THE ORGANS OF THE OUTER GERM-LAYER. 



455 



The formation of the nerve-nbrillae 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- 

 n'brilke 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 BlDDER, KoLLIKER, HlS, SAGEMEHL). Such accounts 



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

 SAGEMEHL. 



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

 nf, nerve-fibril^ ; spk, spinal ganglion; mp l , 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-nbrillse arise in situ, as products of differentiation, 

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

 segment and 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-fibrillse do from the plasma of their muscle-cells. 



