Growth and Differentiation in the Nervous System 265 



column of neurohlasts extends from the cervical to the sacral segment. 

 It is easily identified as the motor column from its position as well as 

 from the direction of the axons which gather in small nerve bundles 

 and emerge from the neural tube as motor roots. The associative and 

 commissural neurons mentioned above give origin to slender fiber 

 tracts; the commissural fiber tract marks the borderline between the 

 compact core of undifferentiated cells which still represent the main 

 component of the neural tube and the differentiated neurons collected 

 in the lateral column. A detailed description of this process is pre- 

 sented in a previous publication (4) ; here we shall briefly outline 

 the developmental patterns of the motor system at the cervical, 

 brachial, and thoracic levels. 



Regional Patterns 



Cervical Segment. The cervical lateral motor column undergoes a 

 dramatic and massive degeneration between 4% anc l 5 days of in- 

 cubation; in these few hours about three-fourths of the neuroblasts 

 which form the column disintegrate and are swept away by macro- 

 phages which gather in the area (4). The combined use of three 

 different techniques — silver impregnation, vital stains, and standard 

 histological techniques — gave a complete picture of this striking 

 process. At 5 days the column is reduced to about one-fourth of its 

 previous width. The surviving cells form the slender medial column 

 which innervates the trunk muscles at that level (Fig. lc). 



The significance of this extensive and short-lasting disintegrative 

 process in the cervical segment of the spinal cord is obscure. We are 

 not aware of the occurrence of similar mass destruction taking place 

 under normal conditions in other sections of the developing nervous 

 system, although we described two other instances of cell disintegra- 

 tion in the cervical and thoracic spinal ganglia of the chick embryos 

 in stages between 5 and 6 days (7). Considerations presented in the 

 previous publication (4) suggested the possibility that the disinte- 

 grating cells at the cervical level might represent an abortive pregan- 

 glionic center. The experiments performed to test this hypothesis will 

 be presented after discussion of the differentiative pattern in other 

 spinal cord segments. 



Brachial and Lumbosacral Segments. In both segments the spinal 

 motor column segregates into a large lateral component which inner- 

 vates, respectively, the anterior and posterior limbs, and into a medio- 

 lateral column of the same width and position as the column described 

 at the cervical level. This column is also present at the thoracic and 



