1. Classical histological methods demonstrate 4 cytoplasmic networks in fixed vertebrate neurones: ‘neurofibrils’, ‘Nissl complex’, ‘Golgi apparatus’, and ‘trophospongium’. The work described in this paper was undertaken to find out whether the 4 networks of classical histology correspond to 4 structures recognizable as such in the living neurone, or to only one structure, which may be coloured in characteristic ways by the classical methods. 2. A single continuous network, comprising features traditionally associated with the four classical networks of the fixed cell, can be isolated by micro-dissection and detected by interference microscopy in living vertebrate neurones. 3. When living neurones are centrifuged at a moderate angular velocity, a single continuous network remains visible under the interference microscope. There does not appear to be enough clear space left for 3 other voluminous structures. 4. When living neurones are centrifuged at a high angular velocity, a single continuous network is pushed to the centrifugal pole of the cell. The remainder of the cell then contains only small separate objects. 5. A single continuous network can be detected by interference microscopy in the cytoplasm of fixed, unstained neurones. When the same cells are dyed by a Nissl method, the cytoplasmic network seen in the unstained cells becomes deeply coloured. When the same cells are bleached and then silvered by a Golgi method, the objects that before had bound the dye now are blackened by the reduced silver. The same effect is obtained when the Nissl method is used after silvering. 6. It is inferred from the results of these experiments that there is only one cytoplasmic network in living normal adult vertebrate neurones. This network is demonstrated in fixed neurones of the same type with varying degrees of faithfulness by the classical methods. It is suggested that the terms ‘neurofibrils’, ‘Nissl complex’, ‘Golgi apparatus’, and ‘trophospongium’ be abandoned.

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