THE SPINAL NEEYES IN ELASMOBEANCH EISHES. 
191 
The presence of transverse commissures connecting the central ends of all the poste- 
rior roots is very peculiar. The commissures may possibly be looked on as outlying 
portions of the cord, rather than as parts of the nerves. 
I have not up to this time followed their history beyond a somewhat early period in 
embryonic life, and am therefore unacquainted with their fate in the adult. 
As far as I am aware, no trace of similar structures has been met with in other 
vertebrates. 
The commissures have a very strong resemblance to those by which in Elasmobranch 
Fishes the glossopharyngeal nerve and the branches of the pneumogastric are united 
in an early embryonic stage*. 
I think it not impossible that the commissures in the two cases represent the same 
structures. If this is the case, it would seem that the junction of a number of nerves 
to form the pneumogastric is not a secondary state, but the remnant of a primary one, 
in which all the spinal nerves were united, as they embryonically are in Elasmobranchs. 
One point brought out in my investigations appears to me to have bearings upon the 
origin of the central canal of the Vertebrate nervous system, and in consequence upon 
the origin of the Vertebrate group itself. 
The point I allude to is the posterior nerve-rudiments making their first appearance 
at the extreme dorsal summit of the spinal cord. 
The transverse section of the ventral nervous cord of an ordinary segmented worm 
consists of two symmetrical halves placed side by side. 
If by a mechanical folding the two lateral halves of the nervous cord became bent 
towards each other, while into the groove formed between the two the external skin 
became pushed, we . should have an approximation to the Vertebrate spinal cord. 
Such a folding might take place to give extra rigidity to the body in the absence of a 
vertebral column. 
If this folding were then completed in such a way that the groove, lined by external 
skin and situated between the two lateral columns of the nervous system, became con- 
verted into a canal, above and below which the two columns of the nervous system 
united, we should have in the transformed nervous cord an organ strongly resembling 
the spinal cord of Vertebrates. 
This resemblance would even extend beyond mere external form. Let the ventral 
nervous cord of the common earthworm, Lumbricus agricola, be used for comparison f, 
a transverse section of which is represented by Leydig J and Claparede. In this we 
find that on the ventral surface (the Annelidan ventral surface) of the nervous cord the 
ganglion-cells (grey matter) (k) are situated, and on the dorsal side the nerve-fibres or 
* Balfour, “ A Preliminary Account of the Development of Elasmobranch Fishes,” Q. J. Micros. Sc. 1874, 
plate xv. fig. 14, v.g. 
f The nervous cords of other Annelids resemble that of Lumbricus in the relations of the ganglion- cells of 
the nerve-fibres. 
J Tafeln zur vergleichenden Anatomie, Taf. iii. fig. 8. 
MDCCCLXXVI. 
2 D 
