THE ORGANS OF THE OUTER GERM-LAYER. 



455 



The formation of the nei've-fibrillae is also a subject of controversy, 

 as has already been stated and as will be farther explained in this 

 connection. According to the view of most observers, the nerve- 

 fibrillae the future axis-cylinders are formed as processes of gang- 

 lionic cells of the spinal cord, the free ends of which grow out from 

 the surface of the latter until they reach their terminal organs 



(KUPFFER UND BlDBER, KoLLIKER, HlS, &AGEMEHL). Such accounts 



Fig. 260. Cross section of a Lizard embryo with completely closed intestinal canal, after 

 SAGEMEHL. 



Ac, Posterior [dorsal], vc, anterior commissure of the spinal cord ; vw, ventral nerve-root ; 

 nf, nerve-fibrillse ; gpk, spinal ganglion; TOP', muscle-plate, muscle-producing layer; 

 nip', outer layer of the muscle-plate ; mp 3 , transition from the outer to the muscle- 

 forming layer. 



are given especially for the development of the motor roots in the 

 higher Vertebrates. 



According to the opinion of DOHRN and VAN WIJHE, on the 

 contrary, the nerve-fibrillae arise in situ, as products of differentiation, 

 fronT the protoplasm of the cords of cells by means of which muscle- 

 segment Imd spinal cord are already united. They do not need to 

 seek out the terminal organ, since there exists already a protoplasmic 

 union with it. They arise in a manner similar to that in which 

 the muscle-fibrillag do from the plasma of their muscle-cells. 



