Growth and Differentiation in the Nervous System 267 



sacral level. In all segments it serves the same function — innervation 

 of the trunk musculature I Fig. 1 ) . 



Thoracic and Sacral Segments. In both segments two massive migra- 

 tory movements take place between the middle of the fourth and the 

 end of the seventh day of incubation. The following description is 

 based on the analysis of this process at the thoracic level. Cell move- 

 ments in the sacral segment present similar features and therefore 

 will not be discussed. 



Before describing the migratory movement, it is of interest to con- 

 sider the motor column, which has barely formed and which consists 

 at 4 days of neuroblasts of similar size and shape in a random dis- 

 tribution. The duality of this apparently homogeneous population 

 becomes manifest if one inspects the peripheral distribution of their 

 nerve fibers. The majority of these fibers end in contact with the 

 primordia of the sympathetic ganglia at the two sides of the spinal 

 cord, whereas only a small fiber confingent is directed toward the 

 trunk muscles. It is therefore evident that the motor column consists 

 of a mixed population of preganglionic and somatomotor nerve cells. 

 In later developmental stages the two populations are topographically 

 well distinct from each other: The preganglionic column is located 

 in birds in a dorsomedial position adjacent to the central canal while 

 the somatomotor column has a medioventral position and is identified 

 as the medial motor column. 



The segregation of the two cell populations is foreshadowed by a 

 change in the texture of the newly formed motor column. At 4% days 

 about three-fourths of the entire cell population assembles in com- 

 pact and parallel rows of neuroblasts oriented toward the central 

 canal (Figs. 1 and 7). A migration of these cells starts immediately 

 after, while the remaining cells continue their differentiation in situ. 

 They form the medial motor column. 



The progression of the migratory cells during the three following 

 days is diagrammatically represented in Fig. 1 and needs no further 

 comment. The intense silver affinity of the migrating cells and the 

 slowness of the entire process, which was studied in close time series 

 of embryos between the fourth and the end of the seventh day, gave 

 the possibility of exploring all the aspects of this movement, which 

 results in the active dislocation of thousands of differentiated nerve 

 cells from their early ventrolateral position to their terminal settle- 

 ment dorsal to the central canal (Figs. 1, 8, and 9). 



At the end of the seventh day the migratory movement is com- 

 pleted, and one can easily identify in the cells which gather in two 



