THE SPINAL NEEVES IN ELASMOBEANCH PISHES. 
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anterior roots of the spinal nerves ; they are opposite the median line of the muscle- 
plates, and do not appear to join the posterior roots ( vide fig. G3, ar). 
At the end of this period or second stage the main characters of the spinal nerves in 
Pristiurus are the following : — 
(1) The posterior nerve-rudiments form somewhat wedge-shaped masses of tissue 
attached dorsally to the spinal cord. 
(2) The cells of which they are composed are typical undifferentiated embryonic cells, 
which can hardly be distinguished from the connective-tissue cells around them. 
(3) The nerves of each pair no longer meet above the summit of the spinal canal, but 
are independently attached to its sides. 
(4) Their dorsal extremities are probably united by commissures. 
(5) The anterior roots have appeared ; they form small conical projections from the 
ventral corner of the spinal cord, but have no connexion with the posterior rudiments. 
The Third Stage of the Spinal Nerves in Pristiurus. 
With the third stage the first distinct histological differentiations of the nerve-rudi- 
ments commence. Owing to the changes both in the nerves themselves and in the 
connective-tissue around them, which becomes less compact and its cells stellate, the 
difficulty of distinguishing the nerves from the surrounding cells vanishes ; and the diffi- 
culties of investigation in the later stages are confined to the modes of attachment of 
the nerves to the neural canal, and the histological changes which take place in the 
rudiments themselves. 
The stage may be considered to commence at the period when the external gills first 
make their appearance as small buds from the walls of the visceral clefts. Already, in the 
earliest rudiments of the posterior root of this period now figured, a number of distinct 
parts are visible (Plate 17. fig. H i). 
Surrounding nearly the whole structure there is present a delicate investment 
similar to that which I mentioned as surrounding the neural canal and other organs ; 
it is quite structureless, but becomes coloured with all staining reagents. I must again 
leave open the question whether it is to be looked upon as a layer of coagulated proto- 
plasm or as a more definite structure. This investment completely surrounds the 
proximal portion of the posterior root, but vanishes near its distal extremity. 
The nerve-rudiment itself maybe divided into three distinct portions : — (l)the proximal 
portion, in which is situated the pedicle of attachment to the wall of the neural canal ; 
(2) an enlarged portion, which may conveniently, from its future fate, be called the 
ganglion; (3) a distal portion beyond this. The proximal portion presents a fairly 
uniform diameter, and ends dorsally in a rounded expansion ; it is attached remarkably 
enough, not by its extremity, but by its side, to the spinal cord. The dorsal extre- 
mities of the posterior nerves are therefore free ; as was before mentioned, they probably 
serve as the starting-point of the longitudinal commissures between the posterior roots. 
The spinal cord at this stage is still made up of fairly uniform cells, which do not 
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