EMBRYO. 24 or the rudiment from which an organized being may be developed. In botanj', 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, i< 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 fcetus (q.v.). Larva? and pupa?, as those of insects, however, are not con- sidered to be embryonal forms. (See Larva.) In mammals, the embryo's existence begins with the formation of the male pronucleus, which consists of the head of a spermatozoSn which has pi'in- t rated 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 oosperm. The next step in the development of the embryo is segmentation of the oosperm; that is. active di- vision of the yolk. A groove forms, dividing the oosperm into two parts. A second groove, ap- pearing at right angles to the Hist, subdivides thi original parts into four. Similar division follows, int. i eight, sixteen, thirty-two, and still re numerous parts. A cleft rilled with clear fluid makes its appearance in the interior of the ma--. :nid this is called the segmentation cavity. A thickening 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 embryonic area, or germinal area, and from tins tin' young animal develops in successive stages. An interesting embryonal structure is the am- nion. This i~ 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- in. I iis opposing edges meel and unite. This enveloping membrane is called t he amnion. - ii it and the foetus apnea > tl iniot ie fluid. In some instano a piece of the amnion teai ofl From thi res! of the membrane, envelop- ing the head of the child as ii i mi rgi from the • a ' in. i. Such ii p.. 1 1 ion of the amnion in called foet il i. unlit ions which are I about i he t ime of birth, For e ulat ion of the, blood of a human embryo impoi i. mi i.-[.. 'i from thai of ill. In the foetus the blood returning from the pi the umbilical v. in divides into two currents. One of these passes through o 1 1;. inf. i T r i . na cai to the ii iricle of the hi ai I thcr proceeds to the liver through the hepat i e to the riglu . limited uippl ifo 1 he i ighi . ... I ion of the heart, while i . right auri cle through the nto the left auricli he |. Minion. ii ... terj . bul moat EMBRYOLOGY. 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, Human. EMBRYOLOGY ( from Gk. %nfjpvov, em - bryon, embryo ---oyla,-logia, account, from X£yeu>, Ii </. in, to say) . That branch of biological science which deals with the formation and development of individual organisms. The starting-point of development is a germ-cell, which may be an egg, or a bud, or a fragment of the body of another individual. But all processes of development, being due to fundamentally the same causes, pre- sent sufficient similarity to be classed together as 'embryological processes.' A germ-cell, or sexual cell, may be generally defined as any cell which can, either alone or by fusion with another cell, reproduce the entire organism. The germ-cell is on the one hand a specialized cell, told off for reproduction, and on the other an undifferentiated one, because it has the capacity of giving rise to the whole body. Sexual germ-cells are of two kinds, essentially alike in the beginning, when they react similarly to microchemieal agents; but later they gain a chemical difference. The one kind acquires a great size, deposits much yolk in its protoplasm, remains passive, and is an egg; the other remains of small size, but gains a powerful locomotor organ, called a tail-lash or flagellum, and is a spermatozoon. Unicellular Development. At the outset it may be well to consider a case of development in I lie Protozoa, or unicellular animals, in order that we may learn to distinguish between the essentia] and the secondary in the process. We may take as a type Podophyra, one of the Sue- toria. Podophyra produces germs of new indi- viduals by the nipping off from the cell-body of a pherical mass, including a nuekrus. At the time that il is set free the germ possesses a girdle of cilia by means of which it swims about. It grows, i.e. increases in size. Finally it be- aes attached ai a point on the equator; be- ..III.- elongated, forming a stalk; develops suck- ing tentacles at the upper extremity of the body, and becomes an adult Podophyra. It is to be 1 that: (1) The germ grows, and of course without any cell division occurring. (2) On the whole il becomes more complex as development proceed ; it is differentiated. (3) During devel- opment the embryo assumes various activities for which its organs lit it: it. is adapted at every tage to meet the environmental conditions of that stage, Growth, differentiation, and constant adaptation to environmental conditions seem to be the essential characters of development. Multicellular Devei opmi hi fn the Meta- h the body is multicellular, the proci . i cell-divi ion ha to be added to the developmental processes, but this may be re- reat ize of the body, which demands nuclear material in every pa 1 1 Here the gei m celh maj lake the form . of an egg or of a spermatozoSn EVOLI mix OP 1 I or ovum, is
