A biography of an embryo. In early 1914 Johns Hopkins gynæcologist William W. Russell removed the uterus from a 25-year-old patient, Mrs R, who was suffering from a fibroid tumour. At the Carnegie Department, the embryologist Herbert M. Evans found in the uterus a perfect specimen estimated at about 28 days (later re-classified as 32). The embryo, catalogued as number 836, came to designate Streeter’s horizon XIII and O’Rahilly’s stage 13, distinguished by the appearance of limb buds, closed neuropores and the division of the heart into primordial atrium and ventricle. The embryo was photographed, drawn and used to produce models, at least one of which was lost during the collection’s difficult times in the 1960s. But in the 1990s, embryo 836 became the prototype for the NIH-funded ‘Visible Embryo Project’. This digitalized sections and then put them together into dynamic virtual 3-D animations. Embryos dead for decades thus came to communicate ‘life’.

From horizons to stages

The Carnegie collection was used to produce stages that remain the authoritative standards of human development.

Normal plates could not easily accommodate new specimens, which might be more advanced in one respect but delayed in another. So in 1914 Mall classified photographs of the external forms of 266 human embryos from 2 to 25 millimetres long into 14 formal stages, each with characteristic anatomical features. He lacked early specimens, but by the time his successor George L. Streeter retired in 1940 the collection had grown considerably. From internal and external features Streeter now constructed 23 divisions for the first seven weeks.

Wary of the precision that the term ‘stage’ implied, Streeter borrowed ‘horizons’ from geology and archæology to indicate a flexible system based on multiple morphological criteria. Streeter’s articles brought together line drawings, photographs of specimens and reconstructions, descriptions of Carnegie embryos and a reference guide to specimens in other laboratories—but only for horizons 10–23. In the early 1970s, Ronan O’Rahilly, who curated the collection, extended the project to the beginning of development, revised the other horizons and renamed them stages.

Streeter’s horizon XIII, 1951


Working with models and photographs, ?1970s