122 THE ORIGIN OF THE NERVOUS SYSTEM 



19 



a 



21 



22 



Figs. 19-22. — Planaria dorotoccphala: reconstitution of pieces from 

 different levels: Fig. 19, diagrammatic, showing general plan of central 

 nervous system and alimentary tract, a and b, two regions from which 

 pieces are taken; Fig. 20, early stage of reconstitution in both a and b; 

 Fig. 21, later stage from a; Fig. 22, later stage from b (Figs. 19-21 from 

 Child, 1915c). 



