2o6 HISTOGENESIS 



cells. More recently Harrison (Amer. Jour. Anat., vol. 5, 1906), experimenting on amphibian 

 larvffi, has shown: (1) that no peripheral nerves develop if the neural tube and crest are re- 

 moved; (2) that isolated ganghon ceUs growing in clotted lymph will give rise to long axis 

 cylinder processes in the course of four or five hours. 



A second theory, supported by Schwann, Balfour, Dohrn, and Bethe, assumes that the 

 nerve fibers are in part differentiated from a chain of ceUs, so that the neurone would represent 

 a multicellular, not a unicellular structure. Apathy and O. Schulze modified this cell-chain 

 theory by assuming that the nerve fibers differentiate in a syncytium which intervenes between 

 the neural tube and the peripheral end organs. Held further modified this theory by assum- 

 ing that the proximal portions of the nerve fibers are derived from the neuroblasts and 

 ganglion cells and that these grow into a syncytium which by differentiation gives rise to the 

 peripheral portion of the fiber. 



The Differentiation of the Supporting Cells of the Ganglia and Neural Tube. 

 —The supporting cells of the spinal ganglia at first form a syncytium in the 

 meshes of which are found the neuroblasts. They differentiate (1) into flattened 

 capsule cells which form capsules about the ganghon cells, and (2) into sheath cells 

 which ensheath the axis cyhnder processes and are continuous with the capsules 

 of the ganghon. It is probable that many of the sheath cells migrate peripherally 

 along with the developing nerve fibers (Harrison). They are at first spindle- 

 shaped, and, as primary sheaths, enclose bundles of nerve fibers. Later, by the 

 proliferation of the sheath cells, the bundles are separated into single fibers, each 

 with its sheath (of Schwann) , or neurilemma. Each sheath cell forms a segment 

 of the neurilemma, the Hmits of contiguous sheath cells being indicated by con- 

 strictions, the nodes of Ranvier. 



The Myelin or Medullary Sheath. — During the fourth month an inner myelin 

 sheath appears about many nerve fibers. This consists of a spongy framework of 

 neurokeratin in the interstices of which a fatty substance, myelin, is deposited. 

 The origin of the myelin sheath is in doubt. By some (Ranvier) it is beheved to 

 be a differentiation of the neurilemma, the myeUn being deposited in the substance 

 of the nucleated sheath cell. By others (KoUiker, Bardeen) the myehn is regarded 

 as a direct or indirect product of the axis cyhnder. Its integrity is dependent at 

 least upon the nerve cell and axis cyhnder, for, when a nerve is cut, the myelin 

 very soon shows degenerative changes. Furthermore, it may form where the 

 sheath is absent. 



In the central nervous system there is no distinct neurilemma sheath invest- 

 ing the fibers. Sheath cells are said to be present and most numerous during the 

 period when myehn is developed. Hardesty derives the sheath cells in the central 

 nervous system of the pig from a portion of the supporting cells, or spongioblasts, 

 of the neural tube, and finds that these cells give rise to the myelin of the fibers. 



