Osborne Heard with some of his embryo reconstructions.

Not only age but size, too, proved a poor way to organize the embryos. An embryo could shrink a full 50% in the preserving fluids. Mall took it upon himself to find a better way. He had more success basing his "staging" scheme on morphological characteristics. To that end, Mall and his colleagues not only prepared and preserved serial sections of the embryos, they also made hundreds of three-dimensional models at different stages of growth. According to Adrianne Noe, who now manages the collection at the National Museum of Health and Medicine, Mall gathered the most renowned scientists, scholars, artists, photographers, and craftspeople ever to apply their interests and skills to embryology. One of the first to be hired, in 1913, was modeler Osborne O. Heard, who spent 42 years at the department and made over 700 wax-based reconstructions. The results of this team effort still stand as the international standard by which human embryos are described and classified.

Although Mall died prematurely in 1917 from complications during gall bladder surgery, his successor was not only a gifted scientist but an organized and efficient administrator who continued the department's research program much as Mall had envisioned. George Streeter, who served as director until 1940, was one of the foremost authorities on the development of the human brain. Under his supervision, hundreds of specimens were added to the collection every year. Notable were the rare, very young normal specimens. (Induced abortions were illegal in the United States and miscarriage usually results in abnormal embryos.) One of the department's early contributors, Elizabeth Ramsey (now deceased), recalls in a 1976 newsletter article her excitement when she and colleagues at Yale discovered a normal 14-day-old embryo during a routine autopsy in 1934. "The work with the Yale embryo was the most interesting professional thing in my life," she said. Later came even earlier embryos, many of them collected by research associates Arthur Hertig and John Rock at a Boston health clinic. One of those embryos was only seven and one-half days old.