NOTE ON THE GEOTROPISM OF ARBACIA LARVAE. 



E. P. LYON. 



As soon as the blastulae of the sea urchin begin to swim, they 

 come to the top of the water. They remain in this locality dur- 

 ing the blastula and gastrula stages but become scattered when 

 they reach the pluteus condition. 



The gathering at the top is not a light effect, for it takes place 

 in total darkness. 



The gathering is not due to lack of oxygen, for the larvae go 

 up in an inverted test-tube filled with sea water and remain at 

 the top a long time. With the dense gathering of thousands 

 of blastulae at the top of such a tube the gradient of increasing 

 oxygen supply must be from above downward, yet the larvae do 

 not go down. A narrow tube closed at one end was nearly 

 filled with water containing young arbacia. An air bubble was 

 introduced into the tube about the middle of its length. The 

 tube was then placed with the closed end directed up. The 

 larvae went up in both ends of the tube, those above the bubble 

 going directly away from the oxygen supply. 



Experiments in tubes kept at constant temperature showed 

 that the gathering was not due to currents in the water. Nor 

 was the gathering due to the larvae being lighter than sea water. 

 Their specific gravity, on the contrary, is considerably higher, 

 being about 1.060. 



That the gathering of larvae at the top is a true gravity effect 

 is indicated by their moving against a centrifugal force. The 

 best way to demonstrate this is by placing sea water containing 

 many larvae in rather long (say, 25 cm.), narrow tubes. On 

 rotating these carefully, it will be found that beyond a certain 

 radius (depending on the rate of rotation) the larvae are precipi- 

 tated, the acceleration in these parts of the tube being greater 

 than they can swim against. Inside this critical circle the larvae 

 will be found to have moved toward the axis of rotation and 

 to be gathered in dense crowds. 



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