SYMPATHETIC SYSTEM IN MAMMALS 



225 



by medullary cells which have broken through the external limit- 

 ing membrane. 



In embryos 7 mm. in length, the spinal ganglia are distinct, but 

 are not completely formed as yet, and have receded but a short 

 distance from the point at which the fibers of the dorsal nerve- 

 roots enter the neural tube. In transverse sections, numerous 

 breaches may be observed in the external limiting membrane in 



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Fig. 5. Transverse section of the ventral part of the neural tube of an em- 

 bryo 9mm. in length, showing cells migrating into the ventral nerve-root, x 250. 



the region of the dorsal nerve-roots. Rows of cells practically 

 touching each other end to end may be traced from the mantle 

 layer, through these breaches, into the proximal parts of the dor- 

 sal nerve-roots (fig. 3, c.m.d.n.r.). Further evidence for the 

 migration of medullary cells into the dorsal nerve-roots is pre- 

 sented by the fact that in many sections where no breaches occur, 

 cells are crowded close to the external limiting membrane in this 

 region. In embryos 9 mm. and over in length, this area is always 



