108 PLANT-ANIMALS [CH. 



no answer until we have discovered experimentally 

 the origin of the plant-like cells occurring in each 

 species of the many animals which possess them. 



This we proceed to do in the case of C. roscoifensis. 

 Two methods are open to us for the purpose. We must 

 either trace back the green cells to the earliest stage 

 at which they make their appearance in the animal 

 and ascertain whether they may be then identified 

 with any known, free-living alga. If we succeed in 

 this, we shall have obtained, not absolute proof, but 

 strong ground for believing that the green cells are 

 of intrusive origin. Or and this is the only certain 

 way we must cultivate the alga, and having ob- 

 tained animals which are free from it, and having 

 demonstrated that such animals remain indefinitely 

 colourless, we must infect the animal with the algse 

 of our pure algal-culture and synthesise the green 

 plant-animal. 



As we have indicated already, all attempts to 

 isolate living green cells from the body of C. ros- 

 coffensis have failed ; and so it would seem that the 

 former, less satisfactory method alone remains. The 

 application of the method is simple enough. It 

 consists in the microscopic examination of larval 

 C. roscoffensis in all stages, from the time of hatch- 

 ing up to the time when green cells, resembling those 

 of the adult, may be recognised within the body. 



