Form Regulation in Harenactis 81 
directive organization along the polar axis as the basis of polarity, 
an organization of the ultimate particles or elements of the proto- 
plasm, such as Driesch assumes. But I believe that these facts 
alone afford not only an inadequate, but an incorrect conception 
of the nature of physiological polarity so-called. Inorder to under- 
stand the organization with reference to the “polar”’ axis it is 
necessary to consider all the essential facts which indicate differ- 
ence in capacity of any kind, quantitative or qualitative, along the 
axis. In addition to the qualitative differences which commonly 
occur at the two ends of the polar axis, there are three other groups 
of facts which must be considered before we can safely draw any 
conclusions with respect to the nature of polarity. ‘These are as 
follows: First, in many forms the regulation of isolated pieces 
from regions near the oral or anterior end, and in some cases of 
pieces from the aboral or posterior regions, does not show the char- 
acteristic qualitative differences at the two ends of the axis. In 
such pieces the structures formed at the two ends are alike, except 
perhaps in size and time of appearance. These phenomena are 
commonly termed axial heteromorphosis. In the cesophageal 
region of Harenactis, for example, the formation of tentacles at 
the aboral end of the piece is just as characteristic as is the forma- 
tion of tentacles at one end and a foot at the other in pieces from 
the subcesophageal region. Similar axial heteromorphosis has 
been observed in various other ccelenterates: in Planaria maculata 
and P. dorotocephala heads frequently appear at both ends of short 
pieces from the region near the old head and the region of fission, 
where the head of the new zooid is at least physiologically speci- 
fied, and in P. simplicissima, which does not undergo fission, hetero- 
morphic heads appear in short pieces from near the anterior end 
(Morgan ’o4). Numerous other cases have been noted. 
In some species we find also that in regions posterior to a given 
level aboral or posterior structures are formed from the oral or 
anterior as well as from the aboral or posterior end. The for- 
mation of tails at the anterior ends of pieces from the posterior half 
of the earthworm (Morgan ’99), and of short posterior pieces of 
Planaria simplicissima (Morgan ’o4) will serve as examples. 
Evidently then the heteromorphic formation of the structures 
