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ORIGINAL ARTICLES. 
him to many merely fanciful results, they have received less attention 
than they might otherwise have met. The alimentary system is encircled 
by the vascular system in all the types of animal form in which blood¬ 
vessels are found. Then, in the nervous system, almost the only 
point of resemblance between the vertebrata and articulata is that 
the great nerves tend, in both, to embrace the visceral cavity; and 
passing further down the scale of life, we find, in the Echinodermata 
and other forms, that the nervous system completes a ring around the 
alimentary system; while, descending even to the lowest forms, we 
still meet with a sensory and motory exterior surrounding a digestive 
interior. 
But, waiving at present that description of evidence, and returning 
to our line of argument, we proceed to prove by the aid of embry¬ 
ology our proposition, that, in the trunk, the elements of the skeleton 
are primarily divisible into those which embrace the visceral cavity 
and those which do not. 
Let us recall to mind that the embryo makes its first appearance in 
the plane of the germinal membrane ; that the layers into which it di¬ 
vides are continuous with, and continued into the germinal membrane; 
and that they fold inwards in such a manner that the products of the 
internal layer are completely surrounded by those of the middle 
layer, and those of the middle by those of the outermost layer.* The 
cerebro-spinal axis, being developed from the most superficial layer, 
must be considered as fundamentally a superficial formation. Doubt¬ 
less it soon closes upon itself to complete a cylinder, and processes 
grow up on each side of it, which join to form the neural arch; but the 
cylinder which the cerebro-spinal axis forms cannot be compared with 
that formed by the epithelial lining of the intestine; for the latter 
is the whole internal layer of the embryo, while the former is the 
product of only a small portion of the external layer, the rest of 
which, forming the epidermis, extends in a complete circle round the 
body, and is the outermost ring, just as the intestinal epithelium is 
the innermost ring. Much less can the whole contents of the costal 
arch, including the products of the musculo-intestinal layer and the 
whole axial part of the vascular system be compared with the contents 
of the neural arch. Neither can the structures which enclose the 
cerebro-spinal axis be compared with those which enclose the visceral 
cavity, for the latter are continuous with the germinal membrane, 
while the former are elevations or processes derived from the latter. 
Confining our view, lastly, to the skeleton; the elements which form 
the costal arch lie in the direction of the dorsal plates, from which 
* Indeed the embryo may with perfect propriety be considered as a bud derived 
from the germinal membrane, after the method of alternate generation. This idea 
has been already put forth by Dr. Ogilvie of Aberdeen, in his work on ‘ The Genetic 
Cycle in Organic Nature,’ p. 157. 
