Vertebrate Embryology - A Text-book for Students and Practitioners (1893) 6

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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Marshall AM. Vertebrate Embryology: A Text-book for Students and Practitioners. (1893) Elder Smith & Co., London.

Marshall (1893): 1 Introduction | 2 Amphioxus | 3 Frog | 4 Chick | 5 The Rabbit | 6 Human Embryo | Illustrations
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Contents

Chapter VI. The Development of the Human Embryo

Preliminary Account

The human embryo is developed from an egg, which, like that of other animals, is a single nucleated cell, derived from the peritoneal cells forming the outermost layer of the ovary. The human egg, or ovum, measures 0'2 mm. in diameter, i.e. is rather less than double the diameter of the ovum of the rabbit.

The ovum, when ripe, is discharged from the ovary, and is taken up by the open mouth of the Fallopian tube or oviduct, down which it travels to the uterus, where it remains during the rest of the period of development. Prior to the arrival of the ovum, the mucous membrane lining the uterus undergoes important changes, and gives rise to a special layer, the decidua, to which the ovum is attached and in which it becomes embedded. During gestation the cavity of the uterus gradually becomes filled up by the growth of the embryo and of its inclosing membranes.

As the ovum is of very small size, the nutriment at the expense of which development takes place must be obtained from without. This is effected, as in the rabbit, by means of the placenta, an organ in which the blood-vessels of the embryo and those of the wall of the uterus are brought into extensive contact, so that free interchange of contents can take place through their walls.

The process of fertilisation of the human ovum, and the early stages of development have not yet been seen ; and of specimens showing the first formation of the embryo only a very limited number have been obtained, and very few of these in fit condition for microscopical investigation. Of later embryos, numerous specimens have been examined and described, and from a stage corresponding to about a forty-eight hour chick embryo, or to a nine-day rabbit embryo, the history of human development has been determined in considerable detail.

For this satisfactory condition of our knowledge we are very largely indebted to the labours of Professor His, whose detailed and careful descriptions, and splendid series of figures, form the basis on which the account in the present chapter has been in chief part founded.

The total period of human development is usually estimated at slightly under ten lunar months. The exact period cannot be ascertained, owing to the impossibility of determining the time at which fertilisation of the egg is effected, i.e. at which development commences.

The details of development of the human embryo are closely similar to those of the rabbit ; the chief points of difference being : (i) the far longer time occupied by the human embryo, more than nine times as long as the rabbit ; (ii) the extreme slowness with which the early stages of development are effected in the human embryo ; and (iii) the early stage at which the allantois is established in the human embryo, and the peculiar mode in which it is formed.


The Human Ovum

1. Formation of the Ovum

The earlier stages in the development of the ova are already completed in the female child before birth ; and after birth the formation of ova only goes on for a very short time, and to a very limited extent. According to Bischoff, Waldeyer, Foulis, and others, the formation of new ova ceases about the age of two years ; in other words, the ovaries of a female child already contain, at the end of the second year, all the ova that will ever be developed in them. As each ovum is morphologically a single cell, this means that an individual cell may live, and retain all its characteristic activities, for a period of forty-five years or more.

The early development of the human ova must, therefore, be studied, not in the woman or child, but in the embryo. The several stages of its formation are so closely similar to those already described in the rabbit that a detailed account will be unnecessary.

The germinal epithelium. In embryos of about the fifth week, the genital ridges appear, as a pair of longitudinal bands along the dorsal wall of the abdominal cavity, close to the inner borders of the Wolffian bodies. The ridges, which at first are merely caused by the epithelial cells becoming columnar in place of squamous in shape, rapidly increase in thickness, partly owing to active division of the cells of the germinal epithelium covering the ridge, and partly owing to ingrowth of connective tissue along their axes.

At an early age, an intimate relation is established between each genital ridge and the corresponding Wolflfian body, a number of rod-like outgrowths arising from the Malpighian bodies of the Wolffian body, and growing into the substance of the genital ridge. These rods subsequently become hollow, and form the so-called tubuliferous tissue of the ovary. This lies, at first, close beneath the germinal epithelium, but soon withdraws into the deeper part of the ovary ; it has nothing to do with the formation of the ova, and merely requires mention on account of its great prominence during the early stages of development.

The germinal epithelium gives rise to the ova in much the same way as in the rabbit. In its earliest stages it is a single layer of columnar epithelium cells, with large nuclei, the cells measuring on an average about O014 mm. in length by 0-007 mm. in width.

By division of its cells, the germinal epithelium rapidly increases in thickness. The surface cells remain columnar, but the deeper cells, which are spherical or polygonal in shape, grow down into the connective-tissue stroma as irregular branching rods of cells, the egg columns (Fig. 171). By further growth inwards of the egg columns, accompanied by active growth outwards of the vascular connective tissue, the structure of the ovary rapidly becomes more complicated. In place of the original arrangement, of a layer of epithelial cells clothing a central connective-tissue core, there is now (Fig. 171) a superficial layer of columnar epithelium, a-, beneath which is a reticular framework of connective tissue, the meshes of which are filled with irregular columns or rods of epithelial cells, arranged for the most part vertical to the surface.

The primitive ova. The columnar epithelial cells of the siirface layer are at first all much the same size, but they do not long remain so. At an early period, about the sixth or seventh week, certain of the cells become conspicuous by their larger size and more spherical shape ; these are the primitive ova (Fig. 171, c, c), each of which is capable of developing into a definitive or permanent ovum, and then, if fertilised, of giving rise to an embryo. Each of these enlarged epithelial cells is in fact a potential human being.



FIG. 171. Part of a vertical section of the ovary of a new-born Infant. (From Strieker's ' Histology.') x 150.

a, superficial layer of columnar epithelium. b, plate of epithelial cells, formed by irregular growth of the ovary, c c, primitive ova. d e, nests of various shapes, containing ova and commencing follicles. /, isolated follicle with its contained ovum, g, bloodvessel.


On the formation of the egg columns, by proliferation of the deeper surface of the germinal epithelium, the primitive ova are carried down into them in large numbers. As the egg columns penetrate deeper and deeper into the substance of the ovary, they become broken up, by further growth of the connective tissue, into groups or nests of cells (Fig. 171, d, e), each nest containing one or more primitive ova as well as a number of indifferent epithelial cells. In these nests a tendency soon manifests itself for the smaller or indifferent cells to arrange themselves round the primitive ova, so as to inclose these in follicles (Fig. 171, d, e,f). At first there may be in a single nest several of these follicles, each containing a single ovum, but the continued growth of the connective tissue stroma gradually breaks up the nests, and tends to isolate the several follicles from one another, forming around each of them a separate connective tissue investment.


