From Big Medical Encyclopedia

GERM, embryo — an organism at the early stages of development which are beginning conception and coming to the end with hatching of egg (at oviparous forms) or the birth (at viviparous). In embryology of animals and the person 3. — the organism formed sexually from impregnated (more rare, at some animals, from unfertilized) ova and developing in egg covers (at oviparous) or in an organism of mother (at live-bearers and the person). In obstetrics of pi of gynecology 3. call an organism only during the initial stage of pre-natal development (the first 8 weeks) when there are main changes its structures (morphogenesis). Other, most part of pre-natal development (with the 9th on 38 — 39 weeks) is called fetal, or fetalis, the period, and the developing organism — fruit (see). Unlike 3., the organization to-rogo continuously and significantly changes (crushing of egg, isolation of germinal leaves, determination and isolation all of new laying of bodies and fabrics), a fruit of hl. obr. grows; this growth can be during the separate periods of development and uneven, leading to change of proportions of parts of a body. Besides, in fabrics of a fruit processes of a differentiation generally come to the end. G. A. Schmidt (1968) distinguishes, except germinal and fetal, still intermediate — the prefetal period of development (at the person — with 46th on the 75th days of an antenatal life), emphasizing that 3., the prefruit and a fruit differ from each other in various sources and ways of food, various type of devices to breath, allocation and other parties of a metabolism.

Studying of a germ of the person, despite religious restrictions, began with ancient times from incidental observations over 3. late stages of development. However prior to the beginning of 20 century of data on the earliest stages of development 3. the person was not. When embryologists Ticher (J. N of Teacher), Bryce (Th. N of Bryce), etc. were investigated by Miller single early 3. (the first 2 weeks of pre-natal development), intravital patol, changes, posthumous maceration and defective gistol, processing led to perverse ideas of initial stages of development. These data held on almost to the middle of 20 century. By 40th years 20 century in Ying-that Carnegie (Baltimore, the USA), and also in high fur boots of Prague and Brno (Czechoslovakia) created the fullest and systematic collections 3. person of different stages of development. These scientific centers appeared also the greatest number of publications about early 3. person. These are Florean's works (J. Florian, from 1929 to 1941), Hertiga and Roca (And. T. Hertig, J. Rock, 1942, 1943, 1945, 1956), Stritera (G. L. Streeter, 1942, 1945, 1948), Mazantsa (K. Mazanec, 1953, 1959), etc. In other countries, including in the USSR, the publication on studying 3. early stages of development are still single, V. S. Burov (1928), G. M. Salgannik (1948), V. K. Beletsky's descriptions (1949), S. D. Shakhov (1950, 1951), A. I. Burak (1954), A concern to them. G. Knorre (1956), B. P. Hvatov (1959), etc. Along with a research 3. early stages of development gistol, methods a number of authors studies a structure 3. the person of later stages of development using methods of a histochemistry, in particular M. Ya. Subbotin with sotr., L. I. Falin, Yu. N. Shapovalov, etc. Also separate systems and rudiments 3 are studied. So, D. M. Golub with sotr. studies a nervous system, L. I. Falin (1968) and A. G. Semenova-Tyan-Shanskaya (from 1967 to 1978) — the sexual rudiment, etc.

Development of a germ

is Distinguished by 5 main periods of development 3. animals and person: one-celled 3., crushing, gastrulation, isolation of the main rudiments of bodies and fabrics, organogenesis and histogenesis.

Fertilisation (see) at the person occurs in the field of a funnel of a uterine tube (oviduct) and comes soon later ovulations (see) since the ovum of the person keeps ability to fertilization not longer than one days.

Period of a one-celled germ, or zygotes (see), proceeds from the moment of penetration of a spermatozoon into an ovum to division of an oospore into the first two daughter cells. Unlike gametes — ova (see) and spermatozoon (see), the parent organisms representing cells, the zygote formed as a result of their merge is already one-celled 3., i.e. organism of new, filial generations. The period of a zygote at many mammals and the person lasts up to one days. During this period in cytoplasm of a zygote there are movements of various organellas (mitochondrions, etc.) and inclusions, the plane of a bilateralism is defined.

