Form Regulation in Harenactis 83 
Planaria are primary heteromorphoses, also the double hydranths 
or partial structures in short pieces of Tubularia (Child ’07c). 
Theoretically primary heteromorphosis may occur at any region 
of the body if sufficiently short pieces are isolated; whether it will 
actually occur or not will depend upon whether the pieces possess 
sufficient energy to produce morphogenic results. 
Secondary heteromorphoses occur when the original axial speci- 
fication of the organism is present in the piece at the time of isola- 
tion, but cannot persist under the conditions of the experiment. 
Under certain conditions secondary heteromorphoses occur in Har- 
enactis, but these will be considered in a following paper. In 
Tubularia the formation of aboral hydranths in long pieces of the 
stem is, as I have shown (Child ’o7a, ’o7c, ’07e), a secondary 
heteromorphosis. 
In the case of primary heteromorphosis, then, we obliterate or 
almost obliterate the differentiation or specification in the axial 
direction by isolating a piece so short that its two ends are almost 
or quite similar physiologically: in secondary heteromorphosis the 
original organization persisting in the isolated piece at firstis 
more or less completely obliterated by conditions arising after 
isolation of the piece. In Harenactis, for example, the union of 
the oral with the aboral end is apparently a factor, probably the 
chief factor, in obliterating the original polarity, and thus, so to 
speak, clearing the way for the establishment of a new polarity. 
But the important point for the present consideration is that 
the phenomena of primary axial heteromorphosis indicate that the 
polar specification is not uniformly distributed along the axis. 
In the whole cesophageal region of Harenactis, for example, there 
is, according to this view, nothing more than a slight quantitative 
difference at the two ends of the axis of any piece, i. e., polarity in 
the usual sense of qualitative polar differences is absent or prac- 
tically absent from this region. If my position 1s correct the same 
must be true for any region of any species in which primary hetero- 
morphosis occurs. And finally, in forms like Planaria simplicis- 
sima where short pieces from regions near the old head, form 
heteromorphic heads and short posterior pieces form heteromor- 
phic tails, these two regions must be so widely different in their 
