22 E. P. LYON. 



Paramoecia will rise to the top of a solution of the same den- 

 sity as themselves and in which they have, therefore, no weight. 

 This fact, to my mind, is proof that gravity acts within the organ- 

 ism and not through the relative densities of water and organism. 

 With the larvae of arbacia, however, I was unable to get any 

 definite gatherings in a solution of gum arabic and sea water of 

 the same density as the larvae. The viscosity of this medium is 

 so great that such minute ciliated organisms as those under con- 

 sideration can hardly move through it. 



While convinced, therefore, that the gathering of the arbacia 

 larvae at the top of the water is a gravity effect, I am unable at 

 the present time to say how gravity brings about the response. 

 The possibility that the orientation is purely passive, a buoyancy 

 effect, must be kept in mind, as well as the various theories pro- 

 posed by Jensen and Davenport. However, on the basis of my 

 paramoecium work, I consider it most probable that gravity acts 

 directly on the cells. 



The blastulae from centrifugalized eggs have all the pigment 

 on one side and are therefore presumably heavier on that side. 

 Later examination shows that this pigment may be in any part 

 of the gastrulae or plutei. Such larvae, although, as stated, pre- 

 sumably unbalanced, nevertheless come to the top of the water. 

 I consider this observation opposed to the theory of orientation 

 by buoyancy, for if the heavier portion were always directed 

 down and this portion sustained no relation to the direction of 

 locomotion, the organisms ought to be scattered through the 

 water. 



