400 EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



the median roof-plate. The dorsal zone is afferent in function 

 and in connection with it the sensory nerves terminate. The 

 roof-plate and the floor- plate take little or no part in forming neu- 

 rones; they lack the mantle layer, and are composed of neuroglia 

 and ependymal cells. 



The Neural Crest (Figs. 16 and 114). (i) The cephalic por- 

 tion of the neural crest becomes broken into five pairs of ganglia, 

 which, during development, shift their positions to the ventral 

 side of the brain. Those ganglia are called the vagus, glos- 

 sopharyngeal, geniculate, auditory and semilunar (Gasseri). 

 Later, the auditory ganglia lie between the geniculate and glos- 

 sopharyngeal ganglia. These five ganglia give origin to the 

 sensory parts of the vagus, glossopharyngeal, intermediate and 

 trigeminal nerves; and to all of the acustic nerve. In all the gan- 

 glia except the auditory, the cells develop into unipolar neurones; 

 they remain bipolar in the auditory ganglia. The unipolar con- 

 dition is produced by the growth of the cell-body toward the sur- 

 face of the ganglion and the shifting of both processes to the same 

 side of the cell-body, together with the elongation of the common 

 point of attachment. The single processes of the unipolar neu- 

 rones immediately divide, T-like, into peripheral and central fibers, 

 which in appearance are axones. The peripheral fibers form the 

 sensory part of the respective nerves and conduct impulses to- 

 ward the cell-body, hence they may be considered dendrites (Cajal) ; 

 the central fibers, the axones proper, form that part of the nerve 

 which extends from the ganglion into the dorsal zone of the em- 

 bryonic brain. All the central axones of the several ganglia 

 divide, T-like, upon entering the brain, and collaterals rise from the 

 undivided fibers and from both branches of them. These axones 

 and collaterals arborize, chiefly, hi the terminal nuclei of the res- 

 pective nerves; but certain of them, the excito-reflex fibers, ter- 

 minate hi nuclei of motor nerves. 



The development of peripheral common sensory neurones, both cerebral 

 and spinal, has been observed in certain lower vertebrates. In the amblystoma 

 punctatum the following phases have~been observed: i. The elongation of the 

 cells in the neural crest to a spindle-form. 2. The projection of a slender 

 central process, the axone, which grows into the neural tube. 3. A thick 



