EPIDERMIS. 147 EPIGENESIS. nerves, and everywhere forming an external cov- ering to the derma, or true skin. It consists of two layers, chemically and morphologically dis- tinct — viz., the mucous layer, which lies imme- diately upon the curium, and the horny layer, which forms the outermost surface of the body. The mucous layer (the rete mucosum or rete mal- pighi) is of a whitish or slightly brown tint (in the negro dark gray or black ) , and is composed of rounded or cuboidal cells, distended with fluid, and likewise containing minute granules, which diminish in number in the more external cells. The horny layer forms the external semi-trans- parent part of the epidermis, which in the white races is colorless, and is composed almost wholly of uniform cells agglutinated and flattened. The color of the epidermis differs in different persons and in different parts of the body. It is deepest around the nipple, especially in women during pregnancy and after they have borne children. A more or less dark pigment is often deposited in persons who are exposed to the sun, in the face, neck, back of the hands, etc. These tints are not produced by special pigment-cells, but are seated in the common cells of the mucous layer, round whose nuclei granular pigment is deposited. In the negro and the other colored races it is also only the epidermis which is colored. Morbid col- oration of the epidermis (freckles, mother's marks, etc.) is produced in the same way as the color of the negro's skin. Numerous instances of partially or entirely white negroes and of black Europeans, not as a consequence of change of climate, but as an abnormal condition of the skin, are on record. The thickness of the epidermis varies extreme- ly. While upon the cheeks, brow, and eyelids, it varies from ^ to ^of a line, on the palm of the hand it ranges from one-third to one-half a line, and on the sole of the foot sometimes even exceeds a line. In some parts of the body the horny layer is thicker than the mucous; in other, the mucous is the thicker of the two. As the chief use of the epidermis is that of affording protection to the soft and tender subjacent parts, it attains its greatest thickness on those portions of the body (the palm of the hand and the sole of the foot) which are most exposed to pressure and friction. The hair and nails belong to the integumentary system, as well as horns in lower animals, and are modifications of epidermis. See Integument. EPIDERMIS. In plants, the superficial layer of cells on the bodies of the higher forms. See Anatomy; Bark; Cobtex. EPIDOTE (from Gk. evi, epi, upon + Sordc, dotos, given, from StS6vai, didonai, to give). A name given to a group of basic orthosilicate min- erals that include zoisite, epidote, piedmontite, and allanite. The mineral epidote proper is an aluminium-iron-calcium silicate that crystallizes in the monoclinic system ; has a vitreous lustre, and is of a green to brown and black color. It is also found massive, fibrous, and granular, and is common in crystalline rocks. Its distribution is world-wide, but fine crystals, which may be cut as gems, occur in Norway, Siberia, the Tyrol, and in the United States in Haddam, Conn., Chester County, Pa., at various places in North Carolina, in the Lake Superior region, and in Pike's Peak region, Colo. EP'IG-fi'A. See Abbutus, Trailing. EP'IGAS'TRITJM (Xeo-Lat., from Gk. cm- yiarptoc, epigastrios, over the stomach, from iiti, cpi, upon + yaorijp, gaster, stomach). The pari ■ it the abdomen (<].v.) which chiefly corresponds to the situation of the stomach, extending fri m the sternum toward the navel or umbilicus (q.v.). It. is called, in popular language, 'the pit of the stomach.' See Abdomen. EP'IGEN'ESIS (Neo-Lat., from Gk. enl, epi, upon -f- j/wnir, genesis, production, from ylyve- aft», giyncslliiii, 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 ecpiivalent 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 Hailer, used the word 'evolution' in the sense that we now em- ploy the term 'preformation.' or the emhoitement 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- formationists. 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-eell, 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 (1651), who maintained that every' living being arose from an egg ('Omne vivum ex ovo'). 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 interpi station, and because he was ahead of his time, naturalists then attaching more importance to abstract rea- soning than to observation.
