The 19th century debates on mammalian classification in the light of the new evolutionary thinking led to controversies between Thomas Huxley and Richard Owen concerning the value of the placenta as a representative key organ. As a main point in his argument, Huxley provided a detailed description of a sectioned rat placenta, highlighting the importance of decidualization of the uterus as an argument supporting an evolutionary relationship between rodents, insectivores and primates, an idea hotly contested by Owen. In addition, he illustrated and correctly interpreted the maternal blood supply from uterus to placenta in striking detail. During the succeeding decades the key role of trophoblast in placenta formation was discovered, and the decidua became neglected in later comparative studies. Nevertheless, at the present time trophoblast-decidual interaction is regarded as an extremely important feature of placental development in both primates and rodents, and Huxley can therefore rightfully be considered as an early pioneer in placental research.