160 yo J. M. D. OLMSTED 
projecting point on the triangular piece becomes rounded off by 
loss of material (cf. Morgan, ’98, p. 393). Immediately after 
the operation there is always a very slight contraction of the cut 
edges, even though the piece is still immersed in chloretone. 
This, no doubt, is due to the direct stimulation of the muscle 
fibers. As soon as the effect of the narcotic is gone, the piece 
contracts greatly, often assuming the form of a hollow cone, the 
apex of which lies approximately at the center of the dorsal 
surface of the piece. Epithelial cells soon cover the wound 
(Lang, 712, p. 272), and after twenty-four hours new white tissue 
can be seen along the cut edges. This new material is never 
evenly distributed along the cut edges, but (figs. 2b, 5c, 6c) 
a greater amount of it appears near the center of the anterior 
edge, a less amount along the posterior edge, and very little at 
the vertex (cf. Morgan, 798, p. 378). <A further noticeable fea- 
ture is the lengthening of the posterior side (figs. 5b, 6c, 7c). 
Even though the two cut edges have the same length immediately 
after cutting, a few days later the posterior edge is almost in- 
variably the longer. These phenomena indicate that there is 
a tendency on the part of the triangular piece to retain its original 
polarity, since the regeneration of a head demands more material 
than that of a tail, and the lengthening of the posterior edge 
would more quickly restore the normal planarian form. 
Several factors influence the subsequent history of these 
triangular pieces: (1) size of the piece, and, closely related to 
this, (2) the angle between the cut edges, (38) the position of the 
‘vertex’ with reference to the original median axis, and (4) the 
relative lengths of the anterior and posterior cut edges. 
The following four rules seem to be obeyed in the regeneration 
of these triangular pieces. 
A. If the vertex of the triangular piece lies beyond the median 
axis of the worm from which it is taken, the new worm which 
is formed by the regeneration of this triangle retains the polarity 
of the original worm, for instance, a new head appears at the 
anterior cut edge, a new tail at the posterior cut edge, and, near 
the old median axis, a pharynx, which points toward the tip 
of the new tail (fig. 1). At first the regenerating worm is cres- 
