He used this term in his 1868 book "Lay Sermons, Addresses, and Reviews" to describe the process of fertilization, specifically the union of the sperm and egg cells. He described it as a "protoplasmic kiss" because of the intimate and essential merging of these two cells, which initiates the development of a new organism.
While the term "protoplasmic kisses" is now considered somewhat outdated, it was a vivid and poetic description of the fundamental process of reproduction at the time.