EP'IGEN'ESIS (Neo-Lat., from Gk. cpi, upon + genesis, production, from aN,Ir, gignesthai, to be produced). A special or technical name for the modern conception of the growth and development of the animal organism from the undifferentiated mass of protoplasm constituting the egg. The word is the equivalent of the word 'evolution,' and is opposed to the preformation views of writers before the time of Harvey, Wolff, and Von Baer, and to somewhat similar views advocated at the present day by Weismann. The older writers, as Bonnet and Haller, used the word 'evolution' in the sense that we now em ploy the term 'preformation,' or the emboitement theory. See PREFORMATION.
The doctrine or phenomenon of epigenesis is the result of the scientific study of the embry ology of animals of all grades from the sponge to man. Before the rise of modern embryology the ablest, most sagacious biologists and philosophers were evolutionists, i.e. pre forma Um] ists. They knew or recognized only the external signs of the process of development. They witnessed the embryo becoming an adult animal, as a bud develops into a blossom. They knew nothing of the nature of the egg-cell, how it became fertilized, subdivided, and how by the multiplication of cells tissues were' formed and the different organs of the embryo became de veloped. They saw the butterfly emerge from the chrysalis, the latter from the caterpillar, and they conceived that the preformed germ of the butterfly and chrysalis and caterpillar existed, in miniature, in the egg laid by the butterfly. Hence they believed that animals in general were a series of cases or wrappings, germ folded within germ, and that the process of birth was a throwing off of these wrappings—a process of evolution.
This ignorance was partially dispelled by Har vey (1fi51), who maintained that. every living being arose from an egg ('Oinne virun• ex ono'). But the founder of embryology was Kaspar F. Wolff, who published his famous Theoria Genera tionis in 1759. He was the first to study the embryology of a vertebrate animal—the barnyard fowl. By means of actual observation of the embryo chick he endeavored to expose the fal lacy of the doctrine of preformation, to show that the animal was not fully formed in the germ, but that all development proceeded by new formation, or `epigenesis.' He maintained that the embryo consisted of unorganized organic mat ter. which only gradually became perfected in the course of its development, and that Nature really was able to produce an organism from an undifferentiated material, simply by her inherent forces. Wolff failed to convince his contempo raries, because he could bring only isolated ob servations and these doubtful of interpretation, and because he was ahead of his time, naturalists then attaching more importance to abstract rea soning than to observation.
The next embryologist to lend by his tions support to the views of Wolff was Von Baer in 1829, and after his time the cell theory was formulated, and the epoch-making works of the later embryologists, J. Milner, Ratlike, Remak, Bischoff, E. Van Beneden, valevskv, the Hertwig brothers. WVIPfillt11111 (in his earlier works), and many others, gradually built up the modern science of embryology (q.v.), and entirely dispelled the old-time preformation Views.