In this paper, we want to present the genesis of Stanisław Lesniewski's mereology. Although 'mereology' comes from theword 'part', mereology arose as a theory of collective classes. That is why we present the differences between the concepts of being a distributive class and being a collective class. Next, we present Lesniewski's original mereology
from 1927, but with a modern approach. Lesniewski was inspired to create his concept of classes and their elements by Russell's antinomy. To face it, Lesniewski had to define the concept of being an element of based on the concept of being part of. Lesniewski showed that in his theory, there is no equivalent to Russell's antinomy. We will show that his solution has nothing to do with the original approach because, in both cases, we are talking about objects of a different kind. Russell's original antinomy concerned distributive classes, and Lesniewski's considerations concerned collective classes.