In control tissue, the sarcoplasmic reticulum usually demonstrated the normal pattern of paired, linearly arranged membranes, although in some cases, the reticulum showed a variation from these membranes to a series of small vesicles. There was no evidence of a T-system of tubules in any of the material examined. Desmosome-Z band complexes were observed in almost all sections of both control and experimental material. A possible role of these complexes in the excitation-contraction mechanism is discussed.

In 3 month cultured material, alterations in normal myofibrillar pattern occurred. Small segments of myofibrils branched from one Z band to join the Z band of an adjacent myofibril, or appeared to be fraying out into the sarcoplasm. In 6 month cultured material, myofibrils were fragmented into short segments from which myofilaments frayed out into the sarcoplasm. This filamentous material may be remnants of myofilaments.

Despite the morphological changes in myofibrils, the heart pulsation rate, established at the beginning, was maintained throughout the culture period. It is suggested that the alterations, observed in the experimental material, occurred in elements not essential for heart beat maintenance, or that these alterations have not yet progressed to a critical point of affecting the heart beat.