* 





THE STKUCTURAL BASIS OF THE BODY 21 



advantage of this resistance to run freely over the surface of water, although 

 their specific gravity may be greater than that of water. The continued 

 existence of protoplasm in a watery environment 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 proto- 

 plasm 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 inactivity, 

 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 im- 

 miscible 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 concentra- 

 tion 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 practi- 

 cally solid and resists any turning of the needle. In consequence of the sur- 

 face aggregation and solidification of the colloidal 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 pro- 

 duced gradually collect to form a solid mass of insoluble protein. Proto- 

 plasm may be regarded as essentially fluid in character, the form and rigidity 

 which are acquired by most cells being due to chemical and physical differen- 

 iation 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 environment, this 

 layer acquires a prime importance for the life of the cell, and we may there- 

 fore consider here at greater length some of the properties of this layer, the 

 Phsmahaut, as it has been called. 



The superficial layer of the protoplasm is not to be confounded with the 



