After many years of working with polychaete worm embryos and completing careful studies of Nereis development at the MBL during the summer, Wilson published his comprehensive cell lineage study, titled The Cell-Lineage of Nereis: A Contribution to the Cytogeny of the Annelid Body, in the Journal of Morphology in1892. By demonstrating that in Nereis, as in several other species, the cells of the early embryo divide according to strict and regular patterns and that these divisions establish lineages of certain tissues and organs, this paper demonstrates that embryologists should pay attention to developmental events prior to the establishment of the germ layers. In Nereis, early cell divisions do not just replicate material that is later organized; rather, these divisions are organize the embryo into cell types that will give rise to the germ layers, and ultimately adult tissues and organs (Wilson 1892). For example, Wilson demonstrates that the mesoderm arises from a single cell, called the mesoblast, that results from the fourth cleavage (Wilson 1892).
In order to study cell lineage, Wilson collected and carefully observed hundreds of embryos. Although he was sometimes able to keep his worms alive through the night and initiate spawning and fertilization in the morning, those embryos usually did not develop normally (Wilson 1892). Thus, he had to follow their early cleavage patterns into the late hours of the night. Due to the extremely slow development of Nereis, this was no small feat. Once fertilized, it takes about an hour and a half before the first cleavage, with the subsequent cleavages occurring in approximately forty-five minute intervals after that. Because he primarily worked in the dark, Wilson had to use a lamp when making observations through the microscope, often posing significant challenges because the heat from the lamp would speed up development. The increased speed made it more difficult to carefully observe and record the cell divisions at each stage. To overcome this challenge, Wilson kept the embryos in a cool aquarium and would pull out and observe new embryos at the completion of each cell division (Wilson 1892).
More recent cell lineage studies of the related polychaete species, Capitella teleta and Platynereis dumerilii, have confirmed many of Wilson’s findings in his studies of Nereis, although no further studies have been published based on Nereis itself (Seaver 2014). Considering that modern cell lineage studies utilize microinjected intra-cellular dyes and 4D microscopy, which allows for semi-automated cell tracking, it is remarkable that Wilson was able to achieve similar results with just careful observation, a microscope, a pencil and paper (Seaver 2014). At approximately 160um in diameter (Fischer et al 2010), these embryos are extremely small and must be studied with at least a 40x objective, which must have made keeping track of cellular movements difficult for Wilson. Today, however, their small size makes them easy to study, as well as their larval forms, because they can be imaged whole with both light and confocal microscopy (Fischer et al 2010).