422 DAVENPORT HOOKER 
their tail buds and, after from two to six days, cut them apart 
again, leaving a portion of the tail of one fixed in reversed posi- 
tion to that of the other. From this reversed piece a nearly 
normal tail regenerated, but the spinal cord of the graft con- 
tained ‘‘few or no ganglion cells or nerve fibers’ and remained 
in a very rudimentary condition. As described in his later 
paper, Harrison made a composite embryo of a head and tail in 
normal orientation, but with a reversed middle section. These 
embryos were at first helpless, owing to the fact that each com- 
ponent reacted independently of the rest. This, however, was 
gradually overcome until almost perfect coordination resulted 
and in some cases the embryos lived for weeks. 
Spemann (’12) removed a portion of the medullary plate as 
soon as it became visible and regrafted it, having turned it end- 
for-end. As the edges of the wound were carefully apposed, 
healing per primum resulted. The portions of the brain anlagen, 
which were thus reversed, retained their original polarity. 
Spemann worked with embryos of Rana, Bombinator and Triton. 
In the present experiments, a piece of spinal cord about 1 mm. 
in length was removed, turned end-for-end and grafted into the 
space from which it has been taken. In the greater number of 
cases the anterior cut passed through the extreme caudal portion 
of the medulla, the posterior cut being 1 mm. caudad to it. For 
the most part the embryos operated upon were those of Rana 
sylvatica, but R. palustris and R. pipiens were also used. All 
these embryos healed well and lived for a considerable period. 
EXPERIMENTS 
Methods. Before each set of operations was performed, a large 
number of embryos were freed from the jelly-mass and egg- 
membranes. The animals to be used in the experiments were 
carefully chosen from this number. The factors upon which the 
selection was based were uniformity of size, of stage of develop- 
ment and a healthy appearance. Especial care was taken in 
matching the normal control animal to each operated specimen. 
The greater number of the embryos were operated upon in the 
just closed neural fold stage and were from 2.5 to 3.5 mm. long. 
