92 C. M. Child 
oral tentacles in the same piece and also less rapid than that of 
oral tentacles from the same level of the body. 
4 In the subcesophageal region aboral tentacles never appear 
except under certain experimental conditions to be considered 
later | 
5 When only the most proximal regions of the body are re- 
moved restitution of a foot region occurs in a short time, but as the 
portion removed becomes larger the restitution of missing parts 
becomes less rapid and less complete, until in cases where most of 
the subcesophageal region is removed, only a slight degree of resti- 
tution occurs. Absence or incompleteness of aboral restitution 
in these cases is undoubtedly due wholly or in large part to the 
environmental conditions. When the animals are kept in water 
without sand in which they can burrow a considerable amount of 
reduction and atrophy occurs and this is greatest in the proximal 
regions and decreases distally. Under conditions in which these 
parts undergo atrophy when they are present it is not to be expected 
that their complete restitution should occur in pieces from which 
they have been removed. ‘The form and constitution of individ- 
uals living in burrows in the usual manner are different from those 
of individuals living in water without sand, and restitution of the 
form characteristic of the existing environment does occur in my 
experiments. If the short pieces could be kept in burrows it is 
probable that at least much more complete restitution of the form 
occurring in nature would take place. 
6 ‘Two groups of factors are involved in the phenomena of 
polarity as they appear in restitution: these are constitution and 
correlation. ‘The regulatory phenomena of polarity may be 
grouped under four heads as follows: 
a Qualitative axial or polar differences in restitution (hetero- 
polar phenomena): the same cells may produce qualitatively dif- 
ferent structures according to their position at one pole or the other 
of the piece. 
b Qualitative regional differences (primary heteromorphosis) : 
short pieces from near one pole or the other, or in some cases both, 
may produce at both ends structures like those present at the near- 
est pole of the original individual. 
