LIST OF ILLUSTRATIONS 



PAGE 



FIGS. 1-3. Ovarian Eggs of Jellyfish, Phialidium gre- 

 garium, Indicating Relation between Polar- 

 ity and Position 55 



FIG. 4. Growing Oocyte of Sternaspis scutata ... 56 



FIG. 5. Semi-diagrammatic View of Ovarian Egg of 

 Frog, Indicating Relation between Polarity 

 and Circulation 5 6 



FIG. 6. Diagram Indicating the Electric Currents 



Arising in Relation to an Excitation Gradient 73 



FIG. 7. Diagram Illustrating Transmission up a 



Gradient 9& 



FIGS. 8-12. Reorganization in Pieces of Planar la dorotoce- 

 phala from Different Body Levels in Relation 

 to Presence or Absence of Head . . . . 101 



FIGS. 13-15. Axial Gradients in Development of the Cen- 

 tral Nervous System 109 



FIGS. 1 6, 17. Transverse Sections of Chick Embryo at Dif- 

 ferent Levels, to Indicate Developmental 

 Gradients m 



FIG. 1 8. Sensory Cells of Sea-Anemone Tentacle and 



Their Outgrowths 112 



FIGS. 19-22. Planar la dorotocephala: Reconstitution of 



Pieces from Different Levels 122 



FIGS. 23-29. Tubularla: Reconstitution of Short Pieces . 124 



FIGS. 30-36. Partial Structures from Short Pieces oiPlanaria 125 

 FIG. 37. Regions of High Susceptibility in Early Larval 



Stage of Chaetopterus, a Polychaete Annelid 133 



FIG. 38. Unsegmented Egg of Frog, Viewed from 

 Apical Pole, Showing Differential Suscepti- 

 bility . 



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