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 56 



Fig. 6. Diagram Indicating the Electric Currents 



Arising in Relation to an Excitation Gradient 73 



Fig. 7. Diagram Illustrating Transmission up a 



Gradient 98 



Figs. 8-12. Reorganization in Pieces of Planar ia 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. 16, 17. Transverse Sections of Chick Embryo at Dif- 

 ferent Levels, to Indicate Developmental 

 Gradients m 



Fig. 18. Sensory Cells of Sea-Anemone Tentacle and 



Their Outgrowths 112 



Figs. 19-22. Planar ia dorotocephala: Reconstitution of 



Pieces from Different Levels 122 



Figs. 23-29. Tubularia: Reconstitution of Short Pieces . 124 



Figs. 30-36. Partial Structures from Short Pieces of Planaria 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 136 



XV 