Fig. 1. The diagrammatic representation of a germ of the person at various stages of development: and — a stage of two blastomeres (up to 1 days); — a transition phase from a morula to a blastocyste (58 cells, the fourth days of pre-natal development): 1 — cells of a trophoblast, 2 — the remains of a transparent cover, 3 — cells of an embrioblast; in — a stage of a hollow free blastocyste (107 cells, the fifth or sixth days of pre-natal development): 1 — embrioblast, 2 — a trophoblast, 3 — a cavity of a bag of waters.

Period of crushing proceeds from the first division of a zygote into two daughter cells before formation single-layer multicellular 3., or blastulas (see). Crushing of egg (see) begins in one days after fertilization and within 3 — 4 days of advance 3. on a uterine tube it is made with a speed of one division a day. During this period 3. it is split up by a series of consecutive mitoses, but without dorastaniye of daughter cells to the sizes maternal after each division as it occurs at reproduction of fabric cells. Therefore, despite increase in number of cells, components 3., general sizes of live weight 3. do not increase, and the sizes of its cells with each following division decrease. Cells of which 3 consists. during the period of crushing, are called to blastomeres and (fig. 1, a). They are characterized by rounded shape, at the beginning of crushing by also large sizes (in comparison with cells of embryonal rudiments and fabrics), small differentiation, rather low level of integration (under a cut understand closer association of parts in one harmonious whole), wide potentialities of development. The main result of the period of crushing in a uterine tube is transformation 3. from one-celled in multicellular — the morula representing a roundish complex from 8 — 12 cells. The morula at mammals and the person comes to a uterus where rates of crushing sharply accelerate (fig. 1, b).

At the small sizes of ova of N the insignificant number of vitelline inclusions in their cytoplasm as it is noted at viviparous mammals and the person, crushing happens full, i.e. the zygote is entirely split up for blastomeres which go for formation of cells and fabrics 3., including and its provisional (temporary) bodies. The blastula (in particular at the highest vertebrata and the person — a discoblastula) arises at the end of the period of crushing, at the person — to the 6th days of pre-natal development. It has the form embryonal, or germinal, a disk — the germinal guard spread over the cavity containing proteinaceous liquid instead of a yolk. Walls of this cavity, to the Crimea also the germinal guard is attached, are formed by cells trophoblast (see) — the special provisional adaptation inherent only 3. placental mammals, including the person. Cells of a trophoblast stand apart extremely early, already during crushing (smaller and light blastomeres), and acquire as a part of a morula group of other larger and dark blastomeres — embrioblast.

Fig. 2. The diagrammatic representation of the site of a mucous membrane of a uterus with the implanted germ of the person at a stage of 7,5 days of pre-natal development: 1 — the regenerating epithelium of a uterus; 2 — the expanding part of a trophoblast which was implemented into a mucous membrane of a uterus; 3 — an ectoderm of amnion; 4 — an entoderm; 5 — a cavity of amnion; 6 — the thin coat of a trophoblast which is not covered with the accruing uterine epithelium yet; 7 — a periblast of a germinal guard (a bottom of an amniotic bubble); 8 — connecting tissue of a mucous membrane of a uterus; 9 — the site of a uterine crypt.

3. strenuously soaks up liquid from a cavity of the uterus and dumps an egg cover (zona pellucida). To 5,5 days of pre-natal development of the person there is a hollow blastocyste consisting of hundred with small cells which vast majority treats a trophoblast and some — an embrioblast (fig. 1, c). On the 7th days implantation (fig. 2) begins; full immersion 3. in thickness of a mucous membrane of a uterus occurs to the 8th days. A trophoblast at implantation 3. in a wall of a uterus produces the proteolytic enzymes destroying in a varying degree tissues of her mucous membrane and soaks up from them nutrients. Destroying further blood vessels and entering thus contact with maternal blood, 3. from anaerobic type of exchange passes to aerobic, from gistiotrofny food to gemotrofny. The trophoblast very much is early differentiated in specialized epithelial fabric. Embrioblast, called differently by «the internal cellular weight», rather long (at 3. the person apprx. a week) remains at a stage of group of blastomeres («a germinal small knot»), edges, being flattened in the form of a disk, and forms a germinal guard. Set of a trophoblast, germinal guard and the cavity filled with liquid received the name germinal (or fetal) a bubble, or a blastocyste.

