Working at the MBL in the late 1880s and 1890s, E.B. Wilson and Edwin Grant Conklin were among the first embryologists to look closely at cellular events in early embryos. By following the lineage of every cell from the single-cell stage to the larval stage, Wilson and Conklin demonstrated that in several species of marine snails and worms entire organs and tissue types arose from individual or small groups of cells in the early embryo.
To accomplish this, they produced hundreds of drawings of embryos at various stages with the aid of a camera lucida. Made up of a series of mirrors that attaches to a microscope, a camera lucida allows specimens seen through the eyepiece to essentially be traced. The process of creating these drawings and the drawings themselves helped Wilson and Conklin to thoroughly observe each specimen and keep track of the behavior of individual cells. Because they worked primarily from slides of fixed and stained embryos, these drawings also played a crucial role in their ability to see cell behaviors in the first place. Wilson and Conklin could infer cellular divisions or movements by comparing drawings of similarly staged embryos and detecting the differences between them.
Once they traced cells and worked out lineages, Wilson and Conklin prepared final drawings that were sent off to lithographers to be copied onto plates and printed. In each their comprehensive publications, Wilson and Conklin described cell lineage in Nereis and Crepidula, respectively, with dozens of full color illustrations.