12 FUNDAMENTALS OF SURMIC ROSCOPIC MORPHOLOGY I 



the methods mentioned and in many cases also by the electron micro- 

 scope. It is seen that there is a continuous transition from the lifeless 

 amicroscopic molecules to the living cells at the limit of microscopic 

 visibility. The smallest particles which exhibit phenomena of life (self- 

 multiplication) are in the submicroscopic region. Theoretical biology^ 

 being concerned with the definition and the essence of life, is therefore 

 called upon to give serious attention to our branch of morphology. 

 On the other hand, these colloid particles often give the impression of 

 consisting of uniform, chemically well-defined substances, and the 

 biochemist attributes molecular weights to them which, depending 

 on the size of the particles, may assume fantastically large values. 



b. Homogeneity 



Real solutions containing amicroscopic particles are designated as 

 uniform or homogeneous from a physico-chemical point of view. Sols, 

 however, are not considered as uniform; they are heterogeneous. The 

 concept of homogeneity applied here is essentially different from the 

 optical homogeneity which plays such an important part in microscopy. 

 A medium is optically homogeneous when its constituent parts have 

 the same refractive index, so that it is impossible to establish their 

 boundary line by means of light. 



Physico-chemical homogeneity, however, requires that two parts 

 taken from the object shall be identical, not only in their behaviour 

 towards light, but also in all other properties. This will be the case if 

 the particles are similarly arranged throughout the whole object 

 (Figs. 3-7). 



Several homogeneous arrangements of particles are possible. The 

 structural elements can be arranged irregularly, like the molecules of 

 a liquid or gas. The distances between the particles are not all equal, 

 but if we proceed through the mass along a straight line, the average 

 distance found will be constant, and equal volume elements will on 

 the average contain an equal number of particles. Such arrangements 

 are called statistically homogeneous in contrast to the distribution of the 

 atoms in a crystal, which are arranged in a certain pattern. As all 

 distances in a given direction are identical, this is called a lattice 

 arrangement. The spacings can be equal in three directions which 

 are mutually perpendicular; in that case the lattice arrangement is 

 isotropic (Fig. 4). Or else, the spacings are different in different di- 



