De ovi mammalium et hominis genesi (1827), by Karl Ernst von Baer

De ovi mammalium et hominis genesi (On the Genesis of the Ovum of Mammals and of Men) is an 1827 pamphlet by Karl Ernst von Baer about the anatomical observation and description of the egg [3] (ovum [4]) of mammals, like dogs and humans [5]. The pamphlet detailed evidence for the existence of the ovum [4] at the beginning of the developmental process in mammals. Prior to von Baer's publication, there was much debate about how organisms develop, as some claimed that organisms grow from a corpuscular element already preformed in the body (preformationism), and others said that organisms developed from a fluid material undergoing a process of progressive formation (epigenesis [6]). Researchers at the time struggled to observe the early stages of development, and those such as von Baer had to observe the phenomenon through microscopes and then provide interpretations of the phenomena they observed.