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Myoblasts obtained by trypsin dissociation of 6-day chick-embryo hearts re-aggregated in rolling tubes and formed a pulsating mass within 1 h. Newly formed intercalated discs (adhesion plaques) were the most frequent type of intercellular contact in the earlier stages of culture. Desmosomes were also present. Focal tight junctions were rare and difficult to identify. In advanced aggregates more extended regions of very close plasma membranes apposition could be observed; lanthanum infiltration did not reveal obliteration of the intercellular gap. The most striking feature, even after 12 days of culture, is the disorder of the contractile units within the sarcoplasm and the irregularity of the cell outlines. The ineffectiveness of the factors responsible for the orientated disposition of myofibrils in culture conditions has been emphasized. The smooth endoplasmic reticulum never attains a regular arrangement in relationship with the myofibrillar banding. Subsarcolemmal cysterns containing a finely granular matrix (peripheral couplings) have been found after the first 24 h of culture; it has been observed that they reach their mature form only in the last stages of culture. The T-system is lacking. Numerous pits, often complicated by beading, are present in the peripheral sarcoplasm. All these features may affect the physiological response that can be recorded from these cells.

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The fixation of dissociated chick embryonic cells in the presence of ruthenium red or alcian blue revealed a remarkable reduction in the binding of these cations to cell surfaces following tryptic digestion at 37 °C, but not after trypsin used at 4 °C or EDTA. The reconstitution of anionic sites present over the cell plasma membrane was studied in retina, liver and heart cells from embryos of different ages, cultivated according to a method which promotes cell reaggregation. All these cells regenerated ruthenium red-stainability within the first hour of culture. Culturing at 15 °C, substitution of saline for the culture medium or addition of puromycin or of neuraminidase to the culture media did not affect this restitutive process. Neuraminidase and hyaluronidase digestion were not able to alter the ruthenium red-binding, once it was present. From these data and from the reports available in the literature it has been concluded that the anionic binding-sites cannot be identified with the cell surface coat, and probably not with sialic acids, which require active metabolic processes and a much longer time interval for regeneration. The presence of these sites probably favours the formation of initial cell contacts but is not sufficient for cell reaggregation.