v] NATURE OF PLANT-ANIMALS 139 



or more of the resting cells of the infecting organism 

 come to be included in the capsule- wall. Further, 

 apart from such chance inclusions, thanks to which 

 we were enabled to produce our pure cultures of the 

 alga, the egg-capsules appear to exert a definite 

 chemical attraction on the motile green cells. Thus, 

 if to a bulk of filtered sea-water containing egg- 

 capsules which have been laid under the cleanest 

 possible conditions, a number of the flagellated cells 

 are transferred, then, after a few hours, one or more 

 of the green cells will be found to have settled down 

 on each capsule. Yet more striking results are 

 obtained if a capsule is suspended in a hanging drop 

 that is, a drop of sea-water which depends from 

 the under side of a microscope cover-glass and if 

 a number of the flagellated cells are added to the 

 drop. On observing such a preparation under the 

 microscope, the motile green cells are seen to approach 

 the capsule, to swarm about it, to press in close ranks 

 into the soft, gelatinous wall and so embed themselves 

 in the envelope. We conclude, therefore, that the 

 egg-capsule exercises an attractive (chemotactic) 

 influence on the flagellated algal cells ; or, in other 

 words, that a definite substance diffusing out from 

 the capsule-walls induces a tropistic (tactic) move- 

 ment in the motile, algal cells of such a nature that 

 they approach the source whence the chemical sub- 

 stance emanates. The behaviour of the green cells, 



