Living chick embryo spinal ganglion neurons grown from 1 to 4 weeks in vitro were studied under the phase contrast microscope. In the peripheral cytoplasm of the earliest stages studied, a homogeneous, phase-dense material is seen which corresponds in location to the cytoplasmic basophil material of the same stages. As maturation proceeds, this material increases in extent, and becomes separated by lighter channels into discrete bodies.

Short fixation by 1 per cent buffered osmium tetroxide followed by post-fixation with neutral buffered formalin does not significantly alter the size, shape, or distribution of any of the cytoplasmic components, and the fixed, hydrated cell is almost indistinguishable from the living cell. Dehydration causes some shrinkage of the fixed preparations, but if the photographs of the stained preparations are enlarged to correspond with those of the living cell, excellent correspondence can be made between at least the larger basophil masses and the larger dark masses seen with phase contrast. Fixation by a formalin-mercuric chloride procedure also results in satisfactory correspondence between the stained Nissl bodies and the phase-dark homogeneous areas. It is concluded that discrete Nissl bodies preexist in the living neuron and are essentially unchanged after good cytological fixation.

Evidence is also presented of the presence of neurofibrils in the living state.

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