494 TEXT-BOOK OF EMBRYOLOGY. 



plasm of these cell-strings and are thus multicellular structures. Still another 

 view is that of Hensen, according to which the fibers are a differentiation in 

 situ from preexisting syncytial bridges uniting the parts connected subsequently 

 by the formed nerve fibers. This differentiation may not be primarily con- 

 nected with the neuroblasts (Apathy, Paton) . An intermediate view between 

 this and the outgrowth view of His is that of Held, according to which the 

 neurofibrillar substance is an outgrowth from the neuroblast body, or at least a 

 differentiation proceeding from that body, but always within the preexisting 

 cellular bridges of Hensen. The differentiating fiber is thus always intracel- 

 lular instead of intercellular as according to the His-Cajal view. The experi- 

 ments of Harrison above alluded to, in which the accompanying migrating cells 

 were eliminated and naked axones (axis-cylinders) nevertheless developed, ap- 

 parently disposes of the cell-string theory of Balfour. The growth of the 

 fibers in the marginal layer of the central nervous system is also unfavorable to 

 this theory. The apparently proven capacity of growing axones to find their 

 way through foreign tissues (aberrant regenerating nerve fibers, Cajal), 

 through ventricular fluid (Cajal), and even through serum (Harrison) seems to 

 throw the weight of evidence in favor of the view of His. The latter is the 

 view adopted in this description, though many of the most important facts of 

 development are not perhaps entirely irreconcilable with any of these views. 

 The general conception of the neurone is affected by these questions and the 

 related question of anastomoses between the nervous elements, whether present 

 at all, and if present, whether primary or secondarily acquired. 



From the above it would seem that the cells of the neural crest have the 

 capacity of differentiating into afferent neurones, efferent (sympathetic) neurones 

 and supporting cells. Other cells of the jneural crest differentiate into the 

 chromaffine cells of the suprarenal glands and similar structures (p. 426). 



There are several views as to the development of the myelin sheath. Ac- 

 cording to one view (Vignal), it is a product of the neurilemma cells, being 

 formed in a manner analogous to the formation of fat by fat cells. Accord- 

 ing to Wlassak, the various substances composing the myelin (fat, lecithin 

 and protagon) are first found in the central nervous system in the protoplasm 

 of the spongioblasts, their probable original source being the blood of the 

 meningeal blood vessels. Later, the myelin is laid down around the axones, 

 appearing first as drops or granules. The same process takes place in the 

 peripheral nervous system. The supporting elements of the nervous system 

 thus would have a chemical as well as a mechanical function. Another view 

 (Gurwitsch) is that the myelin is a product of the axone and is, at its first 

 appearance, quite distinct from the neurilemma cells. 



As the appearance of the myelin sheath is a final stage in the development of the neurone, 

 the various neurone systems would naturally become myelinated in about the same sequence 



