THE NERVOUS SYSTEM. 509 



maxillary process and mandibular arch, respectively (Fig. 435). The central 

 processes, forming the afferent root (portio major) of the V, enter the marginal 

 layer of the alar plate of the rhombencephalon and form a descending bundle, 

 the spinal V (Figs. 438, 439, 440 and 470). 



The trigeminus exhibits its spinal-like character in the behavior of its 

 visceral portion (comp. p. 498). Cells of the ganglionic mass migrate further 

 peripherally and form sympathetic ganglia (ciliary, otic, sphenopalatine (?) 

 submaxillary (?) ). As in the cord, the question has arisen whether efferent 

 roots may not also contribute a portion. Cells have been described as migrat- 

 ing with the oculomotor root fibers and forming part of the ciliary ganglion 

 (Carpenter). 



Besides those already described (cerebrospinal, sympathetic), the only 

 other peripheral neurones of the nervous system are connected with the pros- 

 encephalon and are a part of the eye and nose. The visual receptors (rods 

 and cones) and peripheral afferent neurones (bipolar cells) appear to be repre- 

 sented by portions of the retina and are described elsewhere (Chap. XVIII). 



In the nose there is first a placode (p. 459) from which neuroblasts develop. 

 Some of these migrate toward the neural tube and probably differentiate into 

 lemmocytes, a few becoming ganglion cells.* The majority of the neuroblasts 

 remain in the olfactory epithelium, sending their axones (fila olfactoria) into 

 the olfactory bulb, the peripheral afferent olfactory neurones thus apparently 

 displaying the primitive ectodermal location of afferent peripheral neurones 

 (p. 455 and Fig. 397). (Comp. p. 591.) 



Development of the Lower (Intersegmental) Intermediate Neurones. 



It has already been seen how, by migration and by differentiation of the cells 

 during migration, the nucleated layer comprising the greater part of the thick- 

 ness of the wall of the neural tube is differentiated into two layers— an inner 

 nucleated layer retaining its earlier characteristics, and an outer nucleated 

 (mantle) layer, composed largely of the differentiating neuroblasts and 

 characterized in ordinary staining by more widely separated nuclei. It has 

 also been seen that this differentiation takes place earlier and more rapidly at 

 first in the ventral part of the lateral walls (basal plate), and .that the first cells to 

 migrate and differentiate are those whose axones grow out through the neural 

 wall and pass out as the ventral root fibers. 



Not much later than the above differentiation of the efferent peripheral 

 neurones, axones of other neuroblasts also grow toward the periphery of the 

 tube but do not pass beyond its wall. Such neuroblasts become intermediate 



*The latter are probably transient, but possibly in some forms persist as the ganglion cells of the 

 nervus terminalis of Pinkus. 



