THE NERVOUS SYSTEM. 



525 



The further differentiation, from the alar plate, of terminal nuclei for the 

 afferent peripheral segmental neurones, the axones of the terminal nuclei 

 forming afferent tracts to suprasegmental structures. These tracts and other 

 later forming afferent suprasegmental tracts with their nuclei are laid down 

 ext( rnal to the reticular formation. (4) Formation of efferent (chiefly th ala- 

 mic(?) mid-brain and cerebellar) suprasegmental tracts which act upon the 

 intersegmental neurones or reticular formation. (5) Accession at a late s';age 

 of d svelopment of a descending system of fibres from the neopallium. T.'iese 

 lie 's entral to the preceding structures. 



The Cerebellum. 



It has already been pointed out that at an early period (three weeks) the 

 anterior boundaries of the thin expanded roof plate of the rhombic brain form 

 two lines converging anteriorly to the median line where 

 the roof plate is represented by the usual narrow portion 

 connecting the two alar plates (Fig. 456). It has also 

 been pointed out that the pontine flexure produces on the 

 dorsal surface a deep transverse fold in this thin roof, into 

 which vascular tissue grows later forming the chorioid 

 plexus (Fig. 448) . At this stage, the continuations of the 

 alar plates of the medulla form two transverse bands 

 which, when viewed laterally, are vertical to the general 

 longitudinal axis of this part of the brain (Fig. 448) . At 

 the same time, the rhombic lips are formed along the 

 caudal border of these bands and the latter become 

 thickened into the two rudiments of the cerebellum, a 

 considerable portion of which may be derived from the 

 lips. These rudiments are thus two transverse and 

 vertical swellings and are connected across the median 

 line by the roof plate. The attachment (taenia) of the 

 alar plate of this region to the roof plate of the fourth 

 ventricle is at first along its caudal edge. Later, by the 

 folding back and fusion of this border to form the rhom- 

 bic lips, the attachment is carried forward. Still later, 

 by the growth of the cerebellar rudiment, it is rolled 

 backward and under, as described below. The rudi- 

 ments subsequently fuse across the median line, thus 

 forming externally a single transverse structure, but internally a paired dorsal 

 median projection of the lumen marks the location of the uniting roof plate 

 (comp. Fig. 458). 



FIG. 456. Dorsal view 

 of that part of the 

 brain caudal to the 

 cephalic flexure 

 (human embryo of 3d 

 week, 2.15 mm.). Hh. 

 Cerebellum; J, isth- 

 mus; M, mid-brain; 

 Rf, A7z, medulla. 

 Compare with Fig. 

 416. His. 



