Fig. 5. Theoretical reaction norms for myotube production in fast muscle at different temperatures for (A) the embryonic and (B) the stratified hyperplasia stages of myogenesis prior to hatching. (C) The total number of fast muscle fibres per myotomal cross-section present at hatching at the different temperatures. Two reaction norms are illustrated (open and filled circles). In the situation illustrated by the open circles fast fibre number at hatching is apparently independent of temperature, but this is a consequences of the different reaction norms for embryonic and stratified hyperplasia. The norm of reaction illustrated by the filled symbols shows a temperature optimum for myotube production that differs somewhat for the two phases of myogenesis. Note in this example that studying fish reared at just 4 and 12°C and sampled at hatch would result in the erroneous conclusion that muscle fibre production was independent of temperature.