vertebrates (Fig. 6) resemble very closely those of invertebrates, for their cell-bodies are within the central nervous mass and their neurites extend as motor nerve-fibers to the skeletal muscles. The primary sensory neurones also agree with those of the invertebrates except that their cell-bodies instead of being in or near the integument, as in most invertebrates, have migrated centrally and thus form the dorsal ganglia. At least this appears to have occurred in all vertebrate sensory nerves except the olfactory, which still retains the usual invertebrate condition.
Fig. 6. Diagram of the Primary Neurones of the Vertebrate Nervous System as seen in Transverse Section, c, spinal cord; dg, dorsal ganglion; i, Integument; m, muscle; mn, motor neurone; sn, sensory neurone.
Association neurones, which were met with in the invertebrates, are abundantly present in the vertebrates.
How the neurones in vertebrates are related to one another has been a matter of much dispute. Whether the gray substance of the central organs in these animals contains a true nervous net as seems to be the case in many invertebrates or whether their neurones retain greater individuality and are related morphologically only through contact, is not yet settled. That many embryonic neurones, or neurocytes, are in the beginning widely separated from others with which they are ultimately closely related is true and gives color to the belief that they may never fuse anatomically, though physiologically they do become continuous. The fact that nervous transmission through central organs in adult vertebrates is slow, open to exhaustion, and restricted to one direction as contrasted with transmission through nerve-fibers, is strong physiological evidence of a special central mechanism of interrelation between neurones such as Sherrington (1906) has pictured in the synapse. That no special anatomical condition has thus far been discovered that answers to this physiological requirement can in no sense be taken as an objection to it. That the vertebrate central nervous system is in many of its parts a synaptic organ can not be doubted, but that all its parts are synaptic is not yet proved. Possibly this is a
