Throughout his long career, Ronan O'Rahilly contributed to many areas of Anatomy, but it is his outstanding contribution to human embryology that forms his major legacy. This interest was already evident in the first paper he published in the Journal of Anatomy (1946), in which he described a case of radial hemimelia in the right hand of a cook. In addition to presenting the abnormality anatomically, he discussed its possible developmental origins. Hence although his directly embryological work came later, it is clear that his instinct to ask ‘how did it get like that?’ was fundamental to his intellectual approach from the outset, as was the realisation that natural abnormalities could provide clues to normal development. His work was always absolutely meticulous, as was the manner in which his observations were reported.
Ronan was born in Cork in 1921. His father, Dr Alfred O'Rahilly, was at that time Registrar, and later President, of University College, Cork. After qualifying in medicine at UCC in 1943, Ronan held clinical appointments for 3 years at the Royal Hospital, Sheffield. In 1944, he was sponsored for membership of the Anatomical Society by Professor Francis Davies. He continued his anatomical studies under the guidance of Professor M. A. MacConaill, who had moved from Sheffield to become head of anatomy at UCC in 1942. MacConaill's influence played a crucial part in Ronan's choice of profession.
In a memoir he wrote a few years ago about his involvement with the Anatomical Society, Ronan mentioned several of the then current and former heads of department who influenced him and whose acquaintance he felt honoured to make during his attendance at the Society's meetings in his early years of membership. They included Alexander Low, James Couper Brash, Frank Goldby, Frederick Wood Jones, Sir Wilfred Le Gros Clark, Robert Lockhart and A. J. E. Cave. This reads like a list of famous anatomists of the mid‐20th century; five of them were Presidents of the Anatomical Society.
After completing his MSc in 1946, Ronan was appointed to an anatomy lectureship at the Durham University medical school at Newcastle upon Tyne. In 1948, he returned to Sheffield as a lecturer. In 1950, he took up a professorial appointment at Wayne State University, Detroit, followed by 7 years at St. Louis University, Missouri (1962–1969). Apart from three co‐authored papers on the chick eye, all Ronan's output during this period concerned normal and abnormal human skeletal anatomy and development. His life was transformed in 1969 with his appointment to the prestigious post of Director of the Carnegie Institution's Embryological Collection, which he held at the same time as a second period at Detroit. Although very little of his published work had been purely developmental in subject matter up to this date, this appointment was clearly the catalyst that transformed Ronan from being a human anatomist to a human embryologist.
Ronan launched himself into this new role with enthusiasm and seriousness, starting almost at the beginning of development with a study on the establishment of the embryonic axes. As his studies of the Carnegie embryos progressed, he became increasingly dissatisfied with the accepted definitions of developmental timing, which were at that time based on George L. Streeter's ‘Developmental Horizon's in Human Embryos’ (1942–1957). His patient nature was ideally suited to making a precise analysis of the Carnegie collection to create a more accurate scheme of stages, based on developmental characteristics. He published his proposed ‘Carnegie stages’ in a 1979 paper ‘Early human development and the chief sources of information on staged human embryos’ in the European Journal of Obstetrics, Gynaecology and Reproductive Biology (1979), with the clear intention that it should be noted by clinicians as well as research scientists. The following year, in a paper on the development of the vertebral column published jointly with his wife Fabiola Müller and D. B. Meyer, he wrote that in order to permit valid comparisons, the Carnegie stage should be cited if possible, and that “the term ‘horizon’ is obsolete”. By 1987, further refinement of the criteria and a series of careful papers on successive developmental stages resulted in a book, ‘Developmental Stages in Human Embryos’, written jointly with his wife. It covers the first 8 weeks of development, stages 1 to 23. The book is dedicated to the memory of Wilhelm His, who initiated the study of human embryology, and to his protégé Franklin P. Mall, the founder of the Carnegie Collection. The O'Rahilly–Müller duo's second book, ‘Human Embryology & Teratology’, has a very helpful summary of some of the key events at each developmental stage, with simple drawings. This very comprehensive book was a major undertaking and very successful; first published in 1990, it ran to three editions, the last one published in 2001. His extensive contributions to the nomenclature are evident throughout Terminologia Embryologica.
The Carnegie Collection was initially established at Baltimore but moved to Washington, D.C. in 1960. In 1973–1974 it moved again, to the University of California at Davis, and Ronan moved there, too. He remained at Davis until his retirement in 1990. He then returned to Europe and lived for the rest of his life in Fribourg, Switzerland, where he held an honorary university position for 10 years. Even after that, his analysis of the Carnegie collection continued. His special interest in the nervous system was shared by his wife; they published many papers together on this aspect of development, and also produced a third book, ‘The Embryonic Human Brain: an Atlas of Developmental Stages’ (2006). Their last paper, published in 2013, was entitled ‘The longitudinal growth of the neuromeres and the resulting brain in the human embryo’.
Ronan was an accomplished linguist and many of his scholarly publications were in French. He was widely versed in music, the graphic arts and literature. He delivered numerous prestigious lectures, many reflecting the application of his studies to pressing clinical problems, including the impact of the environment on the developing human. The value of his contributions has been recognised in honorary doctorates from University of Montpellier and the National University of Ireland.
Nowadays, almost all advances in human embryology are made using molecular techniques. Nevertheless, the matter of where the molecular events and interactions take place remains central to understanding their significance. The meticulousness and precision of the work of Ronan and Fabiola fundamentally underpins the importance of this structural framework. Indeed, it will become more and more valuable with the increasing need to refine the exact location of such events, both normal and abnormal.