Fig. 3. The diagrammatic representation of a 14-day germ of the person (in a section): 1 — amnion; 2 — a bottom of an amniotic bubble (a periblast of a germinal guard); 3 — a roof of a vitelline bubble (an intestinal entoderm); 4 — a vitelline entoderm; 5 — clots of liquid in a cavity of a bag of waters; 6 — a connective tissue layer of chorion; 7, 9 and 10 — a trophoblast; 8 — lacunas with maternal blood.
Fig. 4. The diagrammatic representation of cross section of a 15-day germ of the person at the level of primary strip: 1 — an amniotic kidney; 2 — a mesoderm of primary strip; 3 — a cavity of a vitellicle; 4 — an entoderm; 5 — mitotic the sharing cell; 6 — a periblast of a germinal guard; 7 — an amniotic cavity; 8 — an ectoderm of amnion; 9 — connecting fabric of chorion; 10 and 11 — a trophoblast.

Period of a gastrulation consists in transformation already become multicellular, but still single-layer 3. (blastulas) in two-layer — the gastrula (see the Gastrulation) consisting of two germinal leaves (see): outside — primary ectoderms (see), or an epiblast, and internal — primary entoderm (see), or hypoblast. At the highest vertebrata and the person the dvukhsloynost of a germinal disk is reached in the 1st phase of a gastrulation delaminatsiy by its (splitting) on outside (upper) and internal (lower) leaves. This process at the person happens on the 2nd week of development (fig. 3). The third germinal leaf — average, or mesoderm (see), at vertebrata (unlike invertebrates, a lancelet) the cut 3 stands apart not after a gastrulation, and already in its 2nd phase, by the end. becomes three-layered. This phase proceeds several days. At birds, mammals and the person of a cell of future mesoderm originally accumulate in the tail of a periblast of a blastodisc in the form of a medial reinforced tyazh — primary strip and only then plunge into an interval between ekto-and an entoderm. Primary strip appears on the 15th days (fig. 4), thus there comes the 2nd phase of a gastrulation. The chordal shoot is formed on the 16th, an epithelial rudiment of an allantois — an outgrowth of the back department of an intestinal tube supplied with a large number of blood vessels on 17 — the 18th days; at the person it remains underdeveloped. At the same time 3. becomes three-layered: at the expense of plunging into an interval between ekto-and an entoderm of cells of primary strip the mesoderm forms. Until the end of the 3rd week of development it remains not segmented (the prosomitny period). Other result of a gastrulation at chordates, including at the person, is the sformirovaniye of an axial complex of rudiments: a medullary plate, after becoming isolated in a tube, on dorsal side of a body; a rudiment of a back string, or a chord, directly under a medullary plate, and mesoderms, dorsal parts a cut further (on the 4th week) are metamericly dismembered, or segmented, on somites, or back segments which are symmetrically on the right and to the left of a chord (see. Somites ). In the period of the gastrulation accompanied with active movements of cellular masses 3., there are contacts and interactions between the listed bookmarks as a result of which there is a determination of their cellular material, i.e. definition of further ways of their development. The person and subhuman primates long before emergence of primary strip and irrespective of it by eviction of cells from an embrioblast (across Hertigu and Roca, from a trophoblast) have an extraembryonic mesoderm. The peripheral layer of its cells, adjacent to a trophoblast, forms a connective tissue basis of chorion — an outside fleecy cover of a fruit. Arise slabovetvyashchiyesya outgrowths of a trophoblast — primary fibers which more and more deeply are implemented into tissues of a mucous membrane of a uterus. After vascularization of fibers chorion receives the name of true (chorion verum). At mammals and the person, directly contacting to a mucous membrane of a uterus, chorion provides a metabolism between an organism of mother and a fruit, forming a fetal part placentae (see).

Fig. 5. The diagrammatic representation of a germ of the person in an initial stage of isolation of his body from extraembryonic parts: and — a sagittal section of a three-week germ (during formation of the first somites); — sagittal coal mine 22 — a 23-day germ (7 couples of somites); in — 23 — a 25-day germ (17 — 18 couples of somites), a lateral view (chorion and amnion are removed); 1 — a skin ectoderm; 2 — an ectoderm of amnion; 3 — a mesoderm of amnion; 4 — fibers of chorion; 5 — a chorial plate; 6 — an amniotic leg; 7 — blood islands; 8 — an allantois; 9 — a chord; 10 — an intestinal entoderm; 11 — rudiments of heart; 12 — a vitelline entoderm; 13 — a pericardiac cavity; 14 — the becoming isolated neural folds; 15 — a mandibular branchiate arch; 16 — an acoustical pole; 17 — a part of amnion; 18 — a vitellicle; 19 — a cordial ledge. From Hamilton, Boyd and Mossmen, 1952.

