THE SPINAL NEEYES IN ELASMOBEANCH FISHES. 
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rod ; ao, rudiment of dorsal aorta ; so, somatopleure ; sp, splanchnopleure ; 
al, alimentary tract. All the parts of the section except the spinal cord are 
drawn somewhat diagrammatically. 
.Figs. B i, B ii, B hi. Three sections of a Pristiurus-e mbryo. B i is through the heart, 
B ii through the anterior part of the dorsal region, and B hi through a 
point slightly behind this. Drawn with a camera. (Zeiss CC ocul. 2.) 
In B hi there is visible a slight proliferation of cells from the dorsal 
summit of the neural canal. 
In B ii this proliferation definitely constitutes two club-shaped masses of 
cells (pr), both attached to the dorsal summit of the neural canal. The 
masses are the rudiments of the posterior nerve-roots. 
In B i the rudiments of the posterior roots are of considerable length. 
pr, rudiment of posterior roots ; no, neural canal ; mp, muscle-plate ; 
ch, notochord ; x, subnotochordal rod ; ao, dorsal aorta ; so, somatopleure ; 
sp, splanchnopleure ; al, alimentary canal ; Tit , heart. 
Fig. C. Section from a Pristiurus-embryo, slightly older than B. Camera. (Zeiss CC 
ocul. 2.) The embryo from which this figure was taken was slightly distorted 
in the process of removal from the blastoderm. 
vr, rudiment of vertebral body. Other reference letters as in previous 
figures. 
Fig. D a. Section through the dorsal region of a Torpedo-e mbryo with three visceral 
clefts. (Zeiss CC ocul. 2.) The section shows the formation of the dorsal 
nerve-rudiments (pr) and of a ventral anterior nerve-rudiment ( ar ), which at 
this early stage is not distinctly cellular. 
ar, rudiment of an anterior nerve-root ; y, cells left behind on the separation 
of the external skin from the spinal cord ; c, connective-tissue cells springing 
from the summit of the muscle-plates. Other reference letters as above. 
Fig. D b. Section from dorsal region of a Torpedo-embryo somewhat older than D a. 
Camera. (Zeiss CC ocul. 2.) The posterior nerve-rudiment is considerably 
longer than in fig. D a, and its pedicle of attachment to the spinal cord is 
thinner. The anterior nerve-rudiment, of which only the edge is present 
in the section, is distinctly cellular. 
to, mesoblast growing up from vertebral rudiment ; sd, segmental duct. 
Fig. D c. Section from a still older Torpedo-embryo. Camera. (Zeiss CC ocul. 2.) The 
connective-tissue cells are omitted. The rudiment of the ganglion ( g ) on the 
posterior root has appeared. The rudiment of the posterior nerve is much 
longer than before, and its junction with the spinal cord is difficult to 
detect. The anterior root is now an elongated cellular structure. 
g, ganglion. 
Fig. D d. Longitudinal and vertical section through a Torpedo-embryo of the same 
age as D c. 
The section shows the commissures uniting the posterior roots. 
