MOVEMENTS OF FOAM-DROPS 49 



described is, as a rule, connected with a forward movement 

 of the drop in the direction of the axial current. If such 

 drops are watched for some time, a slowing down of the 

 axial current, and the appearance of a new one, directed 

 towards another point of the edge, can often be observed, 

 upon which the drop naturally moves forward in the new 

 direction. 



If the larger drops, which show the above described 

 circulatory streaming, are more or less pressed by the cover- 

 slip, an object best effected by pushing a splinter of a 

 moderately thick cover glass under the edge of the cover- 

 shp, removing the paraf&n feet, and then drawing off the 

 glycerine as reqmred, the phenomena of their streaming move- 

 ment gradually take on the character that has just been 

 described for the smaller ones. Although the currents 

 rushing from above and below towards the equator usually 

 continue for some time longer, there usually appear marginal 

 centres, from which the currents spread out ; a single such 

 centre in the moderately large drops, or frequently several 

 or many in the larger ones. The more or less energetic 

 streamings from these centres finally suppress the original 

 circulatory currents. The middle-sized drops, which develop 

 one such centre of extension-currents, take on, as a rule, an 

 elongate oval shape, for the most part with a slightly 

 widened anterior end, in which the centre of the extension- 

 currents is placed. At the same time, they move for- 

 ward energetically in the direction of the current which 

 passes along the axis to the edge where the streams spread 

 out. I have observed such drops, which in one miaute tra- 

 velled about 0'12 mm. As I only took a few measurements, 

 it is not to be supposed that the measurement here given 

 has reference to a maxunimi of rapidity. We have to 

 deal here not merely with a displacement forwards of the 

 centre of extension, or with a stretching of the elongate 

 oval drop, but rather with a movement of it forwards 

 as a whole, like a simple amceba. This can easily be 

 determined by simple observation, but may also be definitely 

 proved by marking the position of the anterior and 

 posterior end by means of a micrometer. I have already 



4 



