was a Japanese embryologist and cell biologist. He was born in 1904 in Tokyo, the youngest son of Baron Dan Takuma, president of the Mitsui Gomei Kaisha Corporation. Takuma Dan was educated in the United States, graduating from MIT in 1878. He was one of the first foreign students to be educated at MIT and later, as president of the Japan Steel Works, he initiated and maintained close research ties with The Institute. After receiving his undergraduate degree in Japan, Katsuma Dan also came to the United States where he studied embryology with Prof. L.V. Heilbrunn at the University of Pennsylvania. In addition Dan worked and studied at the Marine Biological Laboratory at Woods Hole (MBL) from 1931 to 1934, and later in 1936. It was here that he met his future wife (and longtime scientific collaborator) Jean M. Clark (1910–1978). Clark, who was also a student of Heilbrunn's, studied fertilization in marine invertebrates. The couple raised five children. They also maintained lasting ties to the MBL and returned often in later years as summer researchers and lecturers in embryology. In March 1932, while Dan was studying at the MBL, his father was assassinated in Japan by ultra-nationalist radicals in the 'League of Blood Incident'. Katsuma Dan returned to Japan in the late 1930s and worked at the Misaki Marine Biological Station in Morioso Bay. He and his students maintained a remarkable degree of scientific productivity during World War II. His spirit is reflected in a letter written after the war to a friend in the United States: “...ducking under bombs was not so bad. Rather it was a great excitement. Hide and seek at the expense of your life can't help being exciting. There was, however, an awful side to it too.” Near the end of the war the Japanese Navy took over the Misaki Marine Station and converted it into a base for miniature submarines. Although displaced, Dan and his students set up a crude laboratory nearby and continued their work. At the end of the war Dan posted a hand-written note on the door of Misaki, addressed to advancing America forces, in which he said: “... you can destroy the weapons and the war instruments but save the civil equipments [sic] for the Japanese students. When you are through with your job here notify to the university [sic] and let us come back to our scientific home.”