THE NERVOUS SYSTEM. 531 



lateral fillets, the whole comprising the legmentum. Ventral to this are added 

 later the pons and the descending cortico-pontile, cortico-bulbar and cortico- 

 spinal bundles forming here the pes pedunculi or crusta (probably during the 

 fifth month). 



The alar plate of the mid-brain region forms the corpora quadrigemina 

 (mid-brain roof). 



The further changes in the gross morphology of the medulla are due mainly 

 to further growth of structures already present. The nuclei of the dorsal col- 

 umns by their increase cause the swellings on the surface of the medulla known 

 as the dava and cuneus, and likewise by their increase in size cause a secondary 

 dorsal closing in of the caudal apex of the fourth ventricle which formerly 

 extended to the cervical flexure. The tuberculum of Rolando is produced by the 

 growth of the terminal nucleus of the spinal V, and the restiform body largely 

 by the development of the afferent cerebellar fibers (Fig. 457). 



The growth of the olivary nuclei produces the swellings known as the 

 olives. The above mentioned accession of the descending cerebrospinal tracts 

 to the ventral surface is indicated by the pyramids. 



In the floor of the ventricle there is a longitudinal ridge each side of the 

 median line occupied by swellings produced by the nucleus of the XII and, 

 further forward, the nucleus of the VI, together with other nuclei (intercalatus, 

 funiculus teres and incertus, Streeter) which are not well understood. The 

 furrow forming the lateral boundary of this area is usually taken to be the 

 representative of the sulcus limitans and consequently the area in question 

 would be the basal plate. Lateral to it is a triangular area with depressed 

 edges the ala cinerea. It represents a region where portions of the vago- 

 glossopharyngeal nuclei (dorsal efferent and terminal nuclei of fasciculus 

 solitarius) lie near the surface. Possibly a secondary invasion by surrounding 

 more recently differentiated nuclei may account for their apparent partial 

 retreat from the surface. It is possible that the ala cinerea may be regarded 

 not so much as a part of the alar plate, but that it or rather the branchial 

 nuclei involved in its formation represents an independent intermediate region 

 corresponding to the intermediate region in the cord (J. T. Wilson). The 

 remaining portion of the alar plate, in the floor, is apparently represented 

 principally by the acoustic, especially the vestibular, field. 



In the development of the segmental brain there are thus the following 

 overlapping stages: (i) The differentiation of the inner, mantle and marginal 

 layers. (2) The primary neural apparatus, consisting of (a) the peripheral 

 segmental neurones, the central processes of the afferent neurones entering the 

 alar or receptive plate, the efferent neurone bodies forming two main series 

 of nuclei in the basal plate, and (b) intersegmental neurones composing the 

 reticular formation in which the long tracts occupy external positions. (3) 



