II.

History

Until the second half of the 18th century, embryology was a matter of speculation rather than of knowledge. One generally accepted theory was that of preformation: The complete animal with all its organs was believed to exist in the germ in miniature, needing only to unfold like a flower. It followed that each germ must contain within itself the germs of all its future descendants, one within another, as in a nest of boxes. Many naturalists believed the germ to be contained in the ovum, the female germ cell, but after the microscope had revealed spermatozoa, the male germ cells, in 1677, a school of so-called spermists advanced the hypothesis that the germ was contained in the spermatozoon. Their drawings show the spermatozoon encasing a minute human figure, called the homunculus.

Little attention was given to the theory, called the theory of epigenesis, that the English physician and anatomist William Harvey had stated in 1651. This theory, which had been vaguely expressed much earlier by Aristotle, held that the specialized structures of the individual develop step by step from unspecialized antecedents in the egg. Proof of this theory was not forthcoming, however, until 1759 when the German anatomist Kaspar Friedrich Wolff reported on his study of the development of the chick in the egg and showed that the organs arise from undifferentiated material. The basic potential nature and organization of the structures of the organism are determined by the genetic constitution of the fertilized egg (see Heredity). Wolff is called the founder of modern embryology, a title also sometimes given to the Estonian naturalist Karl Ernst von Baer, who in the 19th century described the principal phases in the development of the chick and pioneered in comparative embryology.

A firm basis for the new science was established by the cell theory formulated in 1838 by the German botanist Matthias Jakob Schleiden, who stated that all plants and animals are made up of cells. A year later his compatriot, the anatomist and physiologist Theodor Schwann, confirmed this theory. In later work these men demonstrated that tissues and organs develop by cell division. See Cell.