Period of isolation of the main rudiments of bodies and fabrics. The diverse rudiments of bodies and fabrics which were outlined earlier as a part of rather homogeneous germinal leaves stand apart from each other (fig. 5). E.g., as a part of an ectoderm the neurotubule (by transformation of a medullary plate into a fillet, edges to-rogo then grow together) and a skin ectoderm stand apart. The first represents a rudiment of a nervous system, the second — a rudiment of a skin epithelium, or epidermis. As a part of a mesoderm dorsal sites — somites, intermediate — segment legs, or nephrotomes, and ventral — side plates, or splanchnotomes anatomically stand apart. On 20 — the 21st days there comes the metamerization of a mesoderm, there is chronologically the first (topographical the 3rd) couple of somites. Somites and nephrotomes (segment legs) are formed and segmented in the kraniokaudalny sequence with a speed of 2 — 3 couples a day, to the 35th days of embryonic development all 43 — 44 couples of somites are formed. The second couple and unpaired first somite arise with delay and in the return sequence are phylogenetic more ancient larvaceous (i.e. inherited from a larval ancestral form) segments. Side plates remain at 3. vertebrata not segmented, but leaves with a slit-like secondary cavity, or tselomy, between them are split on outside (parietal) and internal (visceral). At all highest vertebrata whole trunks (endowhole) proceeds in so-called extraembryonic whole (ekzotsely), being between parietal and visceral leaves of a mesoderm of provisional bodies. Each somite is subdivided further into the dorsolateral site — a dermatome, average — a myotome and medioventralny — a sclerotome. Dermatomas and sclerotomes, being loosened, form a rudiment connecting and skeletal fabrics — mesenchyma (see), consisting generally from otrostchaty and partially of amoeboid mobile cells and the intercellular liquid dividing them. The Dermatomny mesenchyma gives later rise to a connective tissue basis of skin, sclerolanguid — cartilaginous * and bone to tissues of a skeleton. A part of cells of the mesenchyma forming afterwards connecting and smooth muscular tissue, blood and vessels is moved from splanchnotomes. Myotomes are a rudiment of cross-striped skeletal muscles, nephrotomes — a rudiment of epithelial tubules of three generation of kidneys replacing each other — pronephrouses (pronephros), primary kidney (mesonephros) and secondary, or final, a kidney (metanephros) inherent only to the highest vertebrata, including the person. Splanchnotomes are a source of development of a coelomic epithelium, muscular tissue of heart, cortical substance of adrenal glands, an epithelium of gonads (an ovary and a seed plant). But in the period of isolation of the main rudiments of bodies and fabrics the fabric differentiation does not begin yet (except for fabrics of provisional bodies 3.) and rudiments of future bodies and fabrics consist from low-differentiated (embryonal, or zachatkovy) cells. The last possess only the organellas of a general meaning inherent to any cell, and perform the general functions of exchange, cellular reproduction, cellular growth, the movement (migration) etc. However cells of different rudiments are already determined in the direction of certain fabric derivatives and possess chemical and nek-ry morfol, differences from each other.

Fig. 6. The diagrammatic representation of a germ of the person about 10 mm of length (the end of the 5th week of development) with fetal membranes; a part of chorion (a wall of a bag of waters) is removed: 1 — chorion; 2 — amnion; 3 — a vitellicle; 4 — an umbilical cord; 5 — blood vessels.

During isolation of the main rudiments of bodies and fabrics fishes and the highest vertebrata have also an isolation actually of a body 3. from a vitellicle (a vitelline bubble) and other so-called extraembryonic parts which, in fact, represent early the specializing and functioning provisional bodies 3. (fig. 6). At the highest vertebrata — it is amnion, chorion and an allantois. Amnion forms side folds of an extraembryonic ectoderm and an outside leaf of side plates. It is filled with liquid and protects 3. from bruises and drying. 3., before flat and presented only by the central part of an embryonal disk, begins to rise over peripheral parts of the last and then separates from them interception — a so-called truncal fold. A truncal fold, crashing more and more deeply, separates germinal (central) parts of an ectoderm, an entoderm and a mesoderm from extraembryonic; the last depart in structure of a vitellicle and allantois (an entoderm and a visceral mesoderm), amnion and chorion (an ectoderm and a parietal mesoderm). The central (germinal) parts of an entoderm and visceral mesoderm cave in in the form of a fillet which then becomes isolated in an intestinal tube. Short circuit of an intestinal tube, beginning with the front, and then and back ends, extends to its average sites. However communication of an intestinal tube with a vitellicle by means of a vitelline pedicle, or a channel, remains during all germinal period of development. The front and back ends of an intestinal tube are blindly closed in the beginning.

Only at the end of the 4th week on its front end breaks oral, and on back (5 — the 6th week) — kloakalny opening. On each side the tail of the head there are 4 — 5 pair deepenings of an ectoderm in the subject mesenchyma — rudiments of branchial apertures. The front gut forms the corresponding number of protrusions — visceral furrows. Visceral furrows and branchial apertures break each other, and the through message of a front gut with surrounding 3 is formed. Wednesday. At the highest vertebrata breathing after the birth lungs, gills are not formed, however branchial apertures and visceral furrows appear at them 3. in the same look, as at the lowest vertebrata (at 3. mammals and the person do not occur their break). However these rudiments are differentiated in a different way: e.g., the first couple of branchial apertures turns into acoustical passes of an outside ear, the first couple of visceral furrows becomes a rudiment of a middle ear, the epithelium of the 3rd and 4th couples of visceral furrows forms rudiments thymic and parathyroids.

Period of an organogenesis and histogenesis it is characterized by the fact that from cellular material of embryonal rudiments bodies form and fabrics are differentiated. Processes histogenesis (see), and partly and organogenesis (see) proceed, being gradually slowed down, and in the fetal period of development, and partially take also the post-natal (postnatal) period of life.

Provisional bodies exist and function only during development 3. (in egg or a maternal organism) or larvae. These are a vitellicle at 3. the majority of fishes and all highest vertebrata, amnion, chorion and an allantois at 3. highest vertebral (fig. 6). The placenta of the highest mammals represents the provisional body formed not only fabrics 3., but also fabrics of a maternal organism, the site of a mucous membrane of a uterus in particular defined, undergoing special reorganization. Provisional germinal bodies consist from early the specializing and peculiar differentiated fabrics which, functioning in special conditions of germinal life, then, having exhausted potentialities of the development, in the postgerminal period of development die off. The back string, or a chord, edges at the representative of the lowest chordates — a lancelet — plays a role of an axial skeleton during all life of an animal, at the majority of vertebrates (including the person) becomes temporary germinal body, being replaced at further stages of embryonic development with a cartilaginous and bone backbone. Most bodies 3. directly turns into appropriate authorities of the created organism and therefore carries the name definitivny (i.e. final). At all early and average stages of development such definitivny bodies consist of the low-differentiated fabrics which cells vigorously breed a mitosis. The differentiation (specialization) of definitivny fabrics and bodies is made much later, than provisional. At the same time in different definitivny bodies depending on the beginning of their active functioning the specific fabric differentiation comes at different times and goes unequal rates (asynchronously). Besides, asynchrony of a differentiation of different sites of the same system of bodies can depend on the fact that their rudiments are put at different times (e.g., somites are differentiated gradually one by one in process of segmentation of a mesoderm).

By the end of the 8th week at the person the germinal period of development comes to the end, all main laying of bodies and fabrics are created by this time. The fetal period of an antenatal life with dominance of processes of growth and a final fabric differentiation begins with the 9th week.

Features of development of a germ of the person

Features of development of a germ of the person boldly act against the background of the common features stated above, the general patterns of development which are making related the person with all metazoans (the sequence of the main stages of an embryogenesis, potentiality of germinal leaves, etc.), in particular with type of chordates (formation of an axial complex of rudiments, tubular laying of a brain on dorsal side of a body, branchial apertures), and especially with the highest vertebrata (a discoblastula, two-phase gastrulations, formation of primary strip, formation of a vitellicle, amnion, an allantois). 3. the person, it is similar to 3. all placental mammals, has (since the end of the 1st on the 3rd week of development) a form of a blastocyste, chorion forms. As well as at all representatives of a suborder of monkeys in group of primacies, the diskoidalny, haemo chorial vorsinkovy placenta is characteristic of the person.

Unlike the lowest monkeys, at the person (and also subhuman primates) submersible, or intersticial, implantation 3. (the bag of waters entirely plunges into thickness of a mucous membrane of a uterus) and single, but not double, as at monkeys and macaques, a placenta. Amnion arises not by accretion of amniotic folds, as at other highest vertebrata, including at the vast majority of mammals, and accretion of edges chashevidno of a concave periblast of a germinal disk. For 3. the person early and powerful development of the provisional subsidiary bodies (except an allantois) preparing the most favorable conditions for the subsequent development actually of a body 3 is characteristic.

Owing to features of the organization 3. at different stages of development possesses peculiar fiziol. properties, significantly excellent from fiziol, properties of the created organism. During an embryogenesis not only the structure 3 significantly changes., but also its functions, chemical structure, character of a metabolism etc. that is a subject of studying of the independent directions in embryology — physiology of a germ (embriofiziologiya) and chemical embryology (see. Embryology ). Processes of a metabolism in a body 3. much more intensively, than in the differentiated organism; higher sensitivity 3. to various disturbing factors — a penetrating radiation, medicinal and other chemical substances, bacterial toxins, temperature variations etc. In development 3. the periods of the greatest sensitivity to the damaging agents — the so-called critical periods of development matching the most responsible morphogenetic processes are observed; for mammals and the person it is the periods of implantation and formation of a placenta. At the person the general for all organism 3. critical periods fall on the end of the 1st — the beginning of the 2nd week (implantation) and on 3 — 6 weeks of development. However each embryonal rudiment and the body developing from it owing to asynchrony of their differentiation have the critical periods of sensitivity when action of the most various factors selectively breaks a further organogenesis (see. Embryopathies ).

Asynchrony of development of parts 3. does not mean their independence from each other; on the contrary, developments of various bodies and fabrics are interdependent and mutually agreed. At all stages of the formation 3. is a complete organism which not only develops, but also carries out functional departures, characteristic of each stage. Becoming complicated in the organization, 3. respectively there passes a number of the increasing levels of integrity, edges nevertheless is always relative and does not exclude a nek-swarm of limited autonomy of parts. Disturbance of normal interrelations between parts 3., and also various damages caused by action of these or those external factors can come to an end in one cases with a complete recovery of norm (the phenomenon of regulation), in others — to lead to resistant patol, changes and anomalies of development. The direction of embryology studying various disturbances of normal development and patol, changes 3., received the name of pathological embryology. With it it is closely connected teratology (see) as the analysis of ways and reasons of their emergence in an embryogenesis is necessary for understanding of uglinesses.

The age of a germ, and in particular the person, is estimated in hours, and then in days and weeks from the moment of fertilization. From the beginning of segmentation of a mesoderm most precisely the age can be established by the number of the stood apart couples of somites translucent through translucent covers 3. From the moment of isolation actually 3. from a blastodisc and acquisition generally shapes of a body age 3. decides by measurement of length from a darkness to a tailbone (parietal and coccygeal length, growth sitting), and on development of the lower extremities — also lengths from a darkness to heels (temennopyatochny length, costing growth). For 3. person of the first five weeks of pre-natal development by Striter (J.L. (Streeter. 1942, 1945, 1948, 1951) classification of stages («age groups») with their numbering and the characteristic of a number of diagnostic characters is offered (the age for the first seven stages is specified according to Hertiga, Roca and Mazantsa). The sizes for some stages are specified according to Patten (V. M. by Patten, 1959).

I. One-celled egg (within one days from the moment of fertilization). Diameter is 0,1 — 0,15 mm. II. The crushed egg (1 — 4 days). III. Free blastocyste (5 — 6 days). The size is 0,153 X 0,115 mm. IV. The implanted blastocyste (7 — 8 days). V. The blastocyste is implanted, but there are no fibers of chorion yet (9 — 11 days). Three dimensions together with a trophoblast: 0,450 X 0,300 X 0,125. VI. Primary fibers, clearly distinguishable vitellicle (12 — 14 days). The size together with chorion 3,24 X 2,04 mm] a germinal guard 0,23 X 0,2 mm. VII. The branching fibers, an axis of a germinal disk was defined, i.e. primary strip (15 — 16 days) was formed. VIII. Genzenovsky small knot, primitive pit (17 — 18 days). IX. Neural folds, the extended chordal outgrowth (19 — 20 days). X. Early somites — 1 — 12 couples (21 — 23 days). The size together with chorion: 7,6 X 6,7 X 4,7 mm; germinal guard 2 X 0,75 mm. XI. From 13 to 20 couples of somites (24 ± 1 days). XII. From 21 to 29 couples of somites (26 ± 1 days). XIII. A germ length apprx. 4 or 5 mm (parietal and coccygeal length; 28 ± 1 days). XIV. Period of emboly of a lenticular bubble (28 — 30 days). XV. Lenticular bubbles are not opened any more, and a time of their short circuit disappeared. Kidneys of hands are a little subdivided into a distal hand segment and a proximal segment which will form a shoulder and a forearm. Kidneys of legs begin to be differentiated on area. Length 3. mostly 7 and 8 mm, somites (according to Patten) 36 — 39 couples, the largest diameter of chorion are 30 — 40 mm, age of 31 — 32 days of XVI. Eyes get a dark rim with the advent of a retinal pigment. Auricles become noticeable. The area of a brush is differentiated on a shank and a manual plate. Kidneys of legs have three centers of proliferation presenting to area of a hip, a shin and foot. Somites are noticeable outside only kzad from hands. Length is more often than 9 — 10 mm, age 33 ± 1 days. Formation of full number of couples of somites (43 — 44 couples) comes to the end usually during the XVI—XVII stages.

By the end of the 5th week 3. grows to 0,6 — 1 cm and it is strongly bent in the ventral direction. At the XII—XV stages on its back end well noticeable tail, normal later disappearing, but occasionally remaining for life as an atavistic sign is formed at the expense of material of several last couples of somites. The front department of a neurotubule forms expansions — brain bubbles; formation of a brain begins.

Motive powers and causative factors of an embryogenesis are studied by experimental embryology (see. Embryology experimental ) at the development mechanics called also, or physiology of development. By the most widespread methods of experimental studying 3. animals impact on them these or those external factors (chemical, physical, mechanical, etc.), division of parts, change of parts from one 3 are. on others. Also the method of marking (tag) of parts 3 is of great importance. harmless dyes with the subsequent tracing of movement of these tags in development, offered by Vogt (W. Vogt, 1925). Also cultivation 3 is widely applied. and their parts in vitro on artificial mediums, In studying 3. the person find application artificial insemination of ova of in vitro with the subsequent cultivation 3. to stages to a pestilence of l y and an early blastocyste that was shown, in particular, by Shettls (L. Century of Shettles, I960), Edwards (J. L. Edwards, 1967), also method of culture of formative cells, rudiments and fabrics etc.; further development 3. mammals and the person in culture did not manage to achieve so far. Similar researches if they serve interests of mankind (overcoming infertility, cultivation of transplants, clarification of specifics of tissues of the person in comparison with tissues of laboratory animals, studying of development of reflexes at 3. and fruits etc.), have great practical value.

The main result of pilot embryological studies in combination with data of biochemistry, cytology and genetics is establishment of three groups of the factors determining process 3.

1. The genetic factors programmed in molecular composition of DNA, specific to each type of organisms, which is contained in a kernel and chromosomes of cells 3.

2. Interaction of parts 3., napr, determination of a rudiment of a nervous system (medullary plate) as a part of an ectoderm 3. as a result of a growth of material of a chord under it and a mesoderm at a gastrulation, etc. Such contact interactions as «embryonic induction» observed at early stages of formation 3., are replaced with morphogenetic influences of hemadens (endocrine factors), a nervous system later, and also interactions of the developing bodies and fabrics for type antigen — an antibody.

3. Influence external in relation to 3. agents (mechanical, napr, pressure; physical, napr, temperatures, radiant energy etc.; chemical, napr, medicinal substances, etc.; biological, or physiological, napr, bacterial and other toxins, for viviparous mammals and the person — factors immunol, compatibility or incompatibility between organisms 3. and mothers, etc.). However as internal factors of development 3. (heredity, interaction of parts), and its reactions to influence of environmental factors (need for a certain temperature, supply with oxygen and nutrients etc.) developed during evolution. Structure and functions 3. each this species of animals, and also the person can be understood only taking into account an origin and all history of development of a look and, in turn, shed light on specific history.

Ch. Darwin in «Origin of species» (1859) noted that 3. represents «the darkened image of the general primogenitor of this group of animals», i.e. development 3. to some extent reflects history of a look. In more developed form this thought received expression in the fundamental biogenetic law of Müller (F. Muller, 1864) and Gekkelya (E. Haeckel, 1866), according to Krom ontogenesis — development of an individual — is the reduced and distorted repetition of phylogenesis — patrimonial history (see. Biogenetic law ). Reproduction of signs of ancestors germs of descendants received the name of reconciliation (Latin of recapitulatio repetition). So, in development 3. the highest vertebrata, including the person, such signs of ancestral forms — the lowest vertebrata as branchial apertures, a pronephrous and primary kidney, etc. rekapitulirutsya. Signs 3., inherited from far ancestors, received the name palingenez (see), adaptive new acquisitions (a vitellicle, amnion etc.) are named by Gekkel cenogenesises. According to Gekkel, palingeneza allow to recover history of a look on the basis of changes 3. during an ontogeny whereas cenogenesises, on the contrary, darken it. Actually cenogenesises also reflect history of a look, and, e.g., preservation of a vitellicle 3. mammals and the person with obviousness confirms their origin from ancestors with yolky eggs. Initial process is distorted also by geterotopiya (change of provision of laying of bodies in comparison with observed at Z. ancestors) and geterokhroniya (delta time of a bookmark). However the fundamental biogenetic law reflects only one party of interdependence between ontogenesis and phylogeny, namely the fact that process 3. it is caused by history of a look. More fully this interdependence is considered developed by A. N. Severtsov (from 1910 to 1936) the theory of a filembriogenez, according to a cut evolution is made by changes of the course germinal (and in general individual) development. In other words, ontogenesis is caused by phylogenesis.

According to the rule K. M. Bera (1828, 1837), 3. various representatives of the same type (e.g., vertebrata) are most similar at early stages of development («the law of germinal similarity»), and then gradually disperse in signs depending on belonging to this or that class. Development goes from the general to the particular: the first signs of type, then a class, group, at last, of a sort and a look appear. Ch. Darwin highly appreciated this conclusion of K. M. Ber in due time as one of the major embriol, proofs of evolution as such order of emergence of signs at 3. corresponds to divergence (discrepancy) of signs in phylogenesis of each group of animals. However the evolutionary way passed by a look and biochemical, fiziol., morfol, and ekol, specifics of a look leave a mark on all stages of an ontogeny. E.g., 3. a rabbit or the person is the representative of the look beginning already with the moment of emergence and with the earliest stages of development. Specific features 3. are programmed already in chromosomes and structure of cytoplasm of an oospore are implemented during ontogenesis more and more stoutly.

See also Embryonic development .

Bibliography: Dyban A. P. Sketches of pathological embryology of the person, L., 1959, bibliogr.; To N about r r e A. G. A short sketch of embryology of the person (with elements of comparative, experimental and pathological embryology), JI., 1967, bibliogr.; Polezhayev JI. B. Fundamentals of development mechanics of vertebrata, M. — L., 1945, bibliogr.; P e t of t of e of N of B. M. Embriologiya of the person, the lane with English, M., 1959, bibliogr.; Saksen L. and T about y-in about N of e of N of Page. Primary embryonic induction, the lane with English, M., 1963, bibliogr.; Uoddington K. Morfogenez and genetics, the lane with English, M., 1964, bibliogr.; Falin L. I. Embryology of the person, Atlas, M., 1976; Shmalgauzen I. I. An organism as whole in individual and historical development, M. — L., 1942; Clara M. Entwicklungsgeschichte des Menschen, Lpz., 1966, Bibliogr.; H e r-t i g A. T., R o with k J. A d a m s E. C. A description of 34 human ova within the first days of development, Amer. J. Anat., v. 98, p. 435, 1956; MazanecK.> Blastogenese des Menschen, Jena, 1959, Bibliogr.; Shettles L. B. Ovum humanum, growth, maturation, nourishment, fertilization and early development, N. Y., 1960; Wi 1 1 is R. A. The borderland of embryology and pathology, L., 1962.

A. G. Knorre.