BRIEF [O.A.] Hubrecht, Ambrosius A... W... Author: Hubrecht, Ambrosius A... W... Recipient: Haeckel, Ernst Dating: 12.11.1889
BRIEF [O.A.] Vosmaer, G... C. J. Author: Vosmaer, G... C. J. Recipient: Haeckel, Ernst Dating: 19.12.1889
BRIEF OHNE UMSCHLAG Mulder, Lodewijk Author: Mulder, Lodewijk Recipient: Haeckel, Karl; Haeckel, Ernst Dating: 14.02.1889