ii Journal of Comi'akativk Neurology. 



by a conical cap of protoplasm with the nucleus of the cell or neuro- 

 blast. 



During the period of transition when the germinal cells are being 

 transformed into neuroblasts, they are motile. In the first place, they 

 move ectad between the cells of the columnar zone, but find a hinder- 

 ance to their farther migration in the mantle zone. Along this contact 

 line the neuroblasts assemble, forming a mantle layer, of which the 

 dorsal and ventral portions react dissimilarly. The cells of the dorsal 

 half, because of the oblique position of the epithelial frame-work, strike 

 the mantle zone at an angle and turn parallel to the surface. Their 

 fibres describe long curves ventrad and, in part, reach the median line. 



The cells of the ventral region on the other hand impinge more or 

 less perpendicularly upon the mantle layer and, though the cells do not 

 penetrate it but collect in clusters, the fibres make their way through 

 this zone and pass out as motor nerve fibres. 



The migration of neuroblasts is especially extensive in the medulla. 

 The thick lateral walls of the tube here are divided by a groove into 

 a dorsal plate or ala and a ventral half or basal plate. In the latter the 

 groups of neuroblasts form the motor niduli of the hypoglossus, 

 accessory, vagus, and glossopharyngeal. On the lateral margin between 

 the ala and basal plates lies the tractus solifarius containing the 

 sensory fibres from the vagus and glossopharyngeus. The margins of 

 the alee fold outward and the two lips of the fold grow together. From 

 this evaginated portion there arise multitudes of neuroblasts which pass 

 medianly entad from the tractus solitarins to near the median line. 

 Out of the clusters thus derived arise the olives and accessory olives, 

 the fibres from which cross the median line within the raphe. Blood 

 vessels enter the walls of the medullary tube at an early period but it is 

 not until a much later period that ameboid cells transude into the neu- 

 roglia and lodge in its meshes. These connective or wandering cells 

 constitute the third element of the complex. 



The neuroblasts of the medullary plate produce intramedullary 

 fibres as well as motor nerve roots. The sensory roots spring from the 

 spinal ganglia, the cells of which assume a bipolar form and extend 

 into two fibres, one passing centrad, the other peripherad. The central 

 processes collect at first upon the exterior of the medullary tube 

 in special longitudinal fasciculi which, in the cord, form the primary 

 posterior fasciculus, and in the brain constitute the ascending roots (as 

 the ascending root of the trigeminus and the tractus solitarius, or com- 

 mon ascending root of the glossopharyngeal and vagus). 



The origin of the spinal ganglia, though long controverted, is doubt- 

 less from that portion of the ectoderm connecting the medullary plate 

 with the definitive ectoderm where a groove-like depression with num- 

 erous germinative cells forms as the medullary tube separates. These 

 cells are originally motile and multi]>ly by division until they reach their 

 ultimate site. 



