198 PROTOPLASM 



exist.) The same is true of cells within the bodies of plants and 

 animals. Body fluids are aqueous solutions the salt content of 

 which is kept fairly constant. These facts indicate that the 

 osmotic pressure of a solution is of considerable significance for 

 a plant or animal, even though some other facts show that osmotic 

 pressure has no marked influence — certainly little in comparison 

 with the chemical constitution of the solution; thus, animal cells 

 grown in tissue culture thrive equally well in culture solutions 

 of different osmotic values. Experiments by Jacques Loeb 

 on the parthenogenetic development of eggs offset these in tissue 

 culture. 



Parthenogenesis is the growth of an egg without fertilization, 

 i.e., without a sperm having entered. What actually starts 

 the egg dividing in parthenogenetic development it is difficult 

 to say. Loeb was uncertain about it, but he laid emphasis on 

 one of two factors — osmotic pressure and the formation of a 

 fertilization membrane. (Such a membrane is formed at the 

 time of fertilization and becomes a conspicuous structure, con- 

 siderably larger than the egg which it surrounds.) It was 

 possible to cause sea-urchin eggs to develop into larvae by simply 

 exposing them for two hours to hypertonic sea water, i.e., sea 

 water to which salt or sugar had been added. Later experiments 

 showed that other factors may also bring on or hasten partheno- 

 genetic development; hydroxyl or potassium ions and acids will 

 do so. But unfertilized eggs can be activated without these 

 substances and without membrane formation simply by adding 

 a nonelectrolyte such as sugar to the sea water, i.e., by hyper- 

 tonic water. It thus appears that osmotic pressure is the 

 primary stimulus of parthenogenesis when occasioned by 

 solutions. 



The entrance of water into the large vacuole of plant cells is a 

 purely osmotic process. In one-celled animals, and in some few 

 plants in a modified form, there exists a contractile vacuole which 

 pulsates rhythmically, discharging its contents to the outside 

 and then refilling again with liquid from within (Fig. 25). 

 Such contractile vacuoles are especially characteristic of the 

 fresh-water Protozoa. Usually there is but 1, though certain 

 amoebae may have as many as 10. Their function appears to 

 be that of ridding the unicellular organism of waste organic 

 matter and excess water taken in in the process of feeding. The 



