Already in classical antiquity people dealt with the principle of formation, developing different theories. Researchers in the renaissance, working in the conflict zone between tradition and experience, tried to prove one or the other of these theories by the means of new observations, especially of chicken development. Aldrovandi was the first to see the real principle of formation of the hen's egg, i. e. the blastodisc, but he didn't recognize the importance of his discovery due to his close adherence to (...) Aristotle in the theoretical field. Fabricius even thought that traditional knowledge was of more importance than his own excellent observations. Parisano was the first to succeed in making a correct interpretation of the function of the blastodisc, but only by holding to a ‘false’ classical theory. Harvey combined his attempt to restore the developmental theory of Aristotle with a religious interpretation postulating God's intervention in all development. Subsequent to atomism, Highmore evolved a two seed theory of development, which in his view made a permanent engagement of God superfluous. Also the first observations using the microscope did not contribute to any improvement in developmental theory. Malpighi used them to confirm the theory of epigenesis, whereas Croone attributed to a piece of blastoderm the proportion of a whole embryo to demonstrate his ovistic theory of preformation. The founder of animalculism Leeuwenhoek, an amateur researcher, was at first not influenced by the trends of the scientific community. He postulated that the spermatozoa, which he discovered, contained perfect miniature animals. His investigations are a good example of where prejudices can lead, even when the observations are excellent. In the 17th century the tension between experience and tradition shifted in favour of experience, but a final solution had not by any means been reached. (shrink)