Chapter 1. General Introduction The general Introduction begins with a brief description of the natural history of the plaice. This is followed by a review of the literature describing the development of teleost embryos and larvae. The pattern of innervation with development and the differentiation of muscle fibres are both discussed in detail, as are larval locomotion and patterns of swimming behaviour. The remainder of the general introduction concentrates upon the phenotypic plasticity of muscle and the mechanisms by which this may be achieved. Teleost muscle in particular has been found to show an extremely plastic response to changes in ambient temperature. The contractile protein composition, ultrastructure and contraction properties of fish muscle are all influenced by temperature. However most research has concentrated primarily upon adult teleosts and fewer studies have examined the effects of temperature on muscle in larval and juvenile fish. Experiments describing the effect of temperature upon muscle structure and contraction both in the larval and adult stages are discussed at the end of the Introduction. Chapter 3. The influence of temperature on somitogenesis and organogenesis in embryos of the plaice, Pleuronectes platessa L. The development of plaice embryos was examined at two temperatures, 8° and 12°. Temperature influenced the rate of embryonic development but did not affect the order of appearance of morphological characters. The rate of somitogenesis was observed in embryos reared at 5° and 12°. Chapter 4. Muscle development in plaice, Pleuronectes platessa L. The effects of temperature on muscle fibre ultrastructure were examined using larvae reared at 5, 8, 10, 12 and 15°. Two types of muscle fibre could be distinguished in the newly hatched larvae; beneath the skin was a single, superficial layer of smaller diameter muscle fibres, which completely surrounded the larger diameter inner muscle fibres. Larvae reared at 15° only survived for a few days and had significantly more myotomal muscle fibres of larger average cross-sectional area than those hatching at 5-10°. Chapter 5. Electrophoretic analysis of the myofibrillar components of red and white muscle fibres from adult plaice, Pleuronectes platessa L. The myofibrillar proteins of the two main fibre types were identified using various electrophoretic techniques, including SDS PAGE, IEF PAGE, NEPHGE PAGE and alkaii-urea gels. The myosin light chain composition of each of the four histochemically identified fibre types was determined. Chapter 6. Developmental transitions in myosin sub-unit composition from larval inner to adult fast muscle in plaice, Pleuronectes platessa L. The myosin sub-unit composition of plaice deep white muscle fibres was examined at different stages of development. The myosin heavy chain component of the inner muscle fibres was, however, found to be different in group 1 juveniles and adult deep white fibres. Chapter 7. General Discussion. The major findings of the study are discussed in relation to other experiments examining muscle development and differentiation in teleosts. The influence of temperature on the structure and development of plaice myotomal muscle is discussed, with particular reference to larval survival. Possible mechanisms by which muscle development in teleosts may be regulated are described together with suggestions for further work.