Like the fully formed organisms they eventually become, embryos are made of up of cells. The behavior of these individual cells underlies the larger scale tissue and morphological transformations that make up embryogenesis. Since the end of the 19th century, when the technologies first became available to clearly see them, embryologists have produced drawings, micrographs and films to investigate the cellular building blocks of life. By carefully tracing cell movements and lineages, researchers such as E.B. Wilson, Edwin Grant Conklin, Johannes Holtfreter, and John P. Trinkaus made important discoveries about the mechanisms and organization of developing embryos.