162 J. M. D. OLMSTED 
material, perhaps, prevents the complete success of this effort. 
Under all circumstances throughout my experiments, with very 
few exceptions, a new head was formed. It would seem, then, 
that the tendency to produce a head is so strong that the re- 
generation of other structures and the attainment of the typical 
‘werter 
post. 
b c d 
Fig. 2 Stages of regeneration of a small triangle whose angle at the vertex 
was greater than 90°. a, Whole worm showing region from which piece was taken. 
b, 3 days after operation. All tissue to left of broken line is new. c, 5 days after 
operation. d, 11 days after operation. No tail is formed, yet the head is fairly 
near the anterior end of the piece. 
. Fig. 3 Triangle from right side of a small worm, angle at vertex greater than 
90°. 7 days after operation. 
Fig. 4 Similar to 3, but from a larger worm, therefore head almost normal in 
position. 
planarian form is dependent upon the amount of material avail- 
able after the accomplishment of that object. The number of 
cases coming under this group was, in Series I, 10, in Series IT, 3. 
C. If, when the vertex lies at or near the old median axis, 
and the angle at the vertex is 90° or less, the two cut edges are 
