ABSTRACT
The aim of the experiments was to see how different levels in the time-graded head-regeneration field of two species, differing strongly as to the characteristics of the field, might behave at different temperatures.
As was expected, regeneration was retarded for all levels by lowering the temperature.
Regeneration of heads was retarded relatively more strongly at posterior levels of the field.
Some characteristic differences in regeneration rates of the two species were found, leading to certain suggestions as to the possible nature of the field.
By lowering the temperature regeneration may be separated into two main components : formation of the blastema and differentiation of the cells.
Formation of the blastema is not seriously impeded at temperatures below 5–6° C., whereas the differentiation processes are nearly brought to a stop at these low temperatures.
Doubt is therefore cast on the hypothesis of an adaptive value of regenera-tion in view of the fact that several planarian species actually live and reproduce at temperatures well below those necessary for regeneration.
