THE OKGANS OP THE OUTER GERM-LAYEK. 



455 



The formation of the nerve-nbrillae is .also a subject of controversy, 

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

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

 fibrillse 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 



(KUPFFEK UND BlDDER, KoLLIKER, HlS, SAGEMEHL). Such accounts 



J 



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

 SAGEMEHL. 



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

 nf, nerve-fibrillaa ; spk, spinal ganglion ; mp l , muscle-plate, muscle-producing layer ; 

 mp', 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 WJJHE, on the 

 contrary, the nerve-fibrillse 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. 



