492 DAVENPORT HOOKER 
THE FAILURES 
Under this heading may be grouped those cases in which, 
for one reason or another, the spinal cord has failed to reéstablish 
complete anatomical continuity. There are two principal caus- 
ative factors in this failure: excessive separation of the cut ends 
of the cord and the interposition of mechanical obstacles. These 
two factors are frequently found associated with one another. 
As has been noted already, some embryos present abnormali- 
ties in body form following the absorption of the yolk and, in 
most cases, the deformity increases with further growth. It is 
chiefly characterized by the ventral bending of the hind part of 
the body on the front. As a result, the two ends of the spinal 
cord become widely separated from one another. The regenera- 
tive processes begin by the outgrowth of nerve fibers, as in the 
more favorable cases, but the establishment of primary reunion 
between the cord ends is rare. The developing nerve fibers have 
a tendency to remain in bundles, without spreading, for some 
distance from the end of the stump. In those embryos in which | 
primary reunion of the cord is effected, the nerves remain grouped 
across the gap, but where the distance between the cord ends is 
unusually great, spreading of the fibers occurs. They grow in 
all directions and branch frequently. Some of the branches 
occasionally innervate muscle tissue in the vicinity, the others 
lose themselves in the mesenchyme. In certain cases, a few 
fibers may reach the other end of the cord, but it is the exception 
to find nervous connections between the two stumps. The other 
steps toward complete form restoration of the cord proceed even 
in the absence of primary reunion. Neuroblasts migrate into 
the fiber bundles to some extent and the cells of the central canal 
multiply in number. These processes seem to continue for a 
time and then to cease. 
The notochord provides the most effective mechanical ob- 
struction to the restoration of the cord. In many of the de- 
formed tadpoles, one end of the notochord is found turned up 
between the ends of the spinal cord. The upturned end of the 
notochord is usually connected with the skin by a mass of tissue 
which has proliferated from its sheath and which effectually 
