1. The eggs of the cut-throat trout (Salmo clarkii clarkii Richardson) were raised from fertilisation through hatching at constant temperatures of 11.3, 8.25 and 6.35°C. These temperatures are well within the limits of normal development of cut-throat trout.
2. The temperature is not the limiting factor in determining the length of the hatching period, because there are a number of other factors that may cause eggs to hatch either prematurely early or abnormally late, and thus the use of the hatching period as a comparable stage of development for eggs raised at different temperatures may result in error.
3. The average size of cut-throat trout embryos at the moment of hatching was smaller at the higher temperatures and larger at the lower temperatures.
4. The maximum increment of growth in the cut-throat trout occurred about the 41st day at 8.25°C. and about the 28th day at 11.3°C.
5. The percentage of dry weight of the cut-throat trout embryos showed a steady decrease from about 25 to 14 per cent., while that of the yolks showed an increase from about 46 to 55 per cent.
6. Cut-throat trout embryos absorbed water from the yolk and also, at a faster rate, from the environment, both somewhat before and after hatching, so that the percentage of wet weight of the larvae was steadily increasing up to the time the experiment was completed.
The author expresses his thanks to Dr Leonard P. Schultz of the Department of Fisheries, University of Washington, Seattle, Washington, both for the suggestion of the problem and for constant advice and help in all phases of the work. To Dr W. F. Thompson, Director of the International Fisheries Commission (United States and Canada), the author is grateful for permission to use the Commission's constant-temperature machine from April through June of 1933 and 1934, and also for much valuable advice.