Evolution of the Organic Form and Function
Preformation and Epigenesis. Having retired early from a profitable business, Antony van Leeuwenhoek of Delft, Holland, became an amateur microscopist eager to examine anything under his microscope. In 1675 he examined the seminal fluids of several animals, including man. He saw swimming in these fluids the "animalcules," or the spermatozoa as we would say now. This was a discovery enough to make anybody famous, but a few years later another countryman of Leeuwenhoek "improved" on it by publishing a picture of a human spermatozoon in the head of which he saw a "homunculus," a tiny figure of a man ( 1694). This seemed a really magnificent discovery, for it appeared to solve at one stroke the difficult problems of heredity and development. The human body is all ready, preformed, in the male sex cell; all it needs to become an adult man is to increase in size.
To be sure, some of the authorities of that time did not feel convinced that the homunculus resides in spermatozoa, and preferred to look for him in the female sex cell, the egg. But whether they belonged to the school of "animalculists" or to that of "ovists," they believed the idea of preformation to be an excellent one. Especially so when Jan Swammerdam, of Leyden, Holland, developed the idea by supposing that the homunculi in the sex cells contain within them still smaller homunculi; those have more minute homunculi, and so ad infinitum. The reproductive organs of Adam--or of Eve--contained within them the entire mankind to come, packed like boxes within boxes. The utter absurdity of this notion was not obvious at the time, since it was not realized that within a few generations the homunculi would have to be smaller than atoms.
The preformation theory, and its extreme version the "box theory,"