Prenatal and postnatal cardiac muscle from rabbits has been studied by electron microscopy, after osmium fixation and methacrylate embedding. The observations showed that

1. Cell membranes divide the muscle into cellular units from the youngest embryo which was studied (9½ days after coitus) until the adult state.

2. The embryonic muscle cells contain only one nucleus, whereas the adult cell may be multinucleated.

3. At all stages of development, wherever a myofibrillar axis crosses a cellular boundary, the myofilaments are interrupted by an intercalated disc.

4. With age, increase in size and complexity of the discs render them recognisable by the light microscope.

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