256 THE DEVELOPMENT OF THE CHICK 



According to this the axones of peripheral nerves arise as differ- 

 entiations of the sheath-cells in situ; continuity of the axone is 

 established by arrangement of these cells in rows, and union 

 with the neuroblast is essentially secondary. The entire axone 

 is thus by no means an outgrowth of the neuroblast; at most its 

 proximal portion is. 



Bethe (1903) expresses the idea thus: "Between the cord of 

 the embryo and the part to be innervated there is formed primarily 

 a chain of nuclei around which the protoplasm is condensed. 

 This is fundamentally an extended syncytium in which the nuclei 

 of the neuroblasts and of the nerve-primordium lie. Within 

 the denser protoplasm which appears as the body of the nerve- 

 cells, axones differentiate by condensation, and these extend 

 from one cell to the next, and so on to the condensations which 

 are called neuroblasts. The differentiated axones tend more 

 and more to occupy the center of the embryonic nerve, where 

 they appear to lie free, though as a matter of fact they are still 

 embedded in the general plasma which is no longer distinctly 

 visible on account of its lesser density. Since the axones remain 

 in firm connection with the neuroblasts, it appears in later stages 

 as if they were processes of these and had nothing to do with 

 their original formative cells." 



This view is essentially that of Balfour, Beard, and Dohrn; 

 the neurone hypothesis was first clearly formulated in embryo- 

 logical terms by His, and has been supported by the investiga- 

 tions of a considerable number of observers, notably Ramon y 

 Cajal, Lenhossek and Harrison. 



The neurone hypothesis has far stronger embryological sup- 

 port than the cell-chain hypothesis; moreover, it is the only 

 possible hypothesis of the development of nerve tracts in the 

 central system, because cell-chains are entirely lacking here dur- 

 ing the formation of these tracts. In recent years it has been 

 demonstrated that isolated neuroblasts in culture media produce 

 complete axones, sheath cells being entirely absent. Thus the 

 cell-chain hypothesis has received its final quietus, and is now of 

 historical interest only. (Burrows 1911, Lewis and Lewis 1911.) 

 Splanchnic Components (Sympathetic Nervous System). Two 

 views have been held concerning the origin of the sympathetic 

 nervous system: (a) that it is of mesenchymal origin, its elements 

 arising in situ; (b) that it is of ectodermal origin, its elements 



