Mathias Duval on placental development in mice and rats.

Mathias Duval (1844-1907) was one of the pioneers in elucidating the intricate placental histology of different mammalian groups, notably the rodents. Using a well-dated series of mouse conceptuses, he described in detail the successive steps in placental development, and for confirmation he included observations on a (undated) collection of rat specimens. Not only was he able to identify correctly the different extra-embryonic cell layers, but he was also the first to recognize trophoblast invasion in rodents. Not all his interpretations are still valid, however. Re-reading his extensive and detailed work "Le placenta des rongeurs" (1890-1892) confronts us with still existing gaps in our present understanding of placental development, notably the morphogenesis of the different placental layers and the differentiation of invasive trophoblast. His understanding of uteroplacental blood flow was still limited, and he failed to recognize the complexity of the maternal decidua and its vasculature, which is essential for correctly understanding the pathways and extent of trophoblast invasion. Although Duval was active in promoting Darwin's evolutionary ideas, he refrained from extrapolating too quickly his findings in rodents to other mammalian groups including humans. In his view detailed histological studies on complete series of specimens had to come first, and thus provide a firm basis for a proper understanding of placental function and evolution.