Wilhelm His was one of Haeckel's most authoritative and primary opponents advocating physiological embryology. His "Anatomie menschlicher Embryonen" (Anatomy of human embryos) employs a series of his most important drawings chronicling developing embryos from the end of the second week through the end of the second month of pregnancy. In 1878, His begins to engage in serious study of the anatomy of human embryos for his drawings. During the 19th century, embryologists often obtained early human embryos from abortions and miscarriages, postmortems of pregnant women and collections in anatomical museums. In order to construct his series of drawings, His collected specimens which he manipulated into a form that he could operate with.