22 PHYSIOLOGY 



water. The continued existence of protoplasm in a watery environ- 

 ment shows that not only must its composition be different from that 

 of its environment, but that there must be a distinct surface separating 

 the two. The superficial layers of the protoplasm must therefore 

 be in a condition of tension and exercise pressure on the internal 

 portions of the cell, which will tend to diminish the surface of the cell 

 to the smallest possible extent, i.e. to bring it into the spherical form. 



This form is characteristic of free cells in their conditions of in- 

 activity, and the smaller the mass of protoplasm, supposing it to be 

 homogeneous, the greater will be the pressure exerted by its surface 

 layer on its contents and the greater resistance will it present to 

 deformation of the spherical form. A fluid drop, if suspended in a fluid 

 with which it is immiscible, will present greater rigidity the smaller 

 its dimensions. Almost any degree of rigidity can also be imparted to 

 larger masses of fluid protoplasm if their interior has undergone 

 chemical differentiation so as to be made up of two or more immiscible 

 fluids arranged as droplets within alveoli, as in Biitschli's theory. In 

 such a case every droplet will present resistance to deformation and 

 every surface will resist penetration or extension. The resistance of 

 the surface in colloidal fluids is still further increased by a property 

 common to all these fluids, namely, the aggregation in the surface of a 

 greater concentration of the dissolved substance than is present in 

 the underlying fluid. If, for instance, we take a beaker containing egg- 

 white diluted 100 times, and drop a steel magnetised needle on to 

 the surface, it will float although it is much heavier than the fluid, 

 in consequence of the resistance of the surface. If the needle be 

 greasy the same thing will occur on a glass of water. In this case 

 the needle will lie N. and S. On the albumen solution, however, the 

 needle will lie in the position in which it has been dropped. The 

 aggregation of the albumen molecules on the surface of the fluid is 

 such that it is practically solid and resists any turning of the needle. 

 In consequence of the surface aggregation and solidification of the 

 CDlloidal molecules, it is possible to throw out the greater part of the 

 albumen in a solid form from a solution of this substance, if it be shaken 

 up in a bottle with a little air so as to make a surface. As the fluid 

 is shaken fresh surfaces are always being formed, and the albumen 

 aggregating in each of these surfaces has not time to redissolve before 

 a fresh aggregation occurs on a new surface, and the films thus 

 produced gradually collect to form a solid mass of insoluble protein. 

 Protoplasm may be regarded as essentially fluid in character, the form 

 and rigidity which are acquired by most cells being due to chemical and 

 physical differentiation occurring in the fluid. 



THE SURFACE LAYER OF CELLS. Since it is by means of its 

 surface layer that the organism enters into relation with its environ- 



