138 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



brane, in contact with which is one of a more delicate 

 nature. The yolk, too, is enclosed in a very delicate 

 membrane. 



At one end of the egg the two membranes are separated 

 by the air space. Careful removal of the shell permits of 

 the outer one being placed in contact with distilled water 

 by standing the egg on end in a small dish. In this posi- 

 tion the two membranes are adpressed. On testing some 

 of the surrounding water with silver nitrate from time to 

 time, it is seen that chlorides only diffuse through slowly. 

 When, however, the outer membrane is ruptured, a rela- 

 tively rapid diffusion of chloride takes place, showing 

 that the inner is far less resistant to the passage of salts 

 than is the outer membrane. 



The membranes, both outer and inner, are readily 

 penetrated by water; for when the shell has been removed 

 from one end in such a way as not to injure the membrane, 

 and a long, narrow glass tube has been passed through the 

 membrane and cemented in at the other end of an egg, it 

 is seen that the contents rise in the tube when the mem- 

 brane is placed in contact with water. This experiment, 

 described by Bergen and Davis (1906), has been found 

 to be of considerable value as a lecture illustration. The 

 yellow colour of the column, secured by puncturing the 

 yolk, renders it easy to be seen, and a rise of over a metre 

 may be obtained. This height is sometimes maintained 

 for several days. Semi -permeability may also be demon- 

 strated by means of the yoke of an egg. The white and 

 yolk of the egg of the common fowl, Gallus bankiva, are 

 isotonic, and possess an osmotic pressure of 5-5 atmospheres 

 as determined by the cryoscopic method. When the 

 yolk is transferred to water, it swells and at the same time 

 becomes paler in colour. In this distended condition 

 the membrane is very easily ruptured, and cannot even sus- 



