1897 Human Embryology 2

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

Note - this online text is only at a very early draft stage and contains many errors from the original scanning.
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 2. - General Outline of Human Development

This chapter is designed especially for the convenience of students of medicine and biology. Advanced students will find in it little of value to them, since all the subjects it considers are more fully treated in other portions of the volume.

I. Retrogressive History of the Foetus and its Envelopes.

Uterus Eight Months Pregnant

If we examine a pregnant uterus at any time during the sixth to ninth month of gestation, we find essentially the same relations of the parts — the most marked difference being in the size of the uterus, which increases with the duration of gestation, to correspond to the growth of the foetus. A description of a uterus of the eighth month after conception will suffice, therefore, for our present purpose.

Such a uterus is a large, rounded bag, with muscular walls, and measures seven or eight inches in diameter. It renders the abdomen very protulx»rant. Examined externally it is remarkable especially for the nimierous large sinus-liko blood-vessels; its surface is smooth; the texture of the walls is firm to the touch, but the walls jneld to pressure, so that the position of the child can be felt. As the placenta is generally upon the dor^id side, it is usual to open the uterus by a crucial incision upon the ventral side. The walls are about one-half of an inch thick, sometimes more, sometimes less, and as soon as they are cut o])en we enter at once into the cavity of the uterus containing the foetus and nearly a pint of serous liquid — the liquid is the amniotic fluid. The foetus normally lies on one side, has the head bent forward, the arms crossed over the chest, the thighs drawn against the abdomen, and the legs crossed ; it resembles closely the child at birth, but is smaller; its head is relatively to the size of the body larger ; the abdomen is more protuberant, and the limbs proportionately smaller. The inner surface of the uterus is smooth and glistening ; if it is touched with the finger it is found to be covered by a thin but rather tough membrane, CiiUed the amnion, which is only loosely attached. Examination of the uterine wall, where it has been cut through, shows that its thickness is fonned principally by the muscular layer, which is made up by numerous laminae of fibres, between which are the large and ci'owded blood sinuses, similar to those distinguishable on the external surface of the uterus. About one-fifth or less of the wall inside the muscularis lias a different texture and can lx> partly peeled off as two distinct membranes, the innermost of which is the amnion already mentioned, and the outer is the chorion united with the decidua. The amnion and chorion are a))jx}ndages of the embryo; the decidua the modified nmcoiis membrane of the uterus. The inner tMirtion of a microscopical section through the titeriue wall is shown in Fig. 15. The amnion, am, consists of two layers, a cubicalcelle<l epithelium facing tlie embrj-o, ami a connective-tissue stratum facing the uterus. The chorion, C'Ao, is likewise two-layered, but its epithelium, c, is next the uterus, its connective tissue next the amnion ; the amnion and chorion are loosely held together by shreds and bands crossing from one membrane to the other. The decidua occupies the whole space between the chorion, Cho, and muscularis, muse; it contains blood-vessels, l\ and remnants, gl, of gland cavities. Let us return to the embryo. From its abdomen there springs a long, whitish cord, kno\vn as the umbilical cord ; it is usually about one-third to one-half an inch in diameter and 40 cm. long, but its dimensions are extremely variable; it always shows a spiral twist, and contains three large blood-vessels, two arteries, and one vein, all of which can be distinguished through the translucent tissue. The distal end of the cord is attached to the wall of the uterus, usually near the middle of the dorsal side of the organ. It is easily seen that the blood-vessels of the umbilical cord radiate out from its end over the surface of the uterus underneath the amnion, branching as they go; they spread, however, only over a circumscribed area, the placental, where the wall of the uterus is very much thickened. A vertical section through the placental area shows that the amnion and chorion are widely separated from the decidua and muscularis by a spongy mass soaked with maternal blood. This mass consists of numerous trees of tissue, which spring with comparatively thick stems from the chorion and branch again and again. In these stems and branches are to be found the final ramifications of the vessels of the umbilical cord; the trees are known as chorionic or placental villi. Some of their end-twigs are very closely attached to the surface of the decidua. In the centre of the placental area the villi form a mass abc^ut three-fourths of an inch thick, but toward the edge of the area the mass gradually thins out until at the very edge the chorion and decidua come into immediate contact. The mass of villi, together with the overlying portions of the chorionic and amniotic membranes and the underlying ix)rtion of the decidua, constitutes what is known as the placenta. The decidua of the placental area is called the decidua serotina ; the chorion of the placenta is known as the chorion frondosum. When birth takes place the whole 7>/ac^/i/a is expelled after the delivery of the child; the placenta of the obstetrician is therefore partly of foetal, partly of maternal, origin.

