I] 



INTRODUCTION 



Fig. 2. Diagrammatic longitudinal 

 section through Hydra, a tubular 

 animal showing cellular differen- 

 tiation of a simple kind. (From 

 Shipley and MacBride.) 1 Mouth, 

 2 Foot, 3 Tentacle, 4 Digestive 

 Cavity, 5 Outer layer of cells or Ec- 

 toderm, 6 Inner layer of cells or 

 Endoderm, 7 Structureless jelly 

 between the two latter, 8 Groups of 

 cells specialised for offensive pur- 

 poses and containing structures 

 which can be ejected, 9 Testis, 

 10 Ovary. 



