From this concept it has become clear that there is no direct continuity between the protoplasm of one cell and the next and that definite points of contact between nerve fibres can be recognized.
This concept is applicable to the nervous system of all animals. A syncytial organization is recognized only in some of the giant fibres of invertebrates, the enteric plexus of the leech, and perhaps at certain points in nerve nets (Bullock, 1959b: Mackie, I960).
This concept was staunchly supported by the findings of Harrison (1907) but it was championed by the work and arguments of Raymon y Cajal (1909-11, 1934).
Harrison, by his simple hanging drop method, observed growing neurons and noted that the processes extending from them had enlarged ends, from which fine filaments often extended.
These processes grew out of the nerve cell body into the medium, and it appeared that all cells were separate.
It might be noted that the mechanisms by which neurons grow and extend fibrous processes during embryological development are still not known.
Nor are the mechanisms known by which neurons make the proper connections to other neurons or to effector cells (Levi-Mont- alcini, 1965). Recently Le Gros (1963) has tried to observe the connections in the nervous system.
The neuron doctrine was finally substantiated by the electron microscope, which showed that neurons, like most other cells, were bounded by a unit membrane whose presence prevents cytoplasmic continuity between adjoining cells of the nervous system.
The gap separating neurons is small (~75 to 400A wide), and in some cases there is a fusion of the outer layers of two unit membranes at neuronal junctions.