372 MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 
a small portion of the external layer of the embryo," while the inferior cylinder is 
“composed of “the whole internal layer of the embryo," yet in all vertebrate animals, 
when developed, the importance of the contents of the former is not inferior to that of 
the latter. The neural laminæ come to be of primary consequence, even if of secondary 
origin—just as the spinal nerves and their roots in the myelon come to be single and 
continuous structures in spite of the contrast presented by their several modes of 
development. 
The nervous system, indeed, can hardly be considered as ranking below any other in the 
body. The special homologies of the cranial bones are best determined by taking the 
places of exit of the cranial nerves as fixed points; and it is certainly probable that the 
+ relations of the skeleton generally closely resemble those existing between different parts 
of the nervous system. | 
The mode of regarding the embryo which has been adopted by Professor Huxley seems 
to me to be the most simple and natural. According to that the embryo is developed 
from three pairs of antero-posteriorly extending laminz, which radiate from the chorda 
dorsalis. i 
The first pair of laminæ, the dorsal plates, ascend and form the enclosure of the cere- 
bro-spinal centres. 
The ventral plates descend; and each bifurcates, the space formed by the bifurcation 
constituting the pleuro-peritoneal cavity. The outer walls of this cavity (formed by the 
outer bifurcation of each ventral plate) constitute the second pair of laminæ, or external 
ventral plates. | 
The parts within the internal wall of the pleuro-peritoneal cavity (except the lining 
membrane of the alimentary canal, which is formed by the mucous layer of the embryo) 
are formed by the third pair of laminæ, or internal ventral plates. 
Now * such being the case, we might expect à priori, that a branch from each nerve- 
root would ascend into the dorsal lamina, that another would pass directly outwards, and 
two others would follow the laminæ formed by the bifurcation of the ventral plates—one 
passing along the body-wall (or external ventral plate) the other going down the mesen- 
tery to the alimentary canal (the internal ventral plate) ; and this is what we find to be 
really the case”*., E 
For the so-called sympathetic appears to be no separate system distinct from the spinal 
nerves, but merely to consist of those branches of the latter which descend into the third 
pair of laminæ (or internal ventral plates) together with commissural fibres. ; 
This striking and original suggestion was made by Professor Huxley in the twelfth lec- 
ture of his last Hunterian course. Such being the conditions of the embryonic laminæ, 
and the spinal nerves being each arranged in this, at the least, triradiate manner on each 
side, we might expect à priori some parallelism as to arrangement to exist in the skeleton. 
In endeavouring to obtain a good general idea of the vertebrate skeleton, it seems rea- 
sonable, then, to look, in the most completely developed skeleton, for three longitudinal 
series of parts in addition to, and on each side of the common axis. I say “in the most 
completely developed skeletons," because we may expect in many forms a certain coales- 
os * British Medical Journal, for May 15th, 1869, No. 437, p. 445. E 
