340 ALBERT KUNTZ AND O. V. BATSON 



from the cell-masses lying outside the neural tube are present in 

 these segments. In the posterior portion of the region involved 

 in the operation the destruction of tissue was less extensive. 

 In these segments the ventral portion of the neural tube appears 

 nearly normal. The dorsoventral diameter of the neural tube is 

 materially reduced. Two small neural canals each of which is 

 surrounded by an ependymal layer are present. One of these 

 canals occupies the position of the ventral portion of the normal 

 neural canal; the other lies in an asymmetrical position in the 

 dorsal portion of the remnant of the neural tube (fig. 2). In 

 several segments at this level spinal ganglia and dorsal nerve- 

 roots are absent unilaterally. On the opposite side nerve cells 

 lying outside the neural tube but not apparently involved in the 

 ventral nerve-roots are present in all of these segments. These 

 cell-aggregates may be of neural crest origin, but they do not 

 constitute definitive spinal ganglia. Cell-aggregates which con- 

 stitute the primordia of the ganglia of the sympathetic trunks are 

 present in every segment in which there is a well defined ventral 

 nerve-root throughout the entire region affected by the operation ; 

 consequently, the primordia of the sympathetic gangha in those 

 segments in which the spinal ganglia and dorsal nerve-roots are 

 wanting must have arisen from cells which advanced from the 

 neural tube along the paths of the ventral nerve-roots. 



Figure 2 is taken from a section from one of the segments in 

 which the remnant of the neural tube contains two neural canals 

 and from which the spinal ganglia and dorsal nerve-root are 

 absent. In this segment a sympathetic primordhnn of consid- 

 erable size is connected with the ventral nerve-root by means of 

 a fibro-cellular communicating ramus. The presence of num- 

 erous cells of nervous origin along the spinal nerve and the com- 

 municating ramus suggests that migration along this path has 

 not yet ceased. The primordia of the prevertebral plexuses are 

 already present. A few nerve-fibers emerge from the dorsal 

 portion of the remnant of the neural tube in this segment; how- 

 ever, they do not join the ventral nerve-root, but grow into the 

 differentiating myotome. The relation of these fibers to the myo- 

 tome suggests that they are not sensory, but motor fibers; conse- 

 quently, they do not represent the dorsal nerve-root. 



