178 F. KKEULE AND F. W. GAMBLE. 



These cells tniisfc have been derived from one or other or both 

 of two sources — viz. from the body of Convolutas disintegrated 

 during egg-laying or from the environment. In the former 

 case they must be special cells which, unlike the great 

 majority of the green cells of the adult body, have undergone 

 no degenerative changes, and so retain the capacity for 

 development. 



The evidence points to the environment as the place of 

 origin of the green cells of the capsules. In the first place, 

 such cells are by no means uniformly present on the capsules, 

 and in the second place they are present less often on capsules 

 laid by animals which have been freed in some measure by 

 repeated washings from their associated flora and fauna. The 

 effect of this repeated washing is not to render the surfaces 

 of the animals free from extraneous organisms, but to reduce 

 their numbers and to confine them to such as attach them- 

 selves most tenaciously to the slime which covers the surface 

 of Convoluta. Chief among these most intimate associates 

 are the infecting alga, certain other minute unicellular alga3, 

 and various diatoms. Repeated washings, then, reduce the 

 number of competitors for place on the capsules, and though 

 some of the infecting organisms may themselves be swept 

 away those which remain find their tenancy when they succeed 

 in reaching a capsule less disputed than is the case under 

 more natural conditions. 



In the third place, as we show (Section IV), the infecting 

 alga does not depend on chance for its association either with 

 the surface of the animal or with the egg-capsule. It is 

 attracted chemotactically thereto, and hence, though the 

 numbers of infecting alg^ in the water should be but few, 

 they will inevitably distribute themselves upon some of the 

 capsules. 



Summary of Section II. 



(1) Convoluta roscoff ensis commences life as a colour- 

 less (non-green) animal. 



