MODEL MAKING 45 



If true, then, in what way does the act of the hving cell differ 

 from that of the model? 



Herrera and Leduc have made imitation cells with nuclei 

 and mitotic figures. 



A study by Kuwada of the distribution of magnets floating on 

 water in an attempt to explain the orientation of chromosomes 

 in cells is an experiment in model making. 



One of the most fundamental and much studied problems in 

 biology is the mechanism of cell permeability. The manner in 

 which the protoplasmic membrane lets certain substances pass 

 and others not and certain ones enter in greater quantity than 

 others is unknown. There have been many attempts to emulate, 

 the permeability of the plasma membrane. Osterhout and 

 Northrop have made artificial cells which imitate the permea- 

 bility of the living plasma membrane quite well. Northrop 

 constructed thimbles of collodion membranes and found them 

 to be permeable to water, ammonia, and hydrochloric acid; 

 slightly permeable to carbon dioxide, oxygen, hydrogen, 

 and weak acids; and practically impermeable to salts, strong 

 acids, bases, sugar, and glycerin. The permeability resembles 

 that of living cells. 



Another more pretentious type of artificial plant cell is that of 

 MacDougal and Moravek. Thimbles of porous clay or cellulose 

 were impregnated with agar, then dipped in pectin, in imitation 

 of the root-hair cell with its pectin layer in the wall. The inner 

 surface of the wall of the artificial cell was coated with fatty 

 substances (lecithin and cholesterol), and, finally, a layer of 

 "protoplasm" was added, i.e., of agar, or gelatin, and proteins. 

 Diversity in results was obtained by variations in the construc- 

 tion of the cell. One cell, with a lecithin layer, permitted the 

 entrance of sodium chloride, potassium chloride, and calcium 

 chloride, so that permeability was greater for sodium, less for 

 potassium, and least for calcium, thus imitating the selective 

 permeability of the living cell. Omission of the fatty layer and 

 the use of certain proportions of gelatin and agar reversed the 

 relations between the rates of entrance of sodium and of potas- 

 sium. The entrance of ions into the artificial cell shows a parallel- 

 ism to that in the living cell which is very striking. The reactions 

 of the artificial cell to salt and to acidity changes also fairly well 

 duplicate the swelling of living tissues of cacti in similar solutions. 



