136 THE ORIGIN OF THE NERVOUS SYSTEM 



itself segmented is a fact of common knowledge. In 

 the less highly specialized segmented invertebrates a 

 bilateral pair of ganglionic enlargements of the ventral 

 nerve cord, or a single ganglionic mass with more or less 



B 



C 



D 



FIG. 38. Unsegmented egg of frog, viewed from apical pole, show- 

 ing susceptibility gradient to temperature of o C. The stippled areas 

 represent areas of disintegration, the cross-marks the apical pole, the 

 dotted circle the boundary between pigmented and unpigmented 

 regions, and the dotted crescent the outline of the gray crescent, as they 

 appear on the lower hemisphere (from Bellamy, 1919). 



distinct double bilateral structure, appears, but the vari- 

 ous modifications of segmental structure in the different 

 groups are accompanied by modifications of ganglionic 

 arrangement. This is particularly the case as regards 

 the head region which, in the higher invertebrates, 



