THE NERVOUS SYSTEM. 511 



439, 440 and 470) . In the cord the ascending arms grow to a greater length 

 than the descending. In the rhombic brain the reverse is usually the case. 



The longitudinal fibers of the afferent roots and of the intermediate neurones 

 thus form an external layer occupying the marginal layer of the neural tube. 

 This is the beginning of the differentiation into white and gray matter, i.e., 

 into that part of the neural tube containing only the axones of the neurones 

 and into that part containing the cell bodies and the beginnings and termina- 

 tions of the axones. The terminations of axones are formed by a turning of 

 the longitudinal fibers into the mantle layer or gray matter to form there 

 terminal arborizations. Later, the longitudinal fibers develop branches {col- 

 laterals) which also pass into the gray matter. The differentiation of the 

 white matter is completed several months later by the myelination of the 

 nerve fibers. 



The longitudinal axones of intermediate neurones which are formed at this 

 period in the cord and epichordal brain are located ventrally near the median 

 line. These medial tracts occupy the position of the future medial longitu- 

 dinal fasciculi, the reticulo-spinal and ventral ground bundles, and may be 

 regarded on both comparative anatomical and embryological grounds as a 

 primitive system of long and short ascending and descending tracts mediating 

 between cerebrospinal afferent and efferent peripheral neurones, and not 

 having at this period connections with the higher centers. Other more lateral 

 tracts of this character are formed somewhat later, the whole forming the 

 beginning of the reticular formation + ventro-lateral ground bundle system 

 (compare Figs. 442, 449, 452 and 454). 



While merging more or less imperceptibly into the following stages, it may 

 in a general way be said that at this stage of development there is differentiated 

 what might be termed the primary and probably the oldest coordinating mech- 

 anism of the nervous system, most clearly segmental in character and having 

 general features common not only to all Vertebrates, but to many Invertebrates. 

 It is characterized by afferent and efferent peripheral neurones arranged seg- 

 mentally and connected longitudinally in the central nervous system by crossed 

 and uncrossed intersegmental intermediate neurones. (Compare pp. 472 and 

 473) . At the anterior end of this part of the nervous system (epichordal segmen- 

 tal brain) there are also exhibited differentiations due to fundamental vertebrate 

 differentiations in the peripheral receptive and effective apparatus. Some of 

 these are: (1) The differentiation of the splanchnic (visceral) receptive and 

 motor apparatus, giving rise in the nervous system to (a) a separate system of 

 afferent root fibers (tractus solitarius) including the more specialized gustatory 

 apparatus; (b) a distinct series of lateral efferent nuclei. (2) The concentra- 

 tion of the non-specialized somatic afferent innervation into one nerve (tri- 

 33 



