72 J. M. D. OLMSTED 
become the head-forming region.’’ The factors which were 
enumerated above can all'be expressed in terms of Child’s axial 
gradient theory, though one feels that polarity is a more deep- 
seated phenomenon than differences in metabolic rates. 
Morgan (’98, p. 374) makes a statement with reference to 
long narrow side pieces which is very suggestive. “It may be 
claimed that the small, single-headed worms with the long axis 
at right angles to the original long axis, do not really stand in 
this relation to the old part, but the head is at the anterior end 
of the piece, and being fused with the entire inner edge of the 
crescent is unable to swing around later into position.” It 
has usually been considered that in regeneration from oblique 
cuts the polarity of the new material undergoes change, since 
the axis connecting the new head and tail at first hes at right 
angles to the cut edges, but later comes into line with the old 
chief axis. However, the fact that the geometrical median axis 
of the regenerating worm does not pass through the new head and 
tail does not necessarily prove that the structural axis of these 
new parts is discontinuous with the axis of the old part, nor does 
it prove that there is any essential difference in the polarity of 
old and new parts. Sections show that the new systems of the 
worm, especially the nervous and digestive systems, are perfectly 
continuous with the old systems; the median axis, instead of 
being a straight line, as in the fully regenerated worm, is, in the 
early stages of regeneration, a curved line, which may even 
approach a semicircle. At Dr. Rand’s suggestion, experiments 
devised to test the polarity of old and new tissues in regeneration 
were carried out. An oblique cut anterior to the pharynx, and 
quite across the body, was made on thirty planarians (fig. 14a). 
As soon as the head appeared (in its usual position at right 
angles to the cut), a second cut was made as in figure 14b. The 
resulting piece of planarian was thus given a form which closely 
resembled that of the typical planarian, and one which was 
almost geometrically symmetrical, but whose geometrical long 
axis did not coincide with its original long axis. Had the polarity 
of the new head along the obliquely cut edge been truly different 
from that of the rest of the worm, one would expect that under 
