SPINAL CORD REGENERATION. I 479 
cephalad and pass the fibers from the anterior end. They do 
not branch to any extent and do not innervate the myotomes. 
In addition to these fibers from the ventral region of the cord, 
the posterior segment sends out a few small fibers, containing 
neurofibrillae, toward the rostral end of the embryo. They 
are apparently derived from the dorso-lateral or primary sensory 
tract. The anterior segment of the cord has no nerve fibers 
which arise from this region. 
The nerves of the motor tract are less scattered than in the 
24 hour stage. They are grouped into two heavy strands which 
connect the ends of the cord and which pass along the upper 
portion of the mass of notochordal sheath cells, penetrating it 
in some places. Interwoven with the distinctly nervous fibers 
are others which are narrow and granular in appearance. They 
are connective tissue fibers from the mesenchyme cells through 
which the nerves have grown. 
These tadpoles illustrate the second phase of the primary 
reunion of the cord segments. The motor apparatus has bridged 
the gap and the nerves which will later establish the sensory 
connections are just beginning their development. It is impos- 
sible to determine whether the nerves from the anterior segment 
have established their connection with neurones in the hinder 
end of the cord or whether those from the posterior segment 
have reached their goal in the anterior end. The slightly im- 
perfect codrdination of the two ends does not make it seem 
probable that they have done so. 
Larvae 7 days after operation. The larvae at this time meas- 
ured from 7.75 to 9 mm. in length and were typically tadpole- 
like in shape. The yolk had been entirely absorbed and, in con- 
sequence, the deformity of many of the abnormally shaped 
larvae had increased. Those tadpoles which were not deformed 
presented only a slight nick in the dorsal fin to show the location 
of the cut. Physiologically, the undeformed tadpoles were nor- 
mal in every respect. They swam, reacted to stimuli and ate 
as well as did the controls. The deformed larvae had difficulty 
in locomotion due, in part, to the abnormal position of the tail 
and also to the lack of coédrdination between the two portions of 
the body. None of these were killed at this time. 
