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Shape of the myosepts and arrangement of the muscle fibres were recorded in the lateral musculature of the tail of Rana temporaria embryos and larvae. Well developed myomeres are present as early as st. 18-19. The main characteristics-ie. those related to functional properties-of myoseptal shape as well as of muscle fibre arrangement, remain unchanged throughout further development until degeneration of the tail occurs during metamorphosis. The rather simple myoseptal shape observed in Rana-as compared to the multiple cone-form observed in most fishes-shows a close agreement to hypothetical myosept models described in papers by Jarman (1961), van der Stelt (1968) and Willemse (1966). The muscle fibres in the m. lateralis of Rana are arranged in trajectorial patterns that show a close similarity to the trajectorial patterns observed in 'typical teleosts'. Both arrangements agree with trajectorial models based on the mathematical analyses of Alexander (1968). Neurulas anaesthetized with 1:10000 MS-222 and exposed up two weeks to this anaesthetic developed the same shape of the myosepts and arrangement of muscle fibres as in controls. Thus even the details of the function-related features of the myomere structure develop without functioning. In this field possible feedback meachisms are either not affected by anaesthesia or do not exist at all. [ABSTRACT FROM AUTHOR]

In the m. lateralis of Selachii a well-developed perimysium is found, while a similar structure is lacking in Teleostei. The Selachian perimysial fibres are arranged in distinct planes. In such planes two predominant fibre directions are found; the angle between each of them and the body-axis is 60-70°. Additionally the myosepts contain fibre bundles which are arranged in oblique loops. These myoseptal fibre-loops are present in Squalus, well as in the Teleosts we examined. In order to investigate the functional significance of these fibre arrangements we constructed a model, consisting of a steel strip representing the axial skeleton and a bundle of stretched rubber tubes representing the lateral musculature. Both components of this model have to be firmly interconnected. This is easily achieved by winding oblique loops of cotton thread around them, thus tying them to each other. When we now release the stretched rubber tubes, they contract, resulting in a proper bending of the model. An optimal value for the angle between the body-axis and the direction of the fibres is found. On the basis of the morphological similarity between the perimysial and the myoseptal loop fibres on the one hand and the cotton thread loops on the other, it is suggested that a functional similarity exists as well. So it is suggested that the functional significance of the perimysial fibre system, and of the myoseptal fibre loops, is the achievement of a firm contact between the lateral muscle mass and the axial skeleton. [ABSTRACT FROM AUTHOR]