Growth and Differentiation in the Nervous System 269 



paths, merge and segregate again, each to pursue its specific course. 

 In all instances the migrating cells are well differentiated at the time 

 they start their journey. Their slender, elongated bodies, dark-stained 

 in silver preparations, detach sharply on the pale background of the 

 undifferentiated cells, which still represent the major component of 

 the brain stem at this stage. Each population is easily recognizable 

 from adjacent populations by the different shade of its color in silver- 

 impregnated material. All migrating cells trail their axon behind while 

 moving toward their final location; the direction of the fibers repre- 

 sents in the mature organism a valuable guide to detect the route 

 followed by a nerve center to reach its final destination. 



It was the peculiar path traced in the brain stem by the axons of 

 the accessory nucleus of the Vlth nerve (Fig. 2) which suggested that 

 its cells of origin have undergone a migratory movement in previous 

 developmental stages. This movement was actually observed in em- 

 bryos examined between the sixth and 'the eighth day of incubation. 



During this period a considerable number of cells segregate from 

 the main nucleus located near the midline on the floor of the fourth 

 ventricle; they move in a straight line, one after the other in a single 

 row in an oblique direction from their former mediodorsal position 

 to their final ventrolateral location. The neuroblasts cut their way 

 across the dense texture of the brain stem, which consists of closely 

 packed rows of undifferentiated cells with their long axes oriented at 

 45° with the axes of the migrating cells (Figs. 26, 26', 12). Toward 

 the middle of their journey the shifting population crosses and inter- 

 mingles with the population of the facial nucleus, which is also 

 moving from a laterodorsal to a ventrodorsal position (Fig. 12 ) . The 

 two populations dissociate at the beginning of the seventh day and 

 each reaches its destination about 24 hours later (Figs. 2, 13). 



Migratory Movement in the Third Nucleus. This movement, which 

 was first described by Biondi ( 9 ) , consists of the exchange across the 

 midline of one of the four oculomotor nuclei with its partner of the 

 opposite center. The exchange occurs between the two ventromedial 

 nuclei; it starts at the end of the fifth day and is completed at 8 days. 

 Since the two populations moving across the midline are of a con- 

 siderable size, the traffic of the neuroblasts moving in the opposite 

 direction and crossing each other is heavy. At 6 days a continuous 

 trail of neuroblasts in different rows bridge the two oculomotor cen- 

 ters (Figs. 36', 14). When their final settlement is completed at 7 

 days, the axons trailing behind make a ventral commissure on the 

 midline (Figs. 3c', 15). 



Migratory Movements in Suprasegmental Structures. In all supra- 



