THE SPINAL NERVES IN ELASMOBRANCH FISHES. 
179 
From the summit of the muscle-plates (mp) an outgrowth of connective-tissue has 
made its appearance (c), which eventually fills up the space between the dorsal surface 
of the cord and the external epiblast. There is not the slightest difficulty in distin- 
guishing the connective-tissue cells from the nerve-rudiment. I believe that in this 
embryo the origin of the nerves from the neural canal was a continuous one, though 
naturally the peripheral ends of the nerve-rudiments were separate from each other. 
The most interesting feature of the stage is the commencing formation of the anterior 
roots. Each of these arises (Plate 16. fig. D a, ar) as a small but distinct outgrowth from 
the epiblast of the spinal cord, near the ventral corner of which it appears as a conical 
projection. Even from the very first it has an indistinct form of termination and a 
fibrous appearance, while the protoplasm of which it is composed becomes very 
attenuated towards its termination. 
The points of origin of the anterior roots from the spinal cords are separated from 
each other by considerable intervals. • In this fact, and also in the nerves of the two 
sides never being united with each other in the ventral median line, the anterior roots 
exhibit a marked contrast to the posterior. 
There exists, then, in Toipedo-embvyos by the end of this stage distinct rudiments of 
both the anterior and posterior roots of the spinal nerves. These rudiments are at first 
quite independent of and disconnected with each other, and both take their rise as 
outgrowths of the epiblast of the neural canal. 
The next Torpedo-e mbryo (Plate 16. fig. Db), though taken from the same female, 
is somewhat older than the one last described. The cells of the notochord are consider- 
ably vacuolated ; but the segmental duct is still without a lumen. The posterior nerve- 
rudiments are elongated, pear-shaped bodies of considerable size, and, growing in a 
ventral direction, have reached a point nearly opposite the base of the neural canal. 
They still remain attached to the top of the neural canal, though the connexion has in 
each case become a pedicle so narrow that it can only be observed with great difficulty. 
It is fairly certain that by this stage each posterior nerve-rudiment has its own 
separate and independent junction with the spinal cord; their dorsal extremities are 
nevertheless probably connected with each other by a continuous commissure. 
The cells composing the rudiments are still round, and have, in fact, undergone no 
important modifications since the last stage. 
The important feature of the section figured (fig. D b), and one which it shares with 
the other sections of the same embryo, is the appearance of connective-tissue cells around 
the nerve-rudiment. These cells arise from two sources ; one of these is supplied by 
the vertebral rudiments, which at the end of the last stage (Plate 16. fig. C, vr) become 
split off from the inner layer of the muscle-plates. The vertebral rudiments have 
in fact commenced to grow up on each side of the neural canal, in order to form the 
mass of cells out of which the neural arches are subsequently developed. 
The dorsal extremities of the muscle-plates form the second source of these con- 
nective-tissue cells. These latter cells lie dorsal and external to the nerve-rudiments. 