At the time of birth of the infant, the structure of the ovary is as shown in Fig. 171. The germinal epithelium, a, or superficial layer of columnar epithelial cells, is separated from the deeper layers of the ovary, at almost all parts, by a thin layer of connective tissue, the tunica albuginea. A little deeper down are seen large nests of epithelial cells, formed by proliferation from the deeper surface of the germinal epithelium, but cut off and isolated by growth of the connective tissue stroma. In these nests certain of the cells, the primitive ova, are distinguished by their larger size, and round these the smaller cells tend to arrange themselves so as to form capsules or follicles. In the deeper parts of the ovary the vascular connective tissue has, by its further growth, broken up the nests, and separated the follicles more or less completely from one another.


In passing from the exterior towards the deeper parts of the ovary, successive stages in the development of the ova are met with. In the superficial layer of columnar epithelial cells the earliest stages are seen ; certain of these cells, the primitive ova, being of rather larger size than their neighbours.


Beneath this surface layer are large nests, composed of epithelial cells, which, except in the larger size of the primitive ova, differ but little from one another, and present no regularity of arrangement. In the more deeply placed nests, the cells immediately adjacent to the ova have arranged themselves round these latter so as to form follicles ; but there are, in such nests, many cells of indifferent character, whose ultimate fate is still uncertain. Deeper still, the number of these indifferent cells is greatly diminished; and the follicles are larger, more clearly defined, and separated from one another by connective tissue trabeculse.


In such a section, therefore, as in Fig. 171, the most deeply situated ova are the oldest and most mature, and have, in attaining their present position, passed in succession through, the several stages which are met with in passing from the surface to the deeper parts of the ovary.

The primitive ova are spherical cells, from 0'05 to O07 mm. in diameter, with granular and rather ill-defined nuclei, and devoid of nuclear membranes. Each primitive ovum is inclosed in a follicle, consisting of a single layer of small cubical or flattened epithelial cells.

The permanent ova. About the time the egg follicles or capsules commence to form around the primitive ova, these latter undergo changes by which they become converted into the permanent ova. Primitive ova occur in both sexes, and the early stages in the development of the genital organs are the same in both ; but the change to permanent ova occurs in the female only, and marks the establishment of sexuality.

The change, as in other animals, chiefly concerns the nucleus. In the primitive ovum this is uniformly granular, with a rather ill-defined outline ; in the permanent ovum it becomes converted into a spherical vesicular body, of much larger size than before, with a sharply defined double-contoured wall, fluid contents, and a nuclear reticulum with one or more nucleolar enlargements at the nodes.

Besides the changes in the nucleus, the whole egg increases in size; its protoplasm, previously clear, becomes granular ; and around the egg, between it and the follicle, a thin elastic investing membrane, the zona radiata, is formed.

The Graafian follicle. Each ovum is surrounded at first by a single layer of cells, derived, like the ovum itself, from the germinal epithelium. These cells are at first flattened, but very shortly become cubical or columnar in shape. Since they lie between the ovum and the blood-vessels of the ovary, the nutrient matter must pass through the follicular cells in order to reach the ovum ; and it is probable that these cells do not merely transmit the food, but play some part in elaborating it.

A second layer of cells soon appears in each follicle, formed, as in the rabbit, between the original layer and the ovum, and probably by division of this originally single layer of cells into two. Shortly afterwards, by further division, the follicle becomes several cells thick. By splitting apart of these cells, accompanied by rapid growth of the outer layer, a cavity is formed in the thickness of the wall of the follicle ; and this cavity, which is filled with fluid, rapidly increases in size, dividing the follicle into an outer wall, the tunica granulosa, and an inner one, or discus proligerus, which immediately invests the ovum (cf. p. 349).

The fully formed Graafian follicle (cf. Fig. 133, GK, p. 347) is ovoid or ellipsoidal in shape: its walls consist of: (i) an outer investment of vascular connective tissue, derived from the stroma of the ovary, and divisible into a rather ill-defined outer layer, the tunica fibrosa folliculi ; and an inner well-marked layer of fine connective tissue, abundantly supplied with capillary blood-vessels, the tunica propria folliculi. (ii) Within this latter is the tunica granulosa (Fig. 133, GB), a thick layer of granular, spherical or polygonal cells. At one part, the tunica granulosa is much thickened, forming a roundish mass projecting into the cavity of the follicle ; and embedded in the middle of this roundish mass, or discus proligerus, is the ovum, OW. The cells immediately surrounding the ovum are distinctly columnar in shape, while the remaining cells of the follicle are spherical or polygonal. The cavity of the follicle is filled by the watery liquor folliculi.

In the early stages of their formation (Fig. 133) the more mature Graafian follicles lie in the deepest parts of the ovary ; but, as they increase in size, their growth takes place in all directions ; and ultimately the outer walls of the follicles approach very close to the surface of the ovary, or actually push the superficial layer of epithelium and connective tissue of the ovary before them, and so form rounded external projections on its surface.


At the most prominent part of the ripe Graafian follicle is a small spot, the hilum folliculi, distinguished from the rest of the follicle by being devoid of blood-vessels : at this place, shortly after the follicle has reached its full dimensions, i.e. a diameter of from 1'25 to 4 mm., rupture of the follicular wall occurs, and the ovum, together with the liquor folliculi, is discharged on the surface of the ovary.

This rupture of the wall of the Graafian follicle is due in part to fatty degeneration of the cells composing the wall ; and in part to increased pressure on the follicle, caused by a sudden accession of blood to the ovary.

The ripe ovarian ovum. The ripe human ovum is a spherical cell, about 0'2 mm. in diameter. It consists of a granular mass of protoplasm ; within which is a nucleus, or germinal vesicle, about O045 mm. in diameter, containing a nuclear reticulum and a conspicuous nucleolus or germinal spot. The ovum is invested by a transparent elastic membrane, the zona pellucida, which is about O01 mm. thick.


Each Graafian follicle, as a rule, contains only a single ovum ; in exceptional cases two ova, and in a few instances three, have been seen in the same follicle.

2. The Corpus Luteum

After the discharge of the ovum, important changes occur in the Graafian follicle, leading to the formation of the body known as the corpus luteum, which occupies and fills up the cavity of the follicle.

