HISTOGENESIS OF THE NERVOUS SYSTEM 41 



hypothesis, and gives to each axon a multicellular origin, has been supported by 

 Schwann, Balfour, Dohrn, and Bethe, and in modified forms by other workers. 

 There are good reasons, however, for believing that each axon arises as an out- 

 growth from a single cell or neuroblast. This idea, which is in keeping with 

 what is known of the structure and function of the neuron and which forms an 

 integral part of the now generally accepted neuron theory, was first developed in 

 the embryologic publications of His. Convincing experimental evidence has 

 been furnished by Harrison (1906). Using amphibian larvae, this author showed 

 that if the neural crest and tube are removed no peripheral nerves develop. 

 He further showed that isolated nerve-cells cultivated in clotted lymph will 



Roof plate 



Dorsal column 



Dorsal root 



Mantle layer 



Ventral column 



Ependymal layer 



Dorsal Juniculus 



Neural cavity 



Marginal layer 



Floor plate Ventral median fissure 



Fig. 22. Transverse section of the spinal cord of a 20 mm. human embryo. (Prentiss-Arey.) 



give rise to long axons in the course of a few hours. But the ectodermal cells, 

 mentioned above, which migrate outward along the course of the developing 

 nerve, take an important part in the differentiation of the fibers. From them 

 is derived the nucleated sheath or neurilemma of the peripheral nerve-fiber. 

 The myelin sheath is composed of a fatty substance of uncertain origin. It 

 may be a product of the axon, of the neurilemma, or of both. 



The sympathetic ganglia consist of cells derived like those of the spinal 

 ganglia from the neural crest, and, according to Kuntz (1910), also from the 

 neural tube by migration along the course of the cerebrospinal nerves. These 

 cells become aggregated in the ganglia of the sympathetic system and are asso- 

 ciated with the innervation of smooth muscle and glands. 



