88 C. M. Child 
Formative substances, so-called, do not assist us to explain 
these facts, they merely state the facts. It is self-evident that 
where a head is formed there must be “head-forming substances” 
the real problems lie in. the questions of how they come to be there, 
and why in the same cells “head-forming substances”’ are active 
at one time and “‘tail-forming substances” at another. 
My analysis of these qualitative polar differences or heteropolar 
phenomena in restitution is then as follows: first the tissues involved 
possess at the time of operation a constitution capable of either 
reaction; second, their constitution and consequently their reac- 
tion is altered in one direction or the other according to the char- 
acter of their physiological correlations with other parts. ‘These 
correlations may be either chemical or physical or both; in all 
probability they are exceedingly complex, as they are known tobe 
in plants. 
In the primary heteromorphoses, i. e., in those cases of hetero- 
morphosis where the isolated piece is so short that polar differ- 
ences are almost or quite absent at its two ends, we have another 
characteristic restitutional phenomenon. In such pieces the 
two ends are physiologically almost identical, consequently the 
correlations between them can have little or no effect in producing 
a difference between them and any effect must be slight. It 
follows then necessarily that if morphogenic reactions occur at 
the two ends of the piece they will be almost or quite similar in 
character. But the constitution of such pieces is different accord- 
ing as they are taken from one or the other terminal region of the 
original individual and consequently such pieces may give rise 
either to heteromorphic heads or tails, oral or aboral structures 
according to their position in the body. Sometimes, as in the 
case of Tubularia (Child ’07d), the conditions of experiment, in 
this case the formation of free terminal regions, play a part in 
determining the character of the reaction, and in various other 
cases, as in Harenactis, pieces from the extreme aboral or posterior 
region apparently have not sufficient energy, when isolated, to 
produce anything. Under these conditions only primary oral 
heteromorphosis may appear, and this may be limited to the more 
oral region as in Harenactis, or may occur at any level of the body 
if the pieces are sufhciently small, as in Tubularia. 