The corpus luteum is formed by rapid growth of the wall of the empty follicle, which becomes thrown into radial folds, projecting into the cavity of the follicle, and blocking this up almost completely. The folding involves both the follicular epithelium and the connective-tissue wall of the follicle, but the latter takes the most active share in the process. The characteristic yellow colour of the folded wall, which has given rise to the name corpus luteum, is due to large numbers of yellowish cells, derived apparently from the connective tissue stroma of the ovary. Between the two layers of each of the folds, bloodvessels pass in freely ; and the central cavity of the follicle, which, by ingrowth of the radial folds, is reduced to an irregularly stellate space, becomes occupied by a cicatricial fibrous tissue, which is red in the early stages, but in the later ones becomes grey.

The subsequent changes in the corpus luteum differ considerably according to whether the ovum, which has been discharged from the follicle, (i) is fertilised and develops into an embryo ; or (ii) is not fertilised, but dies without undergoing any further development.

In the latter case, i.e. if the ovum is not fertilised, the corpus luteum spurium, as it is then called, increases slightly in size for a few days ; but ten or twelve days after the discharge of the ovum, commences to shrink, and disappears completely in a few weeks' time.

If, however, the ovum that has escaped from the follicle isfertilised, and gives rise to an embryo, the corpus luteum. now spoken of as corpus luteum verum, or corpus luteum of pregnancy, does not reach its full development until two or three months after the discharge of the ovum. It persists throughout the greater part, or the whole, of pregnancy, contracting towards the close of the period to a small white stellate cicatrix, the corpus albicans, which may persist for some months after delivery.

The fully developed corpus luteum verum, or corpus luteum of pregnancy, is a firm body, larger than the original follicle, and attaining one-fourth, or even one-third, the size of the entire ovary (Fig. 255).

The presence of a corpus luteum verum in one of the ovaries is a matter of considerable medico-legal importance, inasmuch as it has been appealed to as positive evidence of pregnancy having occurred ; but the best authorities now agree that there is no infallible sign by which the corpus luteum of pregnancy can be distinguished from that of the non-fertilised ovum. The differences between the two are chiefly those of size, and length of duration, and cannot always be relied on in determining disputed cases. The terms ' true ' and ' false,' as applied to the two kinds of corpora lutea, appear, indeed, to be erroneous ; as the two structures are essentially similar, and in many cases indistinguishable from each other.

3. Ovulation

From the time of puberty, and throughout the whole of the child-bearing period of life, i.e. from about the fifteenth to about the forty-fifth year, the gradual maturation of the Graafian follicles, ending in rupture of the follicles and discharge of the ova, is continually going on ; and in the healthy condition this discharge of ova occurs, not in an indefinite manner, but at regular, and usually monthly intervals, one or more ova being set free at each period.

This periodical maturation and discharge of ova is spoken of as ovulation. It goes 011 independently of sexual intercourse or of any kind of influence from the male ; but it is possible that, as held by many authorities, the discharge of ova, though in no way dependent on sexual intercourse, may yet be hastened by this.

4. Menstruation

Menstruation is the periodical discharge from the uterus of a certain amount of blood, mixed with mucus from the uterine glands, and with epithelial and connective-tissue cells, derived from disintegration of the mucous membrane of the uterus itself.

There is a close connection between menstruation and ovulation. Both processes commence at puberty, and last throughout the child-bearing period. They both recur periodically ; and, further than this, the intervals are the same, and the two processes occur, as a rule, simultaneously. The true nature and extent of the connection between the two will be discussed after the nature of the menstrual process has been considered more fully.

During the period of pregnancy, that is, during the whole time that an ovum or embryo is -developing within the uterus, menstruation ceases, recommencing six or seven weeks after the birth of the child. The normal occurrence of the menstrual periods may also be affected by a variety of accidental or pathological conditions, for the consideration of which reference must be made to works dealing with obstetrics.

Menstruation, i.e. the actual discharge from the uterus of blood and other matters, is not an isolated process, but is the terminal act of a series of changes, which occur at regular intervals in the walls of the uterus, and of which the sequence is as follows.

In the quiescent condition the uterus is lined by a smooth mucous membrane, of a soft, spongy consistence, and pale red colour. It consists of a single layer of ciliated epithelial cells, resting on a very delicate basement membrane, beneath which is the connective-tissue layer of the mucous membrane. This latter is about 1'5 mm. in thickness, and consists of connective tissue, with very numerous connective-tissue cells, and traversed by irregularly arranged muscle fibres. It is attached by its outer surface to the muscular Avail of the uterus.


The epithelium lining the uterus is pitted to form the uterine glands. These (Fig. 175) are tubular glands, embedded in large numbers in the connective-tissue layer of the mucous membrane, vertically to the inner surface of the uterus ; they are straight, or slightly convoluted ; their blind or outer ends are usually slightly dilated ; and they secrete a transparent, glutinous, alkaline fluid.

Changes in the mucous membrane accompanying menstruation. These changes commence with congestion and tumefaction of the mucous membrane lining the entire uterus. This swells up considerably, becoming softer and more vascular than before, and forming ridge-like folds which project into the cavity of the uterus. The connective-tissue cells increase considerably in number, and the uterine glands become longer, wider, and more convoluted. The whole layer of mucous membrane increases in thickness from 1*5 mm. to from 3 to 5 mm. ; while the glands increase in diameter from 0'08 to 0'12 mm. This swollen and hypertrophied mucous membrane forms what is called the menstrual decidua.

At the menstrual period, the superficial layer of the mucous membrane, about a fourth of the entire thickness, breaks down and is thrown off, usually in detached fragments, but sometimes, in cases of dysnienorrhcea membranacea, as a single piece, forming a complete cast of the interior of the uterus. Fatty degeneration has been noticed in these cast-off cells, but only in the later stages, after the menstrual discharge has actually commenced.

This disintegration, and casting off, involves the loss of the epithelial lining of the uterine cavity, of the mouths of the uterine glands, and also of about one-fourth of the entire thickness of the swollen mucous membrane. It of necessity causes rupture of the blood-vessels of the detached portions, and so occasions more or less free hemorrhage ; and the blood so discharged, together with the broken-down mucous membrane of the uterus, and with a certain amount of mucus from the uterine glands, forms the menstrual or catamenial flow.

The menstrual flow lasts, as a rule, from three to five days, but may be protracted for a week or more. It is accompanied by nervous and other disturbances, which are fully described in works on obstetrics.

At the commencement of a period, the menstrual discharge is viscous, consisting largely of mucus from the uterine glands, slightly tinged with blood ; in the. middle of the period the flow becomes almost pure blood ; while towards the end it becomes paler, the mucus again preponderating. Owing to mixture with the uterine mucus, the blood of the menstrual flow does not coagulate. The total amount of the menstrual discharge is usually from four to six ounces ; but this may be widely departed from in individual cases, either in the way of diminution or of excess.

