Observations on initial segments of axons in the prepyriform cortex of the rat

Layers II and III of the prepyriform cortex of rats were studied with the electron microscope and with a rapid Golgi method. Initial segments of axons were identified according to the criteria previously established. The fixative optimum for demonstrating microtubules and differentiating initial segments from dendrites must include glutaraldehyde. Initial segments were found to possess lateral projections, herein referred to as axonic spines and their fine structural features and synaptology are described. The synapses on initial segments and axonic spines predominately contain flattened vesicles and a spine apparatus‐like organelle is often present. The problem of the origin of these synapses is discussed.