EMBRYO, the name applied to the developing young of an animal before birth or before hatching from the egg. (See EM BRYOLOGY.) The term is also used of that part of a seed (q.v.) which will develop into the new plant, in contradistinction to the endosperm of nutritive tissue. (X.) In the present article a condensed summary is given of the development of the human embryo. Further details and a corn parison of the development of man with that of other vertebrates are to be found in the anatomical articles on HEART; NERVOUS SYSTEM; LIVER, ANATOMY OF; etc.
The human embryo, like that of all other vertebrate animals, arises from the union of two minute cells, the ovum and sper matozoon. This union ("fertilization") takes place in the oviduct, or Fallopian tube of the uterus (see REPRODUCTIVE SYSTEM), and results in the formation of a single cell, the oosperm or zygote. It is by means of these two minute cells, the larger of which is less than I/Iooin. across, that hereditary characters are trans mitted from parents to offspring. It is generally believed also that the sex of the new individual is determined at the time of fertilization, although it is not possible to distinguish the sex with certainty until the gonads or genital glands are sufficiently advanced in development to recognize the differences in micro scopical structure between testis and ovary.
During the child-bearing period from the commencement of menstruation to its cessation about the 45th year, it is believed that a mature ovum is normally discharged from the surface of one of the two ovaries with each menstrual cycle. The ovum is received into the oviduct where it may become fertilized. If this does not take place, it perishes and is discharged.
The process of ovulation, or the shedding of a ripe ovum from a ruptured Graafian or ovarian follicle, is associated with menstru ation in the following way : The uterus, stimulated by an internal secretion, perhaps derived from cells of the ruptured follicle, pre pares for a possible pregnancy, its lining membrane thickens and becomes more vascular, but if fertilization does not take place the thickened lining is unnecessary and breaks down ; slight haemor rhage takes place and blood and shreds of membrane are dis charged with the degenerated ovum, the whole constituting the menstrual flow. If, however, ovum and spermatozoon have fused, and a zygote becomes imbedded in the mucous lining of the uterus, this discharge does not take place ; in other words that cessation of menstruation occurs, which is the first recognizable sign of pregnancy.
Certain of the earlier stages of development have not as yet been completely followed out in the human subject, but those which have been observed confirm the general conclusions, based on the study of other mammalian embryos.
The following description is based partly on observations on segmentation of mammalian embryos other than that of man, but with the exception of these earliest stages the account has been drawn from human material.
During the passage of the fertilized ovum through the oviduct into the cavity of the uterus, it undergoes segmentation or divi sion, and becomes a solid mass of rounded cells, the morula. This consists of smaller superficial cells, and larger central cells. The superficial layer, the trophoblast, has the important function of ab sorbing nutriment from the mucous lining of the uterus. The central cells give origin to the embryo. These central cells soon become massed at one pole of the morula, which now assumes an oval form, while at the other pole a cavity appears, so that the solid morula is converted into a hollow vesicle, the blastula. The outer wall of this is formed by the trophoblast, which encloses the eccentrically placed embryonic cell mass, and a cavity occupied by loose tissue containing fluid in its interstices. The embryonic cell mass is attached by a wide pedicle, the body stalk to the inner surface of the epithelial wall of the blastula. Two cavities soon appear within it : the amniotic cavity, and the cavity of the yolk sac. Between these is a disc shaped zone, where the walls of the two cavities are in contact ; this is the embryonic area.
The membrane covering the embryonic area and enclosing the amniotic cavity is the amnion, one of the foetal membranes, and the watery liquid contained within the cavity serves as a protec tion for the embryo. Later the developing child, now called the foetus, is suspended by the umbilical cord and floats freely in the amniotic fluid. The minute yolk sac of the human embryo is a mere remnant of the large yolk sac found in lower types of verte brates such as birds and reptiles; in all of the higher mammals its function has been taken over by the placenta (q.v.). The em bryonic area soon becomes oval in form and two axial grooves ("primitive," and "medullary") appear on its upper or dorsal surface. By a growing upward and coalescence of the edges of the anterior or medullary groove, it is converted into a closed tube, the medullary canal, which gives rise to the central nervous system and cerebrospinal nerves (see BRAIN). This takes place in embryos aged about 25 days and 2.3mm. in length.
Meanwhile the embryonic area becomes encircled by a groove, which by deepening separates the embryo from the amnion and yolk sac. The embryo now assumes a cylindrical form. One end of the hollow tube thus formed is enlarged to form the head and neck; the other, the tail end, becomes curved forward and tapers to a point. On each side of the neck appear f our linear furrows, which represent the gill slits of fishes. Bounding these furrows in front and behind are the visceral arches, which correspond to the lower jaw, gill cover and branchial or gill arches of the fishes. At the sixth week the embryo has become nearly circular in out line, and measures about I o mm. in its greatest diameter. At the head end are developed the mouth, eyes, ears and nasal cavities, and from each side two flattened buds grow outward, which are the rudiments of the upper and lower limbs.
Internally a portion of the yolk sac (archenteron) is separated off from the general cavity and enclosed within the embryo so as to form the alimentary canal. From the forepart of this, glands such as the salivary, thyroid, tonsils and thymus are developed, and as outgrowths from the canal further back, the lungs, liver and pancreas. The main tube gives rise to the pharynx, gullet, stomach and intestine. The heart and blood vessels are formed very early at the 3rd week; and the circulation at first resembles that of a fish, a single tubular heart propelling the blood through a branchial system of vessels.
Afterwards the primary heart and blood vessels are modified so as to form a general circulation for the blood supply of the body and a pulmonary circulation concerned in respiration. The pri mary branchial system adapted for respiration in water has thus been replaced by a pulmonary system, which enables the child from the time of birth, when the placental circulation is cut off, to breathe air by means of the lungs. During the later months of pregnancy the pulsations of the foetal heart may be heard with a stethoscope, and are a valuable sign of the life of the child.
Deposition of calcium salts to form the bones commences about the 6th week of intra-uterine life, and is accompanied by the for mation of the muscles and joints. Development proceeds apace so that movements of the child, spoken of as "quickening," are first felt by the mother about the middle of the fourth month of pregnancy. It must be remembered, however, that although these are a most important sign of life of the child, the new life really commences at the time of fertilization of the ovum.
From the time of the "quickening" until birth, development con sists almost entirely of an increase in size, though a recognizable human being has been in existence since about the 6th week of intra-uterine life, when the embryo was less than I 2mm. in length. The sex of the future child becomes obvious about the loth week, though it is possible to distinguish between the external genital organs of the two sexes at a much earlier age, namely about the 6th week. (R. J. G.)