A STUDY IN POLARITY 171 
It is very evident that such a head-forming tendency is present 
and that, with rare exceptions, it is always in operation. This 
tendency always to produce a head, together with the small 
amount of new material available, are two most potent factors 
in the regeneration of small pieces of planarian tissue of any 
shape. But my experiments show that there are still other 
factors which must be taken into account, since some small 
triangular pieces retain their original polarity and develop both 
a pharynx and a tail in addition to a head, while others, which 
may actually be larger, regenerate heads, and heads only, at 
| Nios 
Fig. 13 The piece within the broken lines, which represent the cuts made, 
regenerated no head. 
Fig. 14 a, First cut indicated by oblique unbroken line. 6, Head regenerating 
after first cut. Second cut indicated by unbroken line. Broken line indicates 
limit between old and new tissue; c, 4 days after second cut; d, final regenerated 
form. Original polarity restored. 
right angles to the old axis. These other factors, brought out 
in the experimental work, are (1) the position of the vertex of 
the triangle. (2) the size of the angle at the vertex, and (3) 
the ratio of the lengths of the anterior and posterior cut edges. 
Child’s (15, p. 165) explanation for the position of the heads in 
these triangular pieces is, ‘“‘In such pieces there is little difference 
in metabolic rate between apical and basal cut surfaces, and the 
cuts are not sufficiently oblique so that the higher level in the 
major gradient of the lateral as compared with the median region 
of the cut surface overbalances its lower level in the transverse 
gradient. Consequently the median regions of both cut surfaces 
represent the region of highest rate or irritability and therefore 
