84 C. M. Child 
physiological specification and capacities that we may call the 
differences qualitative and not quantitative. In other words the 
extreme anterior region is capable of reacting only in a particular 
manner, i. €., soasto produce heads at either or both ends, while the 
extreme posterior region can produce onlytails. In both cases the 
production of the characteristic structures fails to show any indication 
of a directive organization: the organization, specification, or what- 
ever we may call the reactive capacity is, so far as the facts go, very 
clearly regional and direction is determined merely by the rela- 
tions between the cut surface and other parts. I believethen that 
the facts justify the conclusion that the terminal regions of the 
body may differ so widely from each other in their constitution 
as to be incapable of reacting similarly at any point; consequently 
when pieces from these regions are isolated they produce similar 
structures at both ends if they produce anything, and the struc- 
tures produced are those characteristic of that end of the parent 
body from which the piece is taken. Very commonly, as in Haren- 
actis, primary heteromorphosis occurs only in the anterior or oral 
region and short pieces from the extreme proterior or aboral region 
form nothing. ‘This difference is probably due, however, as was 
noted above, rather to a difference in energy or available 
material than to any fundamental difference in organization. 
There is abundant evidence to support the conclusion that in at 
least a very large number of forms the anterior or oral pole is 
primarily the region of greatest formative energy or activity, or 
of greatest rapidity of reaction. 
We cannot then, I believe, escape the conclusion, that qualita- 
tive regional differences in the terminal portions of the body con- 
stitute in at least certain forms a feature of organic polarity, as 
well as the qualitative differences which usually appear at the 
two ends of the axis of the piece. In short the terminal regions 
of the body may be respectively wholly oral or anterior and wholly 
aboral or posterior as regards their “potences’”’ and primary heter- 
omorphosis is the necessary consequence of this extreme specifi- 
cation, provided sufficient energy for morphogenic reactions is 
present. 
In addition to these two features of axial specification which we 