On the cessation of the menstrual flow, the uterine epithelium is very quickly regenerated, spreading over the surface from the necks of the uterine glands. It is completely reformed within three or four days of the end of the menstrual period. After this re-establishment of the uterine mucous membrane, the uterus remains in a quiescent condition for from ten days to a fortnight ; at the end of this time it begins to swell again, and the menstrual process is repeated. This repetition occurs, as already noticed, at intervals, usually of four weeks, throughout the whole child-bearing period ; the only normal disturbing element being gestation, during which menstruation is in abeyance, recommencing a short time after the birth of the child.

5. Explanation of the Menstrual Process

The complete menstrual cycle, occupying in typical cases twenty-eight days, may be divided into four stages, which follow one another in regular sequence.

(i) The first or constructive stage is characterised by swelling of the mucous membrane, enlargement of the uterine glands, and increase in the connective-tissue cells of the mucous membrane ; it results in the formation of a menstrual decidua, lining the entire uterus.

(ii) The second or destructive stage includes what is ordinarily known as the menstrual or catameiiial period. It is marked by abundant discharge of mucus from the enlarged glands, and by the disintegration and discharge from the uterus of the inner layer of the mucous membrane. It involves loss of the epithelial lining of the uterus and of the necks of the glands, and is accompanied by hasmorrhage.

(iii) The stage of repair comes next, during which the uterus is recovering from the destructive changes. The uterine epithelium is restored, by growth from the lips of the deeper parts of the uterine glands ; and the swelling of the mucous membrane subsides.

(iv) The fourth stage is the period of quiescence, during which the uterus, having regained its normal structure, remains without further change until the commencement of the next succeeding constructive stage.

The actual and relative durations of the several stages enumerated above are not determined with certainty, and are subject to individual variations. It will, perhaps, be right to assign about a week to the constructive stage ; rather less than a week (five days on an average) to the destructive stage ; three or four days to the stage of repair ; and twelve or fourteen days to the quiescent period ; the four stages together occupying the twenty-eight days which make up the normal menstrual cycle.

Of the above four stages, the first and second require further attention ; the fourth stage is the normal condition ; and the third stage is merely the return of the uterus to the normal condition after a period of disturbance.

Concerning the first or constructive period, there is hardly any room for doubt that it is to be regarded as a preparation on the part of the uterus for the reception of an ovum.

The several stages of the process correspond closely, in essential respects, with those that occur in the placental lobes ot the rabbit's uterus from about the fourth to the eighth day. In the rabbit, as in the human uterus, there occur swelling of the mucous and submucous tissues, increased vascularity, a large increase in the number of the connective-tissue cells, and a great enlargement of the uterine glands, which become larger, wider, and more freely branched. These changes, in the rabbit's uterus, are clearly related to the nutrition of the embryo, for it is to this hypertrophied and modified area of the uterine mucous membrane that the embryo becomes attached on the eighth day ; and it is from this area that the maternal part of the placenta is formed.

The most important difference between the rabbit's and the human uterus, as regards these stages, is that in the rabbit the ovum, or rather the blastoderrnic vesicle, is present within the uterus during the whole of the series of changes, although it lies quite freely and does not acquire attachment until the eighth day ; while in the human uterus, on the other hand, the menstrual constructive process goes on without the stimulus afforded by the presence of an ovum.

As regards the actual changes in the uterus itself, the resemblance between the two cases is so great that it seems necessary to suppose that their significance is the same ; and it must, therefore, be concluded that the human uterus periodically prepares itself, by the formation of a decidual lining, for the reception of an ovum ; the process occurring at monthly intervals throughout the child-bearing period, and quite irrespectively of the presence or arrival of a fertilised ovum.

The second or destructive stage, constituting the act of menstruation in the ordinary sense of the term, is much more difficult to explain. At first sight it appears to consist simply in a rapid, and somewhat violent, undoing of the work accomplished in the preceding stage.

If, however, it is compared with the changes that take place in the rabbit's uterus during gestation, it is found that the human uterus at the end of the constructive period of menstruation has reached a stage corresponding to that of a rabbit's uterus at the end of the seventh or beginning of the eighth day of pregnancy, when the blastodermic vesicle is still lying freely within the uterus, but is just about to acquire its attachment.

In the rabbit this attachment is effected, early on the eighth day, by fusion of the wall of the blastodermic vesicle with the epithelium of the modified and hypertrophied placental lobes of the uterus (Fig. 169). This fusion is immediately followed, or rather is accompanied, by degenerative changes in the uterine mucous membrane opposite the area of attachment, which rapidly lead to absorption of the uterine epithelium, and of the mouths and necks of the uterine glands.

Similar changes occur during the formation of the human placenta, and will be described in the concluding section of this chapter ; and inasmuch as the portion of the wall of the uterus which is concerned in the changes is the same in menstruation and in pregnancy, the menstrual discharge may be viewed, not merely as a destructive process, but as corresponding in a modified form to the rapid absorption of the same parts which occurs normally during pregnancy.

The constructive stage of menstruation, and, as just seen, the destructive stage as well, may be regarded as phases in the preparation of the uterus for the formation of a placenta ; stages which can be carried up to a certain point without needing the stimulus of the presence of an ovum or embryo, but which, having reached a point at which further development is impossible without an embryo, stop abruptly. The constructive stage has been shown to be an active preparation of the uterus for the reception of a fertilised ovum ; the succeeding or destructive stage is not to be regarded as a simple undoing of this preparation, but as a further continuance, in a modified form, of the act of preparation, which leaves the uterus in a condition in which, for further elaboration to occur, the presence of an embryo is indispensable.

6. The Connection between Ovulation and Menstruation

Ovulation and menstruation, or the discharge of ova from the ovary, and of the disintegrated decidua from the uterus, are processes which occur periodically, and as a rule simultaneously ; and it becomes a matter of interest to inquire into the nature of the connection between them.

The ovaries swell up, and become tender, at monthly intervals. The enlargement commences, as a rule, a few days before the menstrual period, attains its maximum about the time of the period, and gradually subsides after the period is over.

As the ovary is known to become congested just before the rupture of a Graafian follicle and the discharge of an ovum, it appears a fair inference that this discharge occurs about the same time as the menstrual flow, i.e. that ovulation and menstruation are practically simultaneous. However, although this may be, and probably is, the rule, yet it is far from being an invariable one. Thus Kolliker, on examining the ovaries of seven women who had died directly after menstruation, found that in two of the cases there was no fresh corpus luteum in either ovary ; that is, that no ovum had been discharged at the time of menstruation ; and Coste has cited similar instances.

