At Johns Hopkins, Conklin’s dissertation advisor William Keith Brooks assigned him to go to Woods Hole and study siphonophore development. It turned out to be fortunate that there were none to study, and Conklin instead looked at other organisms. He discovered the slipper snail Crepidula. The snail’s eggs are rather large, with cells that are very easy to see as the cells go through cell divisions. Conklin could observe the cell divisions in living embryos, and the snails obliged by providing many fertilized eggs at a time. In addition, Conklin had all the tools he needed to make excellent preserved sections, stained in ways that allowed him to see each cell’s details – like the dividing chromosomes. This careful tracking of cells and their changes was very appropriately called cell lineage study. At the time, Brooks said he didn’t see why anybody would want to count cells, and called the work “cellular bookkeeping.” Conklin went on to study Ascidian eggs as well, and he used his slides for teaching in the MBL Embryology Course for decades. Others at the MBL carried out similar work on other species, which allowed comparison and helped reveal some of the fundamental facts of cells and development. Cell lineage study is still important today for developmental biology.