“Pickled punks” is the carny term for human fetuses preserved in jars of formaldehyde and used as sideshow attractions. Most pickled punks display some sort of anatomical abnormality, such as conjoined twins or polycephaly; however, the deformities present are as varied as the nature of human inflictions. Faked pickled punks, made from rubber or wax, are known as “bouncers” for their tendency to bounce when dropped on the floor. [from Wikipedia]
Many of these were preserved by master anatomist Frederik Ruysch. I have a separate entry on him here.