MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 319 
cence and indeterminateness of skeletal parts—just as nervous structures run together 
or fail to be differentiated. These series of parts may be expected, then, to be :— 
I. Parts related to the first pair of laminæ (that is, in the ascending or dorsal plates of 
the embryo), which may be spoken of as ** dorsal ”’ or eparial parts (Plate LIII. fig. 2, x). 
= ll. Parts related to the second pair of laminæ (that is, in the external ventral plates), 
which may be spoken of as the “ outer ventral" or parazial parts (fig. 2, P). 
III. Parts related to the third pair of lamin (that is in the internal ventral plates), 
which may be spoken of as the “inner ventral" or hypaxial parts (fig. 2, A, B, C). 
Epaxial parts will be parts embracing the cerebro-spinal axis, or parts serially homolo- 
gous with parts which embrace that axis. 
Paraxial parts will more or less embrace externally the pleuro-peritoneal cavity (or at 
least lie without it), or be serially homologous with parts which do so in the trunk, 
These parts may be also expected to include skeletal elements tending to pass directly out- 
wards, just as certain of the nerves pass directly outwards. 
Hypaxial parts will be parts situated between the skeletal axis and some part of the 
pleuro-peritoneal cavity, or serially homologous with parts so situated. 
Now between the skeletal axis and the pleuro-peritoneal cavity we find in the trunk :— 
l. The great dorsal aorta. 
2. 'The alimentary tract. 
9. The heart with the great vessels springing from it. 
It is conceivable then that we might have in a skeleton developed to the most complete 
possible degree, three kinds of hypaxial parts (Plate LIII. fig. 2, A, B, c) :— 
A. Some embracing, or tending to embrace the dorsal vessels (fig. 2, A). 
B. Some embracing, or tending to embrace the alimentary canal (fig. 2, »). 
C. Some embracing or tending to embrace the heart and the great vessels springing 
from it (fig. 2, c). 
These hypaxial skeletal structures might be defined, à priori, as skeletal parts either, 
on the one hand, intervening between the skeletal axis and the pleuro-peritoneal cavity 
together with any prolongations of it (if such there be), or else skeletal parts serially 
homologous with others which do so intervene,—or, on the other hand, as skeletal struc- 
tures placed within the inner wall of the pleuro-peritoneal cavity together with any 
prolongations of it, or else skeletal parts serially homologous with others which are so 
placed. 
Putting aside, then, all exoskeletal parts, with which we have, in this paper, nothing to 
do, and also putting aside the appendicular skeleton, the rest of the endo-skeleton (that 
is to say, the axial skeleton) may, from the. mode of development of the émbryo, and from 
that of the nervous system, be expected to appear (in its most developed condition) asa 
longitudinal axis whence three longitudinal series of more or less developed parts diverge 
on each side for a greater or less extent, namely :—1, epaxial; 2, paraxial; and 3, hyp- 
axial parts. But a certain coalescence or connation of these parts may also be expected 
to occur more or less frequently, especially in lower forms. : vus 
So much for the à priori view; it remains to test its value by-and-by by à posteriori 
considerations. 
