Wilson traced the development of Nereis in minute detail from fertilized egg to the free-swimming larval stage, a pioneer study of cell-lineage.

The first atlas of photomicrographs showing fertilization and cellular development during mitosis. The photomicrographs were taken by Leaming. Digital facsimile from the Internet Archive at this link.

Wilson emphasized the function of cytology in the study of embryology, heredity, evolution and general physiology. The above work has been called the single most influential treatise on cytology of the 20th century. The third edition was extensively revised and enlarged as The cell in development and heredity, 1925.