THE SPINAL NERVES IN ELASMOBRANCH FISHES. 
189 
I have as yet failed to detect any cranial anterior roots like those of the spinal 
nerves*. The similarity in development between the cranial and spinal nerves is espe- 
cially interesting, as forming an important addition to the evidence which at present 
exists that the cranial nerves are only to be looked on as spinal nerves, especially 
modified in connexion with the changes which the anterior extremity of the body has 
undergone in existing vertebrates. 
My results may be summarized as follows : — 
Along the extreme dorsal summit of the spinal cord there arises on each side a con- 
tinuous outgrowth. 
From each outgrowth processes corresponding in number to the muscle-plates grow 
downwards. These are the posterior nerve-rudiments. 
The outgrowths, at first attached to the spinal cord throughout their whole length, 
soon cease to be so, and remain in connexion with it in certain spots only, which form 
the junction of the posterior roots with the spinal cords. 
The original outgrowth on each side remains as a bridge, uniting together the dorsal 
extremities of all the posterior rudiments. The points of junction of the posterior roots 
with the spinal cord are at first situated at the extreme dorsal summit of the latter, but 
eventually travel down, and are finally placed on the sides of the cord. 
After these events the posterior nerve-rudiments grow rapidly in size, and become 
differentiated into a root (by which they are attached to the spinal canal), a ganglion, and 
a nerve. 
The anterior roots, like the posterior, are outgrowths from the spinal cord ; but the 
outgrowths to form them are from the first discontinuous, and the points from which 
they originally spring remain as those by which they are permanently attached to the 
spinal cord, and do not, as in the case of the posterior roots, undergo a change of 
position. The anterior roots arise, not vertically below, but opposite the intervals 
between the posterior roots. 
The anterior roots are at first quite separate from the posterior roots ; but soon after 
the differentiation of the posterior rudiment into a root, ganglion, and nerve a junction 
is effected between each posterior nerve and the corresponding anterior root. The 
junction is from the first at some little distance from the ganglion. 
Investigators have hitherto described the spinal nerves as formed from part of the 
mesoblast of the proto vertebrae. His alone, so far as I know, takes a different view. 
His’sf observations lead him to the conclusion that the posterior roots are developed 
as ingrowths from the external epiblast into the space between the pro to vertebrae and 
the neural canal. These subsequently become constricted off, unite with the neural 
canal and form spinal nerves. 
* [Hay 18, 1876. — Subsequent observations have led me to the conclusion that no anterior nerve-roots are 
to be found in the brain.] f Erste Anlage des Wirbelthier-Leibes. 
