THE NERVOUS SYSTEM. 519 



terminal nuclei of afferent elements from other segments. The latter are 

 external or added to the former and are differentiated from additional prolifer- 

 ations of neuroblasts of the alar plate. In addition to these nuclei, there are 

 certain nuclei forming links between the two great suprasegmental structures, 

 the pallium and cerebellum. These nuclei are the olive* and pons nuclei, 

 both of which form afferent cerebellar bundles and which are differentiatec by 

 still further proliferations and migrations of alar plate neuroblasts. 



It has already been seen that the afferent peripheral nerves (IX and X) 

 c-f the visceral segment form (together with descending fibers of the VII) the 

 tractus solitarius. This is at first (5th week) short, but in six weeks has rea :hed 

 the cord. The terminal nucleus of the tractus solitarius is differentiated irom 

 the neuroblasts of the medial portion of the alar plate. The course of the 

 axones of this nucleus is not known. Judging from comparative anatomical 

 grounds, they would not follow the fillet pathway (C. J. Herrick). The most 

 caudal part of this nucleus is the nucleus commissuralis at the lower apex of the 

 fourth ventricle. 



The formation of the other terminal nuclei lying in the region of this seg- 

 ment is begun by the further developments of the alar plate already alluded 

 to. These are initiated by an expansion and consequent folding of its border 

 (formation of the rhombic lip, p. 513), followed by further cell-proliferation, 

 leading to fusion of these folds and copious formation of neuroblasts in this 

 region. These neuroblasts represent fresh accessions to the neuroblasts 

 already formed in the mantle layer of the more medial part of the alar plate. 

 This latest development of the border portions of the alar plate is the last step 

 in the progressive development of the neural tube from the medial portion 

 (basal plate) to the lateral (dorsal) border of the lateral walls of the tube 

 where further development ceases at the attachment to the roof plate (teenia). 

 (Fig. 45 2 -) 



Many of the neuroblasts of the rhombic lip region migrate ventrally.t 

 Some of those from the medial part of the swelling produced by the fusion of 

 the rhombic lip folds (p. 513) migrate along the inner side of the tractus soli- 

 tarius, while those from the lateral part of the swelling pass outside the tractus, 

 which becomes thereby enclosed in the mantle layer (Fig. 453). Many of these 

 neuroblasts continue their journey, passing along the outer side of the differ- 



* This is conjectural. The origin of fibers to the inferior olivary nuclei is not known. The 

 most conspicuous tract to the olive is von Bechterew's central tegmental tract. Purely a priori con- 

 siderations might be adduced in favor of this being considered a descending tract from thalamic 

 nuclei which in turn receive pallio-thalamic fibers. It may, however, arise from lower optic centers. 



fit is, perhaps, an open 'question whether the formation of the lip is a fundamental feature in 

 this last proliferation and invasion of neuroblasts from the border of the alar plate. The promi- 

 nence of the rhombic lip in man is the early embryological expression of the future great develop- 

 ment of parts subsequently formed from this portion of the neural wall, especially the cerebellum 

 and neurone groups in connection with it. 



