THE NERVOUS SYSTEM. 515 



in its greater thickness above mentioned. This process likewise terminates 

 sooner in the basal plate, few cell divisions being present there at seven weeks. 

 At about the end of the fifth week (see p. 519) the alar plate begins to develop 

 very rapidly. Its period of proliferation is about terminated at the end of the 

 second month. When the cell proliferation near the ventricle has ceased, 

 the inner layer is reduced by outward migration to a single layer of epend] ma 

 cells (compare pp. 485 and 486). 



While the efferent nuclei continue to develop and the central continuations 

 of the afferent neurones continue to grow in length, the principal differential ions 

 now taking place in the rhombic brain are those affecting the intermediate 

 neurone systems. 



The first of these to be considered is the further differentiation of the system 

 of intersegmental neurones (p. 465). The earlier development of this system 

 has been seen to involve especially the basal plate and the further development 

 of the latter leads to the complete differentiation of the formatio reticularis 

 which especially represents this system in the epichordal brain. It has already 

 been seen (p. 504) that many of the intermediate neurones representing the 

 beginning of this system seem to be at first : Jieteromeric and form an internal 

 arcuate system of fibers similar to those seen in the cord (pp. 503, 507). They 

 increase in number toward the median line and are especially numerous in the 

 basal plate, where they, together with the medial efferent neurones (XII and 

 VI cranial nerves) , form an eminence of the mantle layer corresponding to the 

 ventral gray column of the cord (Fig. 449) . Many of the axones of these cells 

 of the arcuate system cross the septum medullae, thus marking the beginning of 

 the raphe, and form on each side a longitudinal bundle in the septal marginal 

 layer (Fig. 449). These longitudinal bundles correspond to the first formation 

 of the ventral funiculi of the cord. They must not, of course, be confused 

 with the pyramids which appear much later. Whether these longitudinal 

 bundles are also partly formed of axones of tautomeric cells is uncertain. 

 Later, as the anterior horn swellings grow and the depth of the septum 

 medullae and of the septal marginal layers increases (compare p. 514), more 

 longitudinal fibers appear in the latter, the new ones apparently being added 

 ventrally. Others also appear more laterally in the marginal layer (Figs. 453, 

 454 and 455). (Compare cord, p. 507.) At this time, also, fibers enter the 

 marginal layer bordering the surface (as distinguished from the septal), pass 

 along parallel with the surface, cross the septum, and proceed to various parts 

 of the marginal layer of the opposite side. These fibers are the first external 

 arcuate fibers as opposed to the preceding internal arcuate fibers which traverse 

 the mantle layer (gray) in the arcuate part of their course (Fig. 453). 



The majority of the longitudinal fibers entering the septal marginal layers 

 during the second month occupy approximately the position of the future 



