18 Ethel Nicholson Browne 
foot close to the line of union. Heteromorphic heads were pro- 
duced in five cases on the exposed aboral surface. King succeeded 
in obtaining heteromorphicstructures by cutting off both ends of a 
head-to-head or a foot-to-foot graft close to the line of union, 
leaving a ring of tissue with two aboral or two oral ends exposed. 
In most cases, she found that normal hydras resulted, one end 
having reversed its polarity so that a head was produced from the 
exposed aboral surface or a foot from the exposed oral surface. 
It has never been definitely shown whether these heteromorphic 
structures are really formed from material whose polarity has been 
reversed through the influence of the complementary structure or 
whether the material has rearranged itself so that polarity is not 
really reversed. 
That such a rearrangement is possible is shown by an experi- 
ment in which I| cut a hydra.longitudinally and reversed the two 
halves so that each free end consisted of half foot and half head 
with tentacles (Fig. 58). Two days later it was evident that 
the foot material of each end was migrating from its position near 
the tentacles to the middle of the body (Fig. 59). ‘That this 
structure was produced by a migration of the foot material and not 
merely by a split along the line of graft, separating the original half 
head and half foot, is shown by the fact that the free foot was not 
of equal length with the hydranth, but was only a small projection 
from the surface. ‘The final result of this graft was the separation 
of the two original half-hydras into two complete hydras (Figs. 60, 
61). In this case, then, there has been a migration and rearrange- 
ment of material. Another instance of migration is the experi- 
ment described in Group C, where the grafted hydranth moved 
down along the stock (Fig. 46) from the middle to the foot region. 
Many similar cases of migration have been described by King and 
Rand and Hefferan. 
Is it not possible that in the case of the supposed heteromorphic 
heads, the aboral material of the graft has wandered in, leaving 
exposed the oral material of either stock or graft, so that the head 
really develops not from an aboral layer of tissue but from the 
oral layer? 
The answer to this question has been definitely determined, I 
