Rhodiginus speaks of a two-headed monster born in Ferrari, Italy, in 1540, well formed, and with two sets of genitals, one male and the other female.

Pare gives a picture of twins, born near Heidelberg in 1486, which had double bodies joined back to back; one of the twins had the aspect of a female and the other of a male, though both had two sets of genitals.
Scientific Teratology .-- About the first half of the eighteenth century what might be called the positive period of teratology begins.
Following the advent of this era come Mery, Duverney, Winslow, Lemery, and Littre.

In their works true and concise descriptions are given and violent attacks are made against the ancient beliefs and prejudices.
From the beginning of the second half of the last century to the present time may be termed the scientific epoch of teratology.

We can almost with a certainty start this era with the names of Haller, Morgagni, Geoffroy-Saint-Hilaire, and Meckel, who adduced the explanations asked for by Harvey and Wolff.

From the appearance of the treatise by Geoffroy-Saint-Hilaire, teratology has made enormous strides, and is to-day well on the road to becoming a science.