SPINAL CORD REGENERATION. I 491 
ism of Amblystoma, that an exteroceptive stimulus in front of 
the cut should excite a motor reflex in the myotomes of the 
opposite side, which would follow down the motor connection 
into the otherwise nervously isolated tail. A proprioceptive 
stimulus set up by the contraction of the myotomes in the ante- 
rior end of the body could then start a motor response in the side 
originally stimulated and thus cause a swimming movement. 
But an exteroceptive stimulus to the posterior end of the body 
causes a Swimming response and there are no sensory con- 
nections between it and the anterior end. Furthermore, in the 
2 day stage, the embryos will swim, though very imperfectly, if 
the head movements become sufficiently vigorous. As yet no 
anatomical basis for these phenomena is at hand. It is possible 
that the independent motion of the hinder end of the 4 day 
embryos is effected through axone reflexes. It must be remem- . 
bered in this respect that some of the myotomes behind the cut 
are innervated by motor nerves from the anterior end of the 
cord in this stage. 
The following hypothesis is offered as a possible explanation 
of the swimming movements exhibited by the 2 day stage. In 
a previous paper (Hooker, ’11), it was noted that an increase 
in tension of the skin over a myotome causes it to contract. 
When the head of an embryo is bent toward one side, it causes 
increased tension of the skin over the opposite side of the poste- 
rior half. If this stimulated the myotomes of that side to con- 
traction, a condition similar to Coghill’s S-reaction would result. 
The return movement of the head would cause the same pro- 
cedure on the other side of the embryo. It is possible that re- 
peated movements of the head should, by summation, excite 
the myotomes to contract alternately on the two sides of the 
body. This would give rise to a very primitive type of swimming 
movement. However this may be, no definite conclusions can 
be drawn legitimately until the correlation of anatomical struc- 
ture to behavior has been as thoroughly investigated in the frog 
as it has in Amblystoma. 
