A STUDY IN POLARITY 161 
cent shaped, but as regeneration proceeds, it straightens out, 
until a perfectly bilaterally symmetrical planarian is produced. 
The number of cases in Series I illustrating this type was 6, in 
Series IT, 10. 
This retention of the original polarity occurs even if the angle 
between the cut edges is quite small, for example 55°, and seems 
also to be independent of the size of the triangular piece. - Sec- 
tions of the early stages of regeneration of such pieces show that 
portions of both the original lateral nerve cords are present, and 
that the new, regenerated nerve cords become linked up with 
the old to form, eventually, an unbroken loop, the new brain 
Fig. 1 Triangular piece from left side of worm, 6 days after operation. Draw- 
ing made by reconstruction from serial sections. The broken line represents the 
boundary between old and new tissue, the new being to the right. The old nerve 
cords are stippled, the new brain crosshatched. One eye and the pharynx are 
present. 
lying at the bend of the loop. Figure 1 illustrates a stage in 
which the nerve cord on the left side of the piece (note that the 
piece was taken from the left side of a worm) and the brain 
have been completed, but in which the brain has not yet estab- 
lished connection with the portion of the original right nerve 
cord. 
B. If the vertex lies at or near the old median axis, and the 
angle at the vertex is greater than 90°, the regenerated worm 
retains its original polarity, unless the’ piece is from a very 
small worm. In the latter case (figs. 2 to 4) there appears tobe 
an attempt to return to the old polarity; but lack of sufficient 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 25, NO 1 
