EM'BRYO (from Gk. eaflptiop, cm bryon, em bryo. from ?v, cm in -- /3ptlEtv, bryri n, to swell). An organized being in a rudimentary condition, or the rudiment from which an organized being may be developed. in botany, the term is ap plied to the germ which forms within the ovule on fertilization, and which increases to become the principal part of the seed. The perisperm of the seed, being regarded as a mere store of nourish ment for the embryo, is not considered to be part of the embryo; while the cotyledons are consid ered as essentially belonging to it, together with the plumule, the radicle, and the connecting parts. In animal physiology, the term is applied to the unborn young from the time the impregna tion of the ovule occurs up to the time of birth, thus including the fetus (q.v.). and pupae, as those of insects, however, are not con sidered to be embryonal forms. (See LARVA.) In mammals, the emliryo's existence begins with the formation of the male pronucleus, which consists of the head of a spermatozoiin which has pene trated the zona pellucida and peri-vitelline space of an ovule, and, entering the yolk, has lost its tail and middle portion, and has enlarged as a result of absorption from the surrounding mate rial. Around each pronucleus a clear spot is formed. Conception takes place when this pene tration and initial change occur. From this spot protoplasmic lines radiate, forming the aster. The fertilized ovum is termed the o3sperm. The next step in the development of the embryo is segmentation of the o3sperm; that is. active di vision of the yolk. A groove forms, dividing the oilsperin into two parts. A second groove, ap pearing at right to the first, subdivides the original parts into four. Similar division follows, into eight, sixteen, thirty-two, and still more numerous parts. A cleft filled with clear fluid makes its appearance in the interior of the mass, and this is called the segmentation cavity. A thiekening of a part of the wall of the ovum now termed a vesicle) appears. oval at first,
pear-shaped later. This structure is called the embryonie area, or germinal area, and from this the young. animal develops in successive stages. All interesting embryonal structure is the am nion. This is developed from the upper leaf of the vesicle membrane, which arises in a ridge about the embryo. This ridge, by a continuous growth upward, arches over the back of the em bryo, and its opposing edges meet and unite. This enveloping membrane is ealled the amnion. Between it and the fo•l's appears the amniotic fluid. In some instanees a piece of the amnion tears off from the rest of the membrane, envelop ing tin- head of the child as it. emerges from the vagina. Such a portion of the amnion b; called a Hull.
There are many foetal conditions which are modified about the time of birth. For example, he eireulat ion of the blood of a human embryo differs in many important respects from that of the adult. In the lulus the blood returning from the pine( eta umbilical vein divides into wo currents. 'One of these passes through I he duel 110S11.1 to the inferior vend corn, and thence to the right auricle of the heart, while he other proceeds to the liver through the hepatic ves els, and thence to the right amielo. A limited supply of blood ii forced into the right wit le (hiring of the heart, while he greater part of it basses from the right auri cle through the foramen of Botal into till' left. auricle. From the right vent rick. ennui. blood artery, butt most of it proceeds through the ductus arteriosus into the aorta. In the aorta this blood mixes with that coming from the left auricle and pul monary veins. Blood is returned to the placenta by the umbilical vein. After birth, obliteration of the umbilical vessels takes place. The ductus venosus becomes a fibrous cord, the foramen be tween the auricles closes, and the ductus ar teriosus becomes also obliterated. For other de velopmental processes in the growth of the em bryo, see EMBRYOLOGY and EMBRYOLOGY, IIUMAN.