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Since the description of a helicoidal pattern of muscle fibres in teleost fishes by Van der Stelt (1968), Alexander (1969) stated that there are two basic patterns. One is found in selachians and primitive bony fishes as Anguilla and Sal mo, whereas it is also found in the caudal peduncles of teleosts, which are considered more advanced. More anteriad a quite different pattern is found, resembling the helicoidal pattern. The elucidation of this pattern was disclosed by both Van der Stelt (1968) and Alexander (1969) by reconstruction from horizontal, as well as vertical serial sections. Van Raamsdonk et al. (1974), in a study of the early development of the myomeres of Brachydanio rerio, observed a transition of a primary longitudinal course of muscle cells to a presumably helicoidal one in transverse sections of the embryo.

As it is more easy to obtain and interpret a series of transverse sections, we have tried to devise a method of reconstruction of the muscle fibre course, starting from transverse sections. With some caution, the results of these studies on Brachydanio may be summarized as follows:

1. The embryo starts with a primordial, longitudinal muscle cell orientation.

2. In the young embryo the muscle cell orientation changes to a helicoidal one.

3. A further change in conformity with Alexander’s primitive pattern is found in later stages of development.

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In: Bijdragen tot de Dierkunde

As a working hypothesis it is supposed that in the teleost Brachydanio rerio, the muscle contractions, the growth, and probably some other factors, cause the first changes in the shape of the somites. Furthermore, the movements of the embryo could yield the forces by which the somites are brought to their theoretically optimal shape.

In order to test this hypothesis, we investigated the somite shape and structure of spontaneously immobile embryos. Although the results are difficult to interpret, they certainly do not contradict the hypothesis.

For further analysis we applied two kinds of lesions in order to immobilize early embryos: removal of the brain, and damage to the midbody somites. The results of these experiments indicate that both the development of the shape of the somite and the arrangement of the muscle fibres are dependent on movements of the embryo.

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In: Bijdragen tot de Dierkunde