The biological meaning of the mass coefficient "a" and mass exponent "b" of the power function relating body mass "M" and a morphometric or physiologic variable "y" is discussed (y = aMb). The mass coefficient represents the effect of intensive or qualitative factors on the considered function or structure. The theoretical mass exponent is a criterion for the constancy of the mass coefficient and the qualitative sameness of the compared animals. The Theory of Homomorphism shows that the power function is a mathematical-physical model for analyzing structure and function in animals of different size.