Biological Studies on Corymorpha. 415 



For instance, the cells in .v-^i (Fig. 2), if forming part of the piece 

 A, would as a whole be positively geotropic so long as the aboral 

 end of ^ tended to develop a hold-fast (thus preserving the original 

 polarity of the piece). If, however, this end should develop a 

 hydranth, the behavior of these cells would be reversed; they 

 would exhibit, as a whole, negative geotropism. 



There are, then, two manifestations of polarity of apparently 

 different sorts: first, polarity expressed through a special mechan- 

 ism involving a single tissue, in terms of osmotic pressure and 

 consequent movements of geotropic orientation ; second, polarity ex- 

 pressed through a mechanism involving many tissues, in terms of 

 regenerative development and differentiation. However different 

 these may seem, they are undoubtedly referable to the same funda- 

 mental causes — the causes of polarity in general, which involve 

 internal factors at present objects of speculation only. Yet it 

 may be possible to determine these internal factors more easily 

 by means of the facts of what may be called functional polarity 

 than by the relatively complex morphological phenomena of devel- 

 opment and differentiation. The simpler mechanism and the 

 simpler effects of polarity as manifested in the axial cells, are 

 bound to bring a true explanation of polarity nearer our compre- 

 hension, although it may still be unattainable. How an organic 

 membrane or its contents may change in order to produce a change 

 in osmotic pressure, while an enormously difficult problem, is yet 

 more hopeful of solution than the problem of how several tissues 

 simultaneously differentiate in different directions to produce a 

 complicated regeneration. 



c. Locomotio72; Amoeboid Cells. 



We come now to a third type of movement, with a new cause. 

 The ectoderm cells of the proximal end of the stem are capable 

 of amoeboid movements, by the aid of which the hydroid may 

 slowly change its location. In this regard Corymorpha closely 

 resembles Hydra. On a horizontal surface, whatever locomotion 

 there is takes place in any direction, with the one qualification 

 that the stem moves always out of its perisarcal investment, 

 which it leaves behind. 



