CHAPTER XIII 



AMEBOID MOTION AND TROPISMS, CELL DIVISION, 

 FERTILIZATION AND PARTHENOGENESIS 



It was shown by Quincke that movements of drops of fluid, 

 resembling ameboid motion, could be produced by localized 

 changes in surface tension. A local reduction in surface tension 

 produces a protrusion of the affected surface — a localized increase 

 produces a flattening. By a combination of such changes in sur- 

 face tension, combined with adhesions to the substratum and to 

 food particles, Rhumbler (1905, 1910) has imitated all forms of 

 ameboid motion. 



In imitating food taking, Rhumbler found that his artificial 

 amebae would take only such food as adhered to them. A chloro- 

 form drop would engulf a piece of shellac but not take a piece 

 of glass. If a piece of glass is coated with shellac, the chloro- 

 form drop will engulf it, but after the shellac is dissolved off 

 it will reject the glass. In this way the process of defecation 

 by the ameba is imitated. Such a process also imitates the shell 

 building of some protozoa. These shells are formed of defecated 

 particles which cover the surface of the protozoon. If chloro- 

 form is shaken with powdered glass and dropped into water, 

 the drops are found to be surrounded by shells composed of 

 particles of glass. 



The ameba apparently does not adhere to all of the food that 

 it takes, shown by the fact that it often takes in a quantity of 

 water with the food particle. The food is separated from the 

 protoplasm on all sides by water, and thus, after ingestion, comes 

 to lie in a food vacuole. Bernstein (1900) was able to make 

 an artificial ameba take food to which it did not adhere. A drop 

 of mercury was immersed in dilute nitric acid and a crystal of 

 potassium bichromate held near it. As soon as some dissolved 

 bichromate reached the mercury the surface tension was reduced 

 and the mercury moved toward the crystal. The momentum of 



