ABSTRACT
Hydra pseudoligactis cultured at 9°C for 3 – 4 weeks are one-and-a-half times larger than those cultured at 18°C. The size of Hydra is correlated with the numbers of epithelio-muscular and digestive cells in the distal portion of the animal and with the diameters of the epithelio-muscular cells in the peduncle.
Counts of mitotic figures and tritiated-thymidine-labeled nuclei and determinations of increase in mass of Hydra populations suggest that the difference caused by these temperatures does not affect mitosis. At 9°C buds are initiated at a lower rate and take longer to develop than at 18°C. The surface-areas of buds raised at the two temperatures are similar.
Because Hydra raised at the two temperatures have similar growth dynamics, the differences in sizes of the animals cannot be due to growth rate. The observed effect of temperature on bud initiation and development is probably relevant to the increased size of animals raised at 9°C, since these larger animals may be accumulating more cells while losing fewer to buds.
