466 EMBRYOLOGY. 



the brain produced by an evagination of the frontal lobe of the 

 cerebral hemisphere (lobus or bulbus olfactorius with tract us olfac- 

 torius). (Enormous development of the olfactory lobes in lower 

 Vertebrates, Sharks, degeneration in Man.) 



Peripheral Nervous System. 



23. The spinal ganglia are developed out of a neural ridge (crest), 

 which grows outward and downward from the raphe of the neural 

 tube on either side between the tube and the primitive epidermis, 

 and becomes thickened in the middle of each primitive segment into 

 a ganglion. 



24. The spinal ganglia therefore arise, like the neural tube itself, 

 from the outer germ-layer. 



25. The sympathetic ganglia of the longitudinal cord (Grenz- 

 strang) are probably detached parts of the spinal ganglia. 



26. Concerning the development of the peripheral nerve-fibres 

 there are different hypotheses : 



First hypothesis. The peripheral nerve-fibres grow out from the 

 central nervous system and only secondarily unite with 

 their peripheral terminal apparatus. 



Second hypothesis. The fundaments of the peripheral terminal 

 apparatus (muscles, sensory organs) and the central 

 nervous system are connected from early stages of 

 development by means of filaments which become nerve- 

 fibres (HENSEN). 



27. Anterior and posterior nerve-roots are developed on tl e 

 spinal cord separately from each other, one ventrally, the other 

 dorsal ly. 



28. The cranial nerves arise in part like posterior, in part like 

 anterior roots of spinal nerves. 



29. The following cranial nerves with their ganglia, which are 

 comparable with spinal ganglia, are developed out of a neural ridge 

 which grows out from the raphe of the brain-vesicles : the trigeminus 

 with the ganglion Gasseri, the acusticus and faeialis with the gang- 

 lion acusticum and g. geniculi, the g-lossopharyngeus and vagus with 

 the ganglion jugulare and g. nodosum. 



30. The oculomotorius, trcchlearis, abducens, hypoglossus, and 

 accessorius are developed like ventral roots of spinal nerves. 



31. The olfactory and optic nerves are metamorphosed parts of 

 the brain. 



