DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 39 



large mass of fibers coming up in the inferior peduncle. The 

 trigeminal nerve in this stage sends its rootlets through these 

 cross fibers in an oblique direction to reach its intramedullary 

 nuclei. The cerebro-spinal neurones which were seen among the 

 relatively thin sheet of nuclei covering the ventral surface of the 

 pontine flexure in the stage just described (35 mm.), are increased 

 enormously during the interval left in this series, and we have a 

 striking similarity to the picture one gets in sagittal sections 

 through the pontine region of the adult brain. The thickened 

 layer of cells (now 0.642 mm.) are invaded by large anastomosing 

 strands of fibers which collect at the cephalic and caudal border 

 of the nuclear sheet into a solid bundle. This behavior is one of 

 the pecularities of the cortical projection system as it lies among 

 the pontine nuclei where, as is well known, the otherwise compact 

 fiber tract is broken up into smaller fasciculi by the cross fibers 

 and nuclei of the pons. Caudad the reunited fibers fuse insepar- 

 ably with the median lemniscus soon after leaving the nuclei 

 pontis; cephalad 1 am unable to trace them beyond the cephalic 

 flexure. 



His has suggested that this interweaving of cross fibers of pons 

 with pyramidal tract points to an alternating time of deposition 

 of the component parts of the two systems — the development 

 proceeding in a direction away from the central canal. This for 

 the most part is true. We find the new cells which have migrated 

 from the ventricular walls, spreading themselves over the surface 

 of those already descended to the pontine flexure, and as the new 

 axones come from the cortex they tend to grow among the younger 

 nuclei, i.e., to grow nearer the surface. Thus each fasciculus 

 when it enters the pontine nuclei, pushes along near the surface 

 but it is soon deeply buried by new cells which are continually 

 streaming down from the ventricle. As a result there is a separa- 

 tion of the pyramidal tract into a series of fasciculi which unite 

 again at their exit from the caudal border of the pons. Some of 

 the cells, however, after passing between the seventh and eighth 

 nerves forsake their superficial position and plunge between the 

 cross fibers of the pons. This is well illustrated in fig. 9 the more 

 deeply staining young cells are seen forcing their way between 



