Historically the exact age of human embryo specimens has long perplexed embryologists. With the menstrual history of the mother often unknown or not exact, and the premenstrual and postmenstrual phases varying considerably among women, age sometimes came down to a best guess based on the weight and size of the embryo.

As the third director of the

Osborne O. Heard was a noted Carnegie embryological model maker for the

As one of the first to work at the

The Carnegie Institution of Washington’s (CIW) Embryology Department was opened in 1914 and remains one of six departments in the CIW. The department quickly became, and remains, world renown for its many embryonic development discoveries. In 1913

Physician and pathologist Elizabeth Maplesden Ramsey was a member of the Carnegie Institution of Washington (CIW) for thirty-nine years. The affiliation began in 1934, when Ramsey discovered what was assumed to be the youngest-known embryo at the time, and donated it to CIW’s massive embryo collection. After studying embryos, Ramsey focused her research on placental circulation in primates.

Franklin Paine Mall was born into a farming family in Belle Plaine, Iowa, on 28 September 1862. While he attended a local academy, an influential teacher fueled Mall’s interest in science. From 1880–1883, he studied medicine at the