ORIGIN OF THE NERVOUS SYSTEM 



261 



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 Nbdhonbs of the Vebtebbate Nervous 

 System as seen in Transvebsb Section, c, spinal cord ; dg, dorsal ganglion ; i, in- 

 tegument ; »i, muscle ; mn, motor neurone ; sra, 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 ulti- 

 mately closely related is true and gives color to the belief that they may 

 never fuse anatomically, though physiologically they do become con- 

 tinuous. 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 dis- 

 covered 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 



