THE SPINAL NEE YES IN ELASMOBEANCH EISHES. 
177 
In most points the sections scarcely differ from Plate 16. fig. A, which, indeed, might 
very well he a posterior section of the embryo to which these three sections belong. 
The chief point, in addition to the formation of the spinal nerves, which shows the 
greater age of the embryo from which the sections were taken is the complete formation 
of the aortse. 
The upper ends of the muscle-plates have grown no further round the neural canal 
than in fig. A, and no scattered mesoblastic connective-tissue cells are visible. 
In fig. A the dorsal surface of the neural canal was as completely rounded off as 
the ventral surface ; but in fig. B hi this has ceased to be the case. The cells at the 
dorsal surface of the neural canal have become rounder and smaller and begun to proli- 
ferate, and the uniform outline of the neural canal has here become broken (fig. B m,pr). 
The peculiar membrane completely surrounding the canal in fig. A now terminates just 
below the point where the proliferation of cells is taking place. 
The prominence of cells which springs in this way from the top of the neural canal 
is the commencing rudiment of a pair of spinal nerves. In fig. B n, a section anterior to 
fig. B hi, this formation has advanced much further (fig. B 11 , pr). From the extreme 
top of the neural canal there have now grown out two club-shaped masses of cells one 
on each side ; they are perfectly continuous with the cells which form the extreme top 
of the neural canal, and necessarily also are in contact with each other dorsally. Each 
grows outwards in contact with the walls of the neural canal; but, except at the point 
where they take their origin, they are not continuous with its walls, and are perfectly 
well separated by a sharp line from them. 
In fig. B i, though the club-shaped processes still retain their attachment to the 
summit of the neural canal, they have become much longer and more conspicuous. 
Specimens hardened in both chromic acid (Plate 16. fig. C) and picric acid give similar 
appearances as to the formation of these bodies. 
In those hardened in osmic acid, though the mutual relations of the masses of cells 
are very clear, yet it is difficult to distinguish the outlines of the individual cells. 
In the chromic-acid specimens (fig. C) the cells of these rudiments appear rounded, 
and each of them contains a large nucleus. 
I have been unable to prepare longitudinal sections of this stage, either horizontal or 
vertical, to show satisfactorily the extreme summit of the spinal cord ; but I would 
call attention to the fact that the cells forming the proximal portion of the outgrowth 
are seen in every transverse section at this stage, and therefore exist the whole way 
along, whereas the distal portion is seen only in every third or fourth section, according 
to the thickness of the sections. It may be concluded from this that there appears a con- 
tinuous outgrowth from the spinal canal, from which discontinuous processes grow out. 
In specimens of a very much later period (Plate 18. fig. L) the proximal portions of the 
outgrowth are unquestionably continuous with each other, though their actual junctions 
with the spinal cord are very limited in extent. The fact of this continuity at a later 
period is strongly in favour of the view that the posterior branches of the spinal nerves 
2 b 2 