Ovulation and menstruation may be assumed to occur as a rule about the same time, but it is by no means clear what is the precise nature of the connection between the two processes. Authorities differ as to the stage in the menstrual period at which ovulation occurs, the majority holding that it takes place two or three days before the commencement of the period, while others maintain that it happens at the middle, or even towards the end, of the period. It is very possible that there is no constancy in this particular respect.

A still more difficult point remains to be considered. The menstrual decidua is to be viewed as a preparation on the part of the uterus for the reception of an ovum ; but it has still to be determined whether the decidua which is broken up and discharged at a given menstrual period is the one prepared for the ovum which is set free from the ovary at the same period, or for an ovum liberated at some previous or subsequent period. The question is one of great importance, as the means of determining the age of human embryos are very materially affected by the answer given to it.

The menstrual cycle has been seen to consist essentially in a periodically recurring preparation of the uterus for the reception of an ovum. It is important to determine, if possible, at what particular phase of the cycle, the uterus is in the condition most favourable for the reception of an ovum. Very different views have been expressed on this point, and two of these call for special notice.

(i) That the end of the constructive period is the natural and most favourable moment for the ovum to enter the uterus.

(ii) That the period of quiescence is the most favourable time.

In support of the former view, it is urged that the formation of the decidua is unintelligible except on the supposition that it is a preparation for the reception of the ovum ; and that the analogy of the rabbit's uterus, in which the sequence of changes is strikingly similar, is in- favour of the end of the constructive period, or perhaps the commencement of the destructive period, being the one specially concerned with the fixation of the ovum to the wall of the uterus.

It must be noticed, however, that if the normal time of attachment to the uterus is, in the human ovum, the end of the constructive period, i.e. the commencement of the menstrual period, then it is clear that the ovum which is to be attached cannot be the one discharged from the ovary at the same period. For the discharge of the ovum is practically coincident with the onset of the menstrual period ; and the ovum, after leaving the ovary, has still, in order to reach the uterus, to travel along the entire length of the Fallopian tube, a passage which is known to take three days in the rabbit, and eight to ten days in the dog, and which in all probability takes at least a week in the human species. It follows that the decidua which is discharged at a given menstrual period cannot have been prepared for the ovum discharged at the same period, but must be the preparation for the ovum which was discharged at the preceding menstrual period.

The second view, that the period of quiescence in the menstrual cycle is the most favourable time for the entrance of the ovum into the uterus, leads to the same conclusion, inasmuch as the only ovum which could reach the uterus during the quiescent stage is the one discharged at the previous menstrual period.

In favour of this second view, that the quiescent period in the menstrual cycle is the most favourable time for the ovum to enter the uterus, the following considerations may be urged.

(a) A much greater range of time is given, within which the uterus is ready for the reception of an ovum. The quiescent period is the longest of the four stages which compose the menstrual cycle, lasting from twelve to fourteen days ; while, on the view that the completion of the constructive process marks the time at which the uterus is best fitted to receive an ovum, the range of time is limited to two or three days at most ; and the longer period is more in accordance with what is known of the range of time within which conception may occur.

(fe) The stages in the formation of the menstrual decidua have been compared, above, with the changes which occur in the uterus of a rabbit, from the fourth to the seventh or eighth day of pregnancy ; and the close similarity between the two cases has been insisted on. It should now be noticed that these changes in the rabbit occur after the entrance of the ovum into the uterus ; i.e. that in the rabbit the ovum enters the uterus while this latter is in the quiescent stage.

Neither of the above arguments is at all conclusive, and the question is still an open one. It must be repeated, however, that if either of these views is correct, the same conclusion follows with regard to the relation between ovulation and menstruation, viz. that the decidua of a particular menstrual period is related, not to the ovum discharged at that period, but to the ovum discharged at the preceding period.

It follows that there is no necessary connection between ovulation and the occurrence of the menstrual flow ; a point which helps to explain the cases quoted by Kolliker, Coste, and others, in which there was no discharge of ova at the time of menstruation.

The fact that the two processes, ovulation and menstruation, occur normally at or about the same time, may perhaps be explained by the consideration that at the time of ovulation there is very considerable congestion of the ovaries and Fallopian tubes ; and this, owing to the free communication between the ovarian and uterine arteries, must almost necessarily cause congestion of the uterus ; and this determination of blood to the large and thin-walled vessels of the decidua is probably an important factor in causing the menstrual haemorrhage.

7. The Duration of Pregnancy

Much has been written on this point, and many elaborate tables have been compiled from which it appears :

(i) That there is no absolutely fixed period of gestation.

(ii) That there is no means of determining with certainty the commencement of gestation, as the precise time of fertilisation of the ovum cannot be ascertained.

It is customary to calculate the duration of pregnancy from the last occurring menstrual period ; and this, if the argument given above is correct, will correspond with the discharge, from the ovary, of the ovum from which the child is developed. The most reliable estimates indicate a normal duration of pregnancy, dating from the last occurring menstrual period, of 270 to 280 days. This is, however, estimated by some authorities from the first day of the period; by others, and more usually, from the last day.

It is possible that the actual limits, in normal pregnancy, are not so wide as indicated above. Apart from the difficulty of determining the date of fertilisation, the chief causes of uncertainty arise from our ignorance of the length of time during which the ova and spermatozoa retain their vital activity, after leaving the ovary and testis respectively.


Concerning the spermatozoa, we have very little precise knowledge. It is known that spermatozoa, introduced into the vagina, may retain their vitality, and presumably their fertilising power as well, for a week ; and the fact that successful impregnation may occur at any time in the menstrual cycle, strengthened by the analogy of other animals, suggests that human spermatozoa may retain their power for considerably longer periods. It is stated that ripe spermatozoa may remain for months in the testis before being discharged, without losing their fertilising power.

The time taken by the spermatozoa to travel along the vagina, uterus, and Fallopian tube to the ovary, is not known, but is probably very short ; in the rabbit it does not occupy more than a quarter of an hour to two hours.

If the ovum is not fertilised it soon dies. How long an ovum may retain its vitality, and capacity for fertilisation, is not known; indeed, no unfertilised human ovum has yet been seen, outside the ovary. Some experiments of Bischoff, on lower Mammals, point to the conclusion that, in these, the ovum, if not fertilised, dies in the lower part of the Fallopian tube, before reaching the uterus. Assuming that the human ovum also dies shortly before reaching the uterus ; and assuming further, as is done by most authorities, that the human ovum takes at least eight days to travel down the Fallopian tube, it may be stated that the human ovum probably retains its vitality, and power of being fertilised, for some time, perhaps a week, after discharge from the ovary ; but ultimately loses it, probably before reaching the uterus. This is, however, at present little more than speculation.

If the above considerations prove well founded, and if, as suggested above, the length of time during which an ovum remains alive and fertilisable, after leaving the ovary, is less than the interval between two successive periods of ovulation, it will follow that there must be certain times during which there are ova ready to be fertilised, and certain times during which there are none ; i.e. that fertilisation can only be effected at certain recurring periods, and cannot occur in the intervals between these periods.

Concerning the respective lengths of these periods we have no certain knowledge, but it is commonly held that the intervals during which there are no ova capable of being fertilised are at least as long as the periods in which there are such ova. In other words, assuming that the ova discharged at a given menstrual period retain their vitality for from ten to fourteen days a pure assumption there would be an interval of about two weeks before the next menstrual period, i.e. before the next discharge of ova, and during this interval there would be no fertilisable ova in the oviduct, and fertilisation could not take place. Any spermatozoa received during this interval would have to wait until its close, -at the next period of ovulation, before they bad a chance of meeting with ova capable of being fertilised.

There seems to be a general consensus of opinion that the first day or days after the cessation of the menstrual period are the most favourable time for fertilisation to take place. This is in complete accordance with what has been said above, both with regard to the ovum and the decidua, for the ovum will be lying within the Fallopian tube in a healthy fertilisable condition and easily accessible to the spermatozoa ; while if the ovum takes another week or so to travel down the tube to the uterus it will enter this latter while it is in the quiescent state, which, it has been shown above, there is reason for regarding as the most favourable one for the reception of the ovum.

8. Estimation of the Age of Human Embryos

It follows from what has been said above, that there is no means of determining with certainty the age of a human embryo prematurely discharged from the uterus; for development dates, not from the discharge of the ovum from the ovary, but from the moment of fertilisation ; and this latter cannot be determined.

Ovulation is a process easily overlooked, but the fact that it occurs simultaneously with the menstrual periods renders its date readily determinable, but within certain limits only. The connection between the two processes is a loose one, and it is probable that ovulation may occur either from two to three days before a menstrual period, or during the period ; giving a possible error of about a week in estimating the age of an embryo from the date of menstruation.


Professor His, in the first part of his monograph on the development of the human embryo, laid down the following rule :

' The age of an embryo is the time that has elapsed since the first day of the first omitted period.'

Thus, supposing the commencement of a menstrual period to be due on January 5, and that when this time comes, the period is omitted ; but that at some subsequent time, say February 9, an embryo is aborted ; then, according to Professor His' rule, the age of the embryo would be the interval between January 5 and February 9, i.e. thirty-five days.

In arriving at this result, Professor His argues in the following manner : The ovum leaves the ovary either at, or shortly before, the menstrual period;- if it is fertilised, presumably by spermatozoa previously introduced, menstruation does not occur ; but the changes in the uterine mucous membrane, instead of, as usual, becoming retrogressive, either remain stationary or else continue to be progressive ; and so prepare the uterus for the reception of the ovum. Hence the first omitted menstrual period corresponds in point of time with the fertilisation of the ovum ; and hence the age of the embryo may be taken as the time that has elapsed since the first omitted period.

This method of calculation is, however, open to very grave objections, the more important of which are as follows :

(i) There are strong reasons, which have been fully considered in the previous portion of this chapter, for regarding the decidua which is broken up and discharged at a menstrual period to be related, not to the ovum discharged from the ovary at the same period, but to the ovum discharged at the preceding period.

(ii) Professor His' rule assumes that the ovum is invariably fertilised on the first day of the first omitted period. There is no direct evidence in support of this ; and the loose nature of the connection between ovulation and menstruation renders it highly improbable.

(iii) The rule assumes that the act of fertilisation of an ovum, which in all probability will not reach the uterus for at least a week, is able to arrest the degenerative changes already commenced in the decidua, to suddenly stop the menstrual flow that is on the verge of taking place, or has actually commenced, and to convert the retrogressive changes of the uterus into progressive ones.

(iv) The rule is not in accord with the well-established fact that, in order to insure pregnancy, the most favourable time for intercourse is shortly, or immediately, after the conclusion of a menstrual period. This is intelligible enough if the ovum to be fertilised is the one discharged at that period ; but is hard to understand if, as the rule requires, these spermatozoa have to wait for a period of three weeks or more, until the next discharge of ova.

These objections are serious ones, and Professor His, in the second part of his work, recognises that the rule as originally formulated cannot apply to all cases. He quotes instances in which the dates were accurately recorded, and in which the fertilised ovum must have belonged to the last occurring period, and not to the first omitted one ; he is of opinion, however, that the rule as stated above will still apply to the majority of cases.

This more recent view may be expressed graphically, thus. If I. is the first day of the last actually occurring menstrual period, and II. is the first day of the first omitted period ; then the possible days of fertilisation are as follows :

L, 2, 3, 4, 5, 6, 7 26, 27, 28, II.

That is, an ovum discharged during an actually occurring period remains capable of fertilisation for a certain number of days, expressed in the formula as a week, commencing with I., and ending at 7. During this time it may be fertilised, either by spermatozoa received after the period is over, or received before the period and retained in the oviduct during it. In the case of these embryos the age should be calculated from I., the first day of the last actually occurring period.

On the other hand, Professor His, and others, maintain that there are possibilities of fertilisation at the other end of the series ; and that an ovum, discharged from the ovary a day or two before the next period, II., is due, may, if fertilised, stop that period from occurring; and in such cases, if they really happen, the age of the embryo should be calculated from the first omitted period, and not from the last occurring one. It is not yet certain which of these two possibilities is the normal mode of occurrence, but such evidence as we have is in favour of the former.

It is customary, however, to adopt His' original rule, and to estimate the age of human embryos from the first day of the first omitted period, and this method will be followed in this chapter. It must be repeated, however, that this is done merely from convenience, and from the absence of any other precisely formulated system. Viewed on its own merits, His' rule will certainly not apply generally.

The General History of Development of the Human Embryo

In this section the earliest stages in the formation of the human embryo will be described, so far as they are at present known ; and an account will be given of the external characters of the embryo at the several stages up to the time of birth.

These descriptions are in the great majority of cases taken from Professor His' monograph, and, as explained in the preceding section, the ages given are those assigned by him to the several stages. In the following sections the development of the nervous, digestive, and other systems will be considered in detail ; and in the concluding section the placenta, the foetal membranes, and the relations of the embryo to the uterus will be described.

The actual length of an embryo is not always easy to determine, owing to the varying amount of flexnire of the head and body at different stages. By the length of an embryo, in the following descriptions, is always meant the longest straight line that can be drawn through it in the sagittal plane. In the earliest stages of development this coincides fairly well with the longitudinal axis of the embryo (Figs. 176 to 179); from the beginning of the fourth week to the end of the fifth week (Figs. 200, 203, and 205) it is a line drawn from the prominent hump at the junction of the head and body, to the pelvic region ; and from the end of the fifth week onwards, as the head is gradually lifted up by straightening of the neck (Figs. 211, 212), the line once more approximates to the longitudinal axis of the fcetus.

"With regard to the general course of development, the first fortnight is occupied in preliminary processes, no trace of the embryo appearing until the twelfth or thirteenth day. From the end of the second to the end of the fourth week, the embryo is acquiring definite form, and the various organs and systems are being established. From the fourth to the sixth or seventh week there is a gradual change from the embryonic to the foetal form ; the head becoming uplifted, the nose, ears, and lips established, the limbs divided by joints, and the fingers and toes formed. By the end of the second month, the general form is as shown in Fig. 212, and from this time onwards the further changes consist chiefly in increase of size, and in proportionately greater development of the limbs.

The changes that occur in the shape and size of the embryo up to the end of the second month are well shown in the series of outlines given in Figs. 176 to 178, 189 to 195, 199 to 203, 205, 211, and 212. These figures, which are borrowed from Professor His, are in each case five times the linear dimensions of the embryos themselves.

1. The First Week

The fertilisation of the human ovum has not been studied. A single observation, by Nagel, of a ripe ovarian ovum, removed by operation and examined in a fresh condition, showed that two polar bodies were present, lying on the surface of the ovum within the zona pellucida.

There is no reason for supposing that fertilisation is effected in other than the normal manner; and it is probable that it takes place at or about the time the ovum leaves the ovary and enters the oviduct.

The segmentation of the human ovum has not been seen. It is highly probable, from analogy of other Mammals, that it occurs during the passage of the ovum along the Fallopian tube towards the uterus.

The ovum of the dog, which is slightly smaller than the human ovum, travels quickly along the first part of the oviduct but stays some days in the distal or lower part, where it undergoes segmentation, entering the uterus eight or ten days after leaving the ovary. Bischoff and others believe, though there is no direct evidence on the point, that the human ovum agrees in this respect fairly closely with that of the dog ; undergoing segmentation in the lower part of the oviduct, and not entering the uterus until from eight to ten days, or perhaps longer, after the time of discharge from the ovary.

2. The Second Week

Of ova or embryos which are believed to belong to the end of the second week a few examples have been described. These are of great interest, although there is room for doubt in some of these cases whether the specimens can be regarded as perfectly normal.

Reichert's ovum. The best known instance is an ovum described by Reichert, and believed to be of about the twelfth or thirteenth day. This ovum, which is represented four times the natural size in Figs. 172 and 173, was found, in situ, in the uterus of a woman who had committed suicide, and gave every indication of being perfectly normal.


FIGS. 172 and 173. Front and side views of Reichert's Ovum. (From Kolliker, after Reichert.) x. 4.


The ovum was a vesicular body, lenticular in shape, and measuring 5 - 5 mm. across its greater diameter, and 3'3 mm. from side to side. Of the two surfaces, the one turned towards the wall of the uterus, the upper one in Fig. 173, was more convex than the opposite surface, which faced towards the cavity of the uterus. The margin of the vesicle was thickly fringed with villi, the largest of which were 0'2 mm. long, and slightly branched ; the middle portions of both surfaces were smooth, and devoid of villi ; and in the centre of the more convex or uterine surface was a small circular spot (Fig. 172), 1-6 mm. in diameter, and of a darker colour than the rest of the vesicle.

The relations of the ovum to the uterus were as follows. The entire uterus was lined by a decidua, described as not differing in any special manner from an ordinary menstrual decidua, and forming the usual ridge-like projections into the cavity of the uterus. To one of these ridges, on the dorsal surface of the fundus of the uterus, the ovum was attached, the decidua spreading over it as a thin layer so as to completely encapsule it (cf. Fig. 175). The marginal villi were described and figured by Reichert as penetrating a little distance into the enlarged uterine glands.

In the ovum itself there was no indication of primitive or neural grooves, nor of any other part of the embryo.


IlG - . 174> Diagram i> -i ^ i matic section of

Ihe wall of the vesicle was described by Reichert's Ovum.

Reichert as consisting of a single layer (From His.) x o.

, i -iiTi n i n a, the embryonal area.

of flattened epithelial cells, prolonged

outwards to form the hollow villi. In the circular patch on the uterine surface, spoken of as the germinal or embryonal area, a second or inner layer of finely granular nucleated cells was present. The cavity of the vesicle was occupied by a gelatinous fluid, traversed by a network of fibres, and containing within it a rounded body attached to the germinal area.

Lining the whole vesicle was a second, fairly coherent membrane, with which the fibres were continuous. By Reichert this second membrane, the network of fibres, and the central rounded body were all alike considered to be artificial products, due to coagulation of the fluid contents of the vesicle by the alcohol in which the specimen was preserved.

Ova of similar appearance, and of apparently about the same age, have been described by Wharton Jones, Breuss, Kollmann, and others ; and in none of these cases was any trace of an embryo present.

The position held by the ovum in relation to the uterus, in the case recorded by Kollmann, is shown in Fig. 175. The whole uterus was lined by a decidual membrane, DV ; this was greatly thickened about the middle of the ventral wall, forming the decidua serotina, DW, to which the ovum, CV, was attached ; the decidua extending over the ovum so as to completely encapsule it. The ovum itself was in the form of a hollow, thinwalled vesicle, with short branched villi projecting from its surface, apparently on all sides. The villi were stated not to penetrate into the uterine glands. The minute structure of the ovum, or blastodermic vesicle as it may more properly be called, was not ascertained.

The chief additional points learnt from these further



Fig. 175. A longitudinal section of the Uterus with an Ovum in situ, estimated as about the thirteenth day. (After Kollmann.) x 1.

CV, cavity of ovum or blastodermic vesicle. CW, wall of blastodermic vesicle. CZ, villi projecting from wall of blastodermic vesicle. DV, decidua vera. D~W, decidua serotina. 33X, decidua reflexa. G"V, vagina. TJA, cavity of uterus. TTB, dorsal wall of uterus. TJC, ventral wall of uterus. TTO, os uteri. TJK, uterine glands. TJX, cervix uteri.

specimens are : (i) that the rounded body described by Eeichert as lying within the vesicle is made up of nucleated cells, and is apparently solid, and attached to the embryonal or germinal area ; (ii) that there is strong reason for thinking that the wall of the vesicle really consists, not of a single layer of cells, but of two layers, of which the inner one, regarded by Reichert as a coagulation product, is of the nature of connective tissue, and therefore of mesoblastic origin. This latter point is. however, a doubtful one.

In the present state of our knowledge it is hardly possible to make any satisfactory comparison between these early human ova, and the stages already described as occurring in the rabbit. The difficulty is much increased by the absence of detailed histological description, and by the doubt as to whether the ova are in all respects normal. It must also be borne in mind that we are absolutely ignorant of the mode in which segmentation of the human ovum, and the immediately succeeding stages are effected ; and that great uncertainty still exists with regard to the details of these processes in the rabbit.

As, however, the several human ova of the stage in question agree in a number of important points, and as, in the case of Reichert's ovum, there is every reason for regarding the specimen as normal, it is advisable to make such comparison as is possible between these ova and the several stages of development of such a Mammal as the rabbit.

In the first place, the complete absence of any trace of an embryo indicates that the stage is a very early one. In ova which there are strong reasons for regarding as but one or two days older, an embryo is present ; so that the stage represented by Reichert's ovum may be described as one shortly before the first appearance of the embryo, and as corresponding in this respect with the blastodermic vesicle of a rabbit at about the fifth or sixth day.

In its vesicular character, the thinness of its walls, and the presence of a central embryonal area of different constitution to the rest of the wall, there are additional points of resemblance between Reichert's ovum and" the blastodermic vesicle of a rabbit of the sixth day, or of a dog in the early part of the third week. There is also a close correspondence in actual size between these stages in the three instances.

Reichert was of opinion that this comparison was a true one ; and the view is supported by His, who gives in illustration of it the diagrammatic section (Fig. 174). His considers that the outer wall of the vesicle consists of epiblast only, and that the hypoblast forms the inner circular patch of cells in the embryonal area ; he also regards the central rounded mass of cells as hypoblastic, and as destined to become hollowed out at a later stage to form the yolk-sac.

Concerning this comparison, it must be borne in mind that we have as yet no satisfactory knowledge of the histological structure of these early human ova, and that the stage is one about which much doubt exists even in the case of the rabbit. It must further be noticed that there are some points of importance which tell directly against the interpretation suggested by Reichert and His.

In the first place, there is nothing in the blastodermic vesicle of a rabbit on the sixth day that can be compared with the central mass of cells in Reichert's ovum.

Secondly, if His is right in interpreting this central mass of cells as the yolk-sac, then the yolk-sac of the human embryo is developed in a manner entirely different from that of the rabbit. In the rabbit (Fig. 146), the yolk-sac is part of the blastodermic vesicle itself, while in the human embryo it appears to be, from the first, independent of this. In other words, if the central mass of cells in Reichert's ovum is the yolk-sac, and the later stages strongly support this intei'pretation, then the wall of the vesicle of Reichert's ovum cannot correspond to the wall of the rabbit's blastodermic vesicle (cf. Figs. 146 and 188).

Thirdly, there is strong reason for thinking, as already noticed, that the wall of Reichert's ovum is double ; an inner mesoblastic lining being already present, as well as the outer epithelial layer. To this inner layer, if it really exists, there is nothing corresponding in the rabbit's blastodermic vesicle until a later stage.

On the whole, then, the evidence, while not excluding a general correspondence in grade of development between Reichert's ovum and a rabbit's blastodermic vesicle of about the sixth day, appears to be against a close or exact agreement between the two. There are features in Reichert's ovum which do not fit in with the processes of development as known in the rabbit, or in other Mammals ; peculiarities which will probably not be understood until opportunity has occurred for study of the segmentation of the human ovum, and of the stages immediately following it. Light will perhaps be thrown on the question by investigations on Mammals more nearly allied to man than are rabbits or dogs.


Embryos of the Thirteenth and Fourteenth Days.

His' embryo, E. One of the youngest human ova containing a distinct embryo was obtained by Professor His in 1869, and was carefully described by him under the distinguishing letter E. This embryo, which is at present deposited in the Anatomical Museum at Basle, is estimated to be about thirteen days old : it is represented from the right side in Fig. 176 ; and in diagrammatic sagittal section in Fig. 188.

The entire vesicle (Fig. 188) is a thin-walled sac, measuring 8'5 mm. by 5'5 mm. and covered all over with branched villi. The contained embryo (Fig. 176) is 2*1 mm. long, and is attached at its hinder end, by a short thick stalk, to the inner surface of the vesicle. A slight constriction separates the




FIG. 176. FIG. 1 77. FIG. 178.

FIGS. 176, 177, 178. Outline figures, from the right side, of three Human Embryos, estimated to be of the thirteenth or fourteenth days. (From His.) x 5.

FIG. 176. Embryo lettered by Professor His, E (of. Fig. 188).

FIG. 177. Embryo described by Allen Thomson.

FIG. 178. Embryo lettered by Professor His, SR (cf. Fig. 179).

embryo ventrally from the yolk-sac, which measures 2'3 by 1'6 mm. Covering the embryo, but at a short distance from it, is a membranous fold, which is clearly the inner or true amnion. The embryo itself presents along its dorsal surface a shallow neural groove, bounded by prominent neural folds ; and the only other organs visible on the surface are a pair of longitudinal folds, formed by the two halves of the heart, and lying between the anterior end of the embryo and the yolk-sac. From the heart, vessels can be traced, running over the surface of the yolk-sac.

His' embryo, SR. This is a well-preserved embryo of the thirteenth day, slightly older than the embryo E, but very similar to it in all important respects.

The entire vesicle measures 8 to 9 mm. in diameter, and


is covered over its whole surface by branched villi, as in the case of the embryo E (cf. Fig. 1 88). The embryo (Figs. 1 78 and 1 79) is 2'2 mm. long : it is attached to the inner surface of the vesicle by a short thick stalk, TZ, and is separated from the yolksac, YS, by a slight constriction.

In the embryo itself, the head end, HD, is more markedly raised above the yolk-sac than in the embryo E ; and the neural groove, NG, is widely open along its whole length. The dorsal


FIG. 179. Human Embryo lettered by Professor His, SR, and estimated as of the thirteenth day. The wall of the blastodermic vesicle has been removed, except the part to which the allantoic stalk is attached. (After His.) x 25.

AN, inner or true amnion. HD, head end of the embryo. R, heart. Tf Gr, neural groove. TL, tail. TZ, allantoic stalk, connecting the embryo with the wall of the blastodermic vesicle. VN, villi. YS, yolk-sac.

surface of the embryo is somewhat sinuous in outline, presenting alternate convexities and concavities