¢,) with clear spots, scattered here and there, throughout the stra- 
tum. The appearance of the third layer (c) is so similar to the two outer ones, 
and to the second one. in particular,—which only differs in showing very faint, 
