Form Regulation in Harenactis 89 
But the essential point for our present purposes is that the 
phenomena of heteromorphosis in short pieces —primary heteromor- 
phosis—are the necessary consequence of the constitution of the 
material, when the length of the piece is so short that its axial 
differentiation is reduced practically to zero, and the correlations 
between parts consequently almost or quite eliminated. But, as 
pointed out above, the physiological state at the two ends of 
such pieces must after isolation become different from that at the 
middle, consequently the two opposed polarities result. 
These cases of primary heteromorphosis show us, according to 
this view, something of the differences of constitution along the 
axis. In such a case as that of Planaria simplicissima (Morgan, 
’04), where short pieces from near the old head form double heads 
and those from the posterior region double tails, it is evident that 
the two terminal regions of the body are qualitatively different 
in constitution. Moreover, these phenomena afford in my opin- 
ion the strongest evidence against the hypothesis that polarity 
consists in a directive organization or orientation. 
But correlations between the two terminal regions are not en- 
tirely absent in cases of primary heteromorphosis. [In Haren- 
actis, for example, the aboral tentacles in the cesophageal region 
always appear, not only later than the oral tentacles, at the 
opposite end of the piece, but later than oral tentacles at their 
own level. Evidently there is some slight physiological difference, 
at least a quantitative one, between the two ends of the piece, 
for the processes at the oral end retard those at the aboral end. 
Similar differences between the two ends occur sometimes or 
always in various other forms. In primary heteromorphosis in 
Tubularia I found that at least very commonly the aboral primor- 
dium or partial primordium was considerably delayed in the earlier 
stages (Child ’07d), though after emergence the two were usually 
more nearly alike. In Planaria the two heads are often different 
in size and one may appear before the other. 
Here again in the retarding influence of the oral or anterior end 
upon the aboral or posterior end.in short pieces we find a correla- 
tive factor of polarity. The fact that the development of the 
aboral structures is slower than that of oral structures at the same 