Uterus Three Months Pregnant

The uterus measures about 3^ inches in transverse diameter, and shows well-marked inlaid sinuses on its external surface. If it is opened, as before, b}" a crucial incision on the anterior side, its walls will be foimd about half an inch or more in thickness ; it contains a grayish-i'ed bag (decidua reHexa), which nearly fills the cavity of the uterus and incloses the embryo, so that ui)on opening the womb we do not encounter the foetus directly. The inner bag has a smooth surface, but shows a few small pores ; it is without blood-vessels and is attached to the dorsal wall of the uterus. Tlie inner surface of the uterus shows a rich network of blood-vessels, many of which are large, irregular sinuses. The walls are seen to consist of an outer muscular layer, and an inner decidual layer, which takes up nearly half the thickness of the wall, and is known as the clecidna vera. As compared with the eightli-month uterus the proportion of the layers shows us that during gestation the muscular layer increases and the decidual layer diminishes in thickness. The inner bag when opened shows the large cavity in which the emluyo lies, floating in amniotic fluid. The bag is formed by three very distinct membranes, of which the outermost, decidua reflexa, is the thickest and opaque; the two inner ones are thin and transparent; the innermost is the delicate amnion; the middle membrane is the chorion and is quite distinct from both the amnion and reflexa; with the latter it is connected by a number of small branching villi scattered at some distance from one another over the surface ; the villi adhere firmly to the reflexa by their tips. The embryo resembles a child in its general appearance; the length of the head and rump together is about eight centimetres, and the head is approximately of e(iual bulk to the rump. The umbilical cord is 5-7 mm. in diameter and usually about 12 centimetres long. From its distal end the bUxxl-vessels spread out over the placental area, and around the edge of the area rises the decidua reflexa, which does not extend on to the placenta. Floating in the amniotic fluid is a pear-shaped vesicle, the yolk-sack, which is about 8 mm. in diameter ; it has a fine network of blood-vessels upon its surface, and is connected at its pointed end with a long slender pedicle, the yolkstalk, which runs to the placental end of the umbilical cord, there enters the cord itself, and runs through its entire length to its attachment to one of the coils of the intestine of the embr}- o. Over the whole of the placental area the chorion gives oflp large villous trunks, each of which has numerous branches, with ramifications of the foetal vessels ; the villi fill a space about one centimetre wide between the membrane of the chorion frondosum and the surface of the uterine decidua serotina, to which the tips of some of the villi are attached. With care the villi may be separated from the decidua, which is seen, when it is thus uncovered, to be cavernous ; the caverns are rounded in form and may be followed on the one hand until thev connect with the blood sinuses of the uterus, and on the other until they open into the inter\'^illous spaces, which therefore receive a direct supply of blood from the mother.


The principal difference to be noted in the relations of parts between the uterus before and that after the fifth month is in the presence or absence of the decidua reflexa as a distinct membrane. During the fourth month the reflexa stretches as the membranes expand and becomes thinner and thinner until by the end of the fourth month it is as delicate and transparent as the chorion and lies close against the wall of the uterus (decidua vera) . It is probable that the decidua reflexa degenerates and is resorbed, compare p. 19.


Uterus Five Weeks Pregnant

The relations are best shown by a median antero-posterior section. Fig. 4. The arrangement of the uterine parts is essentially the same as at three months. The mucosa uteri is changed into the decidua graviditatis. On the dorsal side from s to 5 is the decidua serotina of the placental area, where the villi of the chorion are fastened by their tips to the uterus. From the edge of the placental area on all sides rises the decidua reflexa, r ?•, which is much thinner than the other parts of the decidua, and which forms a closed dome over the embryo; hence when we pass through the cervix uteri, c, we enter, not the cavity containing the ovum, but the fissure-like space between reflexa, ?* r, and the vera, q g, which includes the whole of the modified mucosa of the body of the uterus, except that part to which the OA^im is attached and which produces the reflexa and serotina. The vera is that portion of the decidua which is not in direct contact with the ovum. No stage of gestation earlier than the completed formation of the reflexa has been observed.


The embryo differs greatly from the three months' foetus. Beginning with the envelopes we notice that tho chorion ia beset w^ith weU-deveioi)eil vilH over its entire surface, but the villi over the placental area arc larger than thoije over the ])arts adjacent to the decidua reflexa. The amnion does not lie cIoko to the chorion, but close around the embryo, loaving a wide space between the two membnines, which sjiace, as we have seen, ia snbse<iuently obliterated by the expansion of the amnion. Tho embryo itself is very small and not human iu ajipearance, and its orgjins are only partially differentiateil. The umbilical cord is very sliort; the amnion springs from it at a short distance from the embr>"0. The yolk-stalk leaves the cord just beyond tlie amnion, is compimitively short, and ends in the pear-8hape<l yolk-sack, which is about the same size as at three months. Beyond the point M'hero tho amnion and yolk-stalk part from it, tho umbilical coitl continues a short distance with its bloodvessels, which ramify over the entire cliorion and penetnito all tho villi thereof. To pnxhice the i-eliitions foimd at three months tho blood-vessels and villi of tho chorion mnt^t alxjrt except over tho placental area; the umbilical cord must elongate greatly; the amnion must expand tmtil it touches the cliorion, and the fcetus must grow and change.


We must now trace back the history of the embryo still farther, in order to understmul the relation of the embryo to the embryonic membranes and appendages.

Ovum of Three weeks

Normal human ova of this ago very rarely indt-eil reach the embryologistx, but a few have Iteen described. The eluirion forms a closed vesicle beset on all sides with crowded, clumsily -branching villi ; the vesicle meiisiires aliout 3 cm. in diameter; the villi are about 3 mm. long, and as yet show no regional inequality in their development. If the vesicle in oiK-ned llio (?mbrj'o is found within rolled uji, t'.ie b;ic"k being convex; it measures in its natural attitude about -14 mm. The head is bent toward the right; the caudal extremity toward the left ; the head and tail are almost in contact, so tliat it is difficult to ohwcrvo the insertion of the mnliilical cord. With care this may be done, and it will then be seen that the amnion arises from the emhrj-o, if the IxMly-wall ; the amnion itself is aUiut the embrvo. The umbilical cord unites with the abdomen; in front of it, i.e., headword, is a Kinall opening thi-ough which the stalk of the yolk-sack enters the body to unite with the intestine; it is from the etlgesof this opening that the amnion arises, and as the amnion passes around the umbilical cord, it may be said that the cord and the j-olk-stalk both enter the body thn)ugh tbe opening, but whereas the cord is in contact with the amnion the yolk -stalk is not. The opening may be called the umbilical foramen. The yolk-sack is pear-shaped, measures about 3 mm. in diameter, and is attached by its pointed end to a loop of the embryonic intestine. The 3"olk-stalk is developed by the subsequent prolongation of the pointed end of the sack.



and is, in fact, a i>ridoiifj:atioii extremely thin and lies chfr


• Fur llKunf* am ChuiHiT XIII.



In an embryo a little younger the relations can be more clearly recognized. Fig. IG. The einbrjois nearl\ straight although both head and tail are bent over ventralh The umbilical foramen, from the edge of which the amnion __

arises, is very wide and long; at its taiiward edge runs out the umbihcil cortl {Banchstiel), to which the am uion is attached, and which jjins the chorion a chort distance from the embrjo- The neck of the jolk sack, n.*, is also much wider if the sack ia cut open we findilsnwkto be a lai^ opening into the cavity of tbe intestine; in fact, the yolk-sack is an appendage of the intestinal ciiiial which at this stage is verj- simple being liardly more than a straight tube running lengthwise: the opening between the sack and intestine -— - — --^ may be called the vitelline foramen " '

The younger the embryo the fiq i — e i ry i m iodk An*r longer are — relatively to tlie size of the embryo — the foramen umbilicale and the foramen ^ itelhmun, as is well shown in Fig. 17. The line of ittaihmeut of the amnion extends almost the entire length of the embryo beginning mst in front of the heart, and ending upon the umbilical cord (Bauchstiel OT allantois-stalk), close to tbe chorion The ■\olk lacfc has also a long attachment, h^inning just behind the heart, and extending nearly to the allantois stalk, which now appears t« the eye very much what it is morphologically, a prolongation of the posterior extremity of thft body of the embr>'o.


Going back still farther, we find the relations to be as represented by the accompanying diagram. Fig, 18. The embryo, Emb, rests upon the yolk-sack, and is scarcely longer than the umbilical foramen ; the end of the embryo is prolonged posteriorly as the relatively large allantois-stalk, .41, by which the embryo is attached to the chorion. The amnion springs from the sides of the embryo and of its allantois stalk, and forms a closed sack over the embryo. This Ntage is almost the youngest in the series of known human embryos, and has been only im|)erfeclly described.

■ AI iliis Blttji"; morr pr«|>rrly Ui be tnlled Iht Bniichilirl. see Chapter XVI. 3


The following generalized diagram, Fig. 10, of a young amnioto vertebrate embryo is intended to render clear the essential relations of the embryo and its appendages. The figure represents a transverse section of the embryo, together with all the membranes. The embryo consists of an axial mass, from wliich runs out on each side a lamina or plate of tissue, Sum, to form the body-wall; this plate extends beyond the embryo to form the amnion, Am; as the date from one side joins that from the other, the amnion makes a closed sack over the biick of the embryo. From the axial mass there run out two other plates, Spl, to form the walls of the intestinal canal, In; these plates are likewise prolonged beyond the body to form the large yolk-sack, Yolk, upon the U>p of which the embryo rests. The apsice between the walls of the intestine and the bodywalls is of course the body-cavity, Coe^ Where the body-wall, Soiii, passes over into the amnion. Am, there is an opening by which the body-cavity communicates directly wHth the space Iwtween the amnion and yolk-sack on one side and the chorion on the other; this opening is the umbilical foramen. Similarly there is a passage by which the cavity of the intestine, In, communicates with that of the yolk-sack. Yolk: this passage is the vitelline foramen.


For our conceptions of the probable history of the human ovum up to the fourteenth (Liy, wo must rely mainly on analogy, drawn from our knowledge of the development of other manunals and of birds and reptiles. From these sources wo leani that the amnion and chiirion are originally portions of the same membrane, which is an extension of the IxHly-wall of the einbrio. lu i-eality the differentiation of the amnion is (juite a complex process, as is shown by the detailed history given in Chapter XV. The essential stejis can be niiuUt clear, however, by a bi-ief description. Fig. iO is a diagram of a stago in the development of amuiota a little earlier than that shown in Fig. I'.K Both the vitelline and umbilical fonimens are much wider than in the pifceding figure. The VxMly-wall of the embryo, Soiit, iHisses over as before into the amnion. Am. but the amnion of one side does not join that of the other, but instead bends over and is continuous with the chorion, Cho, Thus the amnion and chorion conjointly form a fold on each side of the embryo ; if the two folds enlarge and arch over the embryo until they meet and unite by their edges the condition illustrated by the preceding diagram, Fig. 10, will be established. ^^ /rr?^. ,, Returning to the earlier condition, Spl ^^ o Fig. 20, we may say that the ovum consists of two closed vesicles united together by the axial mass of the embryo. The membrane, which forms the outer vesicle, is 8ubdivide<l into three principal regions, to wit : the body- wall of the embryo, the amnion, the chorion, each having its separate history. The membrane which forms the inner vesicle is subdivided into two principal regions, to wit : the wall of the intestine and the wall of the yolk-siick, each having its separate history. It will be remembered that the posterior end Fig. ao. -Generalized diagram of an amniote c\i fVift aTnV»rvr\ ia T^rrklnnfrofl aa vertebrate embryo before the separation of tJti© OI ine embryo is proiong ea as amnion, ^Im, and chorion, CAo.

the allantois-stalk, by means of which it remains permanently and directly united with the chorion.

It is unnecessary, for our present purpose, to follow back the earlier history step by step. Suffice it to say that in 3'ounger stages the two vesicles are represented only by one, and earlier yet there is merely a cluster of cells.

The stages of development preceding this are not to be found in the uterus, but in the Fallopian tubes. They exhibit to us merely an agglomeration of a few cells, the so-called segmented ovtm[i. The earlier the stage the fewer the cells, until we reach the condition when there are but few cells, then two, and finally one only. This cell is the impregnated ovimi, the beginning of all development, but is itself formed of two separate parts, very different in their origin and constitution, namely, the egg-cell or ovum and the spermatozoon, whose union is the act of impregnation — the beginning of a new existence.

II. Progressive History of the Foetus and its Envelopes

The ovum enters the upper end of the Fallopian tube, and is there impregnated.* Very slowly it moves down the Fallopian tube, undergoing meanwhile the process of so-called segmentation, by which it is separated into a gradually increasing number of cells, that arrange themselves so as to begin the formation of the embryo and its appendages. Probably about the eighth day the ovum reaches ihe uterus, where it becomes adherent to the mucosa upon the dorsal side of the uterus usually, and by an unknown process of agglutination. The decidua reflexa grows up around it by a process not yet observed. The amnion is differentiated from the chorion. The portion of the mucosa uteri in contact with the ovum is transformed into the decidua serotina; the remaining portion of the mucosa becomes the decidua vera. The allantois-staJk unites the embr>'o with the chorion, and carries the blood-vessels of the foetus to ramify upon the chorion. The embryo is enclosed by the amnion ; the amnion is enclosed by the villous chorion ; the chorion is enclosed by the decidua reflexa and serotina. The vesicle formed by the close adherence of the chorion to the reflexa is suspended from the wall of the uterus. The mass of tissue resulting from the union of the chorion with the serotina forms the placenta. The umbilical cord (allantois-stalk) is always attached to the placental area, and later the ramifications of the umbilical vessels are restricted to that area. During the fifth month the decidua reflexa coalesces with the decidua vera, and the space between them is of course obliterated. Finally, we find that the amnion enlarges, lays itself against the chorion, and, uniting loosely witJi it, becomes the innermost constituent of the vesicle enclosing the embryo.



  • It is poftsihle that impreimation may occur while the ovum is passing from the ovary to the flaibfiate opening of the FaUopian tube.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)


Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History



Cite this page: Hill, M.A. (2018, September 26) Embryology 1897 Human Embryology 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/1897_Human_Embryology_2

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