Fig. 2 Slightly later cleavage of mammalian egg. Fig. 3 Section of early blastoFig. 1 Early cleavage. Four-cell stage of monkey. After Selenka. Fig. 4 Section (slightly diagrammatic) of Peters' ovum, one of the cyst of mammalian embryo. The "Embryonic mass" gives rise to the entire body. Fig. 5 Frontal section (diagrammatic) of human uterus and embryo about the end of the second youngest human embryos which has been carefully studied. month of pregnancy, to illustrate the relations of the embryonic membranes, and the vascular relations of embryo and mother in the placenta. The placenta is formed by the Decidua basalis and maternal blood vessels, together with the Chorion frondosum

EMBRYOLOGY, HUMAN

chemical interaction the embryonic vesicle undergoes "implantation," sinking into the superficial layer of the decidua, which grows over and encloses it, thus separating it from the uterine cavity. Implantation is usually on the posterior wall of the uterus, though it may occur elsewhere, and in abnormal conditions the embryo may become attached in the oviduct, causing a tubal (extrauterine) pregnancy.

The earliest human embryos studied had already become implanted. In one of these (Peters' ovum) the entire blastocyst was about one millimeter in diameter, the embryo onefifth millimeter, or about one one hundred and twenty-fifth of an inch in length. The age was estimated by Peters at three or four days, but it is now believed to be several days older. The three germ layers are distinguishable and the amnion and yolk-sac are already formed. The chorion is covered with villi which have invaded the capillaries of the uterine mucosa, thus bringing the embryo even at this early stage into nutritive relation with the maternal blood by osmosis. (Fig. 4).

In a slightly older embryo (Graf Spee's embryo) measuring 1.54 millimeters in length, the neural plate or rudiment of the spinal cord and brain is formed. Blood-channels representing some of the chief veins and arteries are distinguishable, and also the two heart rudiments, not yet united in the median line. The chorionic villi already contain blood vessels. The yolk-sac, though quite empty of yolk, is of considerable size, and the allantois has appeared. Several embryos of the third week have been described. At this age the spinal cord and brain form a closed canal, the heart is a twisted tube, and from comparison with other mammals there can be no doubt that the heartbeat is already established. By the 21st day the embryo is four or five millimeters in length, and head, tail, gill-clefts and rudiments of eyes and inner ears are clearly distinguishable. By the end of the first month the arms and legs appear as lateral buds and the rudiments of the face are formed. There is a well-marked tail, and head and tail are so flexed as nearly to meet. Measured from neck to rump the embryo is about one centimeter in length. The yolk-sac has not kept pace with the body and is a small pedunculate vesicle. Practically all the great organs are indicated by the beginning of the fifth week. For example, from the alimentary canal the rudiments of thyroid, thymus, lungs, liver and pancreas have budded out. In general structural plan the embryo up to this stage is rather more like a fish than like the adult human. This is especially true of the bloodvascular and urinary systems.

During the second month growth is rapid, and by the end of this period the embryo is about 30 millimeters in length. Even the layman could now identify it not merely as a mammal but as human, or at least as a primate. The face is now fairly well formed, even to mouth and nostrils, and the external ear is taking shape. The tail diminishes after the sixth week and has almost disappeared by the eighth. Elbow and knee flexures are well marked and hand and foot exhibit digits. The third month witnesses an increasing humanness in the appearance of the embryo- or "foetus," as it is commonly called after the establishment of its ex

ternal form. About the 11th or 12th week it becomes possible to distinguish the sex from the external genitalia. Before this time these organs were present but different in development, though sex is actually determined at fertilization and can be distinguished about the end of the first month by microscopic examination of the genital ridges, the structures which later give rise to ovary or testis. About the middle of pregnancy, toward the fifth month, muscular movements of the foetus become strong enough to be felt by the mother, a fact which has given rise to a vulgar belief that life begins at this period of "quickening as it is called. About this time the face and most parts of the body become covered by a dense growth of fine hair, the "lanugo." This foetal hair increases for a month or two, but is shed to a great extent before birth. With the growth of the fœtus great changes have taken place in the embryonic membranes, the later conditions of which and their relations to the decidua or uterine mucous membrane are illustrated in the diagram (Fig. 5). It may be stated in brief that the amnion enlarges greatly, becoming adherent to the chorion. The yolk-sac and allantois virtually disappear. The chorionic villi disappear except on the portion of the surface directed toward the original site of attachment, where they persist, forming the chorion frondosum, which comes into close relation with the corresponding part of the decidua, and with it forms the placenta, the vascular organ by which the fœtus, physiologically a parasite, derives its nourishment and its oxygen from the maternal blood.

Birth occurs approximately 280 days after fertilization, though a fœtus born as early as the seventh month may survive. The average weight at birth is near seven pounds. The fœtus is expelled by involuntary contraction of the uterus, aided by contraction of the abdominal muscles. Rupture of the amnion by muscular pressure precedes birth, and shortly after the child is born the placenta, torn loose by further uterine contraction, is expelled, together with the amnio-chorion. The entire mass is called the "afterbirth.» Tremendous physiological changes occur suddenly in the child at birth. Cessation of placental oxygenation of the blood stimulates the lung-breathing reflex. Dilation of the lungs at the first breath brings into service the pulmonary circulation, including the functioning of the left side of the heart, and also effects the closure of the foramen ovale, an opening between the two auricles. Certain arteries and veins, hitherto very important, suddenly become non-functional and undergo rapid atrophy. Thus, almost in an instant a fundamental alteration is effected in the respiratory, circulatory and nutritive mechanisms by which the physiologically passive fœtus is transformed into the active breathing and feeding infant.

Bibliography. Many excellent works on human embryology have been published. Among the best textbooks are McMurrich, J. P., The Development of the Human Body) (Philadelphia 1907); Bryce, T. H., Embryology' (Vol. I of Quain's 'Elements of Anatomy,' London and New York 1908); Keibel and Mall, 'Manual of Human Embryology (Vols,

I and II, Philadelphia and London 1912), a very exhaustive work; Bailey and Miller, 'Textbook of Embryology' (3d ed., New York 1916).

J. H. MCGREGOR, Professor of Zoology, Columbia University.

EMBRYOLOGY OF PLANTS. That phase in the life history designated as the embryology begins within the fertilized egg, but its end is not marked by any such definite feature. In general, the embryo represents the early stages in the development of an individual from the egg. In the ferns and their allies, somewhat later stages, in which one or more leaves are visible to the naked eye, are called sporelings. There is no definite feature to mark a line between the sporeling and the adult plant. In the seed plants, the series is embryo, seedling, adult, with no features to mark the transitions. The difficulty is the same as that in defining baby, boy and man. The early stages in the development of the embryo are fairly well known in all groups from the liverworts to the

highest flowering plants. In the liverworts and mosses, the development of the embryo from the fertilized egg up to the adult stage, and even to the death of the individual, is rather short. The embryo, and even the adult, are small, are parasitic upon the egg-bearing plant (gametophyte), and do not produce any leaves. In the lowest liverworts the egg divides into halves, then into quarters and continues dividing until a spherical mass becomes differentiated into an outer protective layer enclosing a large number of spores. In the higher liverworts and in the mosses, the embryo starts in the same way, but later becomes differentiated into three regions called the foot, stalk and capsule, the latter producing the spores. In the lower liverworts, the adult is a small spherical body not more than one-sixteenth of an inch in diameter; in the higher liverworts and in the mosses the diameter is not much greater, but there is considerable elongation. A couple of inches is rather long; but a few liverworts reach a length of five or six inches and one of the higher mosses is said to reach a length of 10 or 12

inches. (Figures of some of these features may be found under SPOROPHYTE, EVOLUTION OF). In the ferns and their allies, the embryo begins to develop in the same way, forming a spherical mass of cells, but definite growing regions soon appear, marking the root, stem, leaf and foot. The embryo is parasitic upon the gametophyte until the root becomes developed and begins to get nutrition from the soil and the leaf begins to secure materials from the air. When this stage has been reached, we no longer call the young plant an embryo, but a sporeling. In the seed plants, which include the Gymnosperms and Angiosperms, the development of the embryo presents great variation and complexity. In the cycads (q.v.) which represent the lower living Gymnosperms, the fertilized egg does not immediately give rise to a mass of cells, but nuclear divisions, without any separating walls, take place, until there may be as many as 1,000 nuclei lying free in the cytoplasm of the egg (Fig. 1, A). Cell walls then appear at the lower part of the egg (Fig. 1, B). The cells, thus formed, become differentiated into three regions, (1) a group of cells remaining within the limits of the egg, (2) a region of rapidly elongating cells called the suspensor and (3) at the tip of the suspensor some small cells with dense protoplasmic contents (Fig. 1, C). The root, stem, cotyledons and leaves of the embryo come from these small cells, the other two regions being temporary structures which function only during the early development. After the embryo breaks out from the seed and becomes independent, it is usually called a seedling. The eggs of the cycads are very large, reaching one-eighth of an inch or even one-fourth of an inch in length. In the higher Gymnosperms, the eggs are much smaller, in most Pines not more than one-onehundredth of an inch in length. In these higher forms there is a constant tendency to reduce not only the size of the egg, but also the number of free nuclei. There are still the three regions mentioned above, but each consists of only a few cells. In a few Gymnosperms, the free nuclear period is entirely eliminated, a cell wall following the first division of the egg nucleus. In the Angiosperms the eggs are still smaller, all being microscopic in size, and in all the cases the first division of the nucleus of the fertilized egg is followed by the formation of a cell wall so that there is no free nuclear stage. Even under the microscope, the eggs of this group look so exactly alike that it hardly seems possible for one to develop into an herb, another into a shrub and another into a tree. We say the course of development is determined by heredity, and those who are satisfied with the mere naming of a phenomenon may be satisfied with this explanation. Although the eggs and embryos are very small, modern technic is so efficient that the embryology is well known from the willows and crowfoots to the sunflowers and orchids. A simple and fairly typical type of embryology is illustrated by the Shepherd's Purse (Capsella), a familiar and widely-distributed weed (Fig. 2). The first division of the fertilized egg is transverse (A). Divisions then take place so that a filament consisting of a single row of cells is produced (B); the terminal cell of the row then divides vertically and from the two resulting cells the stem, cotyledons, leaves and nearly all the root

are produced (C). The cell in which the vertical division has appeared is generally called the embryo cell, and the rows of cells below it, the suspensor. A second vertical wall at right angles to the first one gives rise to four cells, each of which immediately divides transversely, so that eight cells, apparently just alike, are produced (D). Each of the eight cells now divides, forming a wall parallel to its outer surface (E). These outer cells (dotted in the illustration) continue to divide, but all walls are perpendicular to the surface, so that the result is an extensive layer of cells only one cell in thickness. Since this layer, at maturity, is the epidermis, it is called the dermatogen, which means the epidermis producer. In the lower half of the more or less spherical embryo, the four central cells, inside the dermatogen, divide longitudinally (F). The four inner cells resulting from this division (dotted in the illustration) constitute the plerome and give rise to

the vascular system of the root; the outer four give rise to the periblem which gives rise to the cortex of the root. In the upper half of the embryo, which is to form the stem and leaves, the differentiation into cortex and vascular region takes places much later, after a large number of cells has been produced. Thus there are three embryonic regions, one producing epidermis, another producing the vascular system and the third producing cortex. These three regions, established in the early development of the embryo, are also found in the adult plant.

There are other types of embryology in the flowering plants. Many have no filamentous stage; some have a single, very large suspensor cell, while some have a massive suspensor. In many the differentiation into the three embryonic regions takes place much later; some do not differentiate at all until the seed germinates; while in others, like the bean, the embryo, while still in the seed, has not only cotyledons but well-developed leaves. Some special features of embryology will be found under PLANTS, RECAPITULATION IN, and SPOROPHYTE, EVOLUTION

OF.

Consult Morphology of Gymnosperms,' by John M. Coulter and Charles J. Chamberlain; 'Morphology of Angiosperms, by the same authors; College Botany, by G. F. Atkinson; 'Mosses and Ferns,' by D. H. Campbell. CHARLES J. CHAMBERLAIN, Professor of Cytology and Morphology, University of Chicago.

EMBURY, Philip, Methodist clergyman: b. Ballygaran, Ireland, 21 Sept. 1729; d. Camden, N. Y., August 1775. He joined John Wesley's society and became a local preacher at Court-Mattress in 1758. Emigrating to New York in 1760, he began to preach in his own house in 1766 and two years later erected a chapel on the site of the present "Old John Street Church." Being a carpenter by trade, he worked on the building with his own hands and completed the pulpit, in which he preached the sermon of dedication 30 Oct. 1768. This was the first Methodist chapel of the New World and he has been called "the founder of American Methodism." It was, however, at Camden, Washington County, N. Y., that he did his greatest work, forming there a congregation which grew into the flourishing and influential Troy Conference. Consult Buckley, 'History of Methodism' (Vol. I, New York 1898).

EMDEN, Germany, town, in the province of Hanover, on the Ems, near where it discharges itself into the Dollart estuary. Emden has an excellent roadstead and its harbor is connected with this by a canal admitting large vessels. The Dortmund-Ems and other canals connect it with the interior. The town has a Dutch appearance due to its quaint architecture and the dykes which protect it from inundation. The town hall, dating from the 16th century, has a remarkable collection of ancient armor and is one of the finest public buildings in Germany. The town contains also a 12th century church, a museum, art gallery, barracks, a public library, trade and industrial schools, and a deaf and dumb institute. Emden has cable communication with Great Britain, America, and other countries. Its export trade includes grain, dairy produce, cattle, tallow, wool, hides, etc.; and it imports coal, timber, wine and colonial produce. A considerable number of vessels are built here annually; and the manufactures include leather, paper, dairying instruments, basketware, cement, wire ropes, bricks, soap and tobacco. There are also oil-mills, breweries and distilleries. Emden was founded in the 10th century or earlier and in 1433 was added to Hamburg. It became a free city in 1595, and a free port in 1751. In 1806 it was taken by Holland, but nine years later was added to Hanover, which in 1866 was itself made part of Prussia. Pop. 24,038.

EMELÉ, ā-ma-lā, Wilhelm, German painter: b. Buchen, Odenwald, 1830; d. 1905. He first adopted a military career but studied art with Dietz at Munich and later at Antwerp and Paris. His canvases are noted for exact knowledge of military detail and are spirited in conception, his subjects being military. He lived in Vienna after 1861 where he attained great popularity as a painter of equestrian portraits and hunting scenes. Among his works are 'Battle of Stockach'; 'Capture of Heidelberg Bridge in 1799) (1857), purchased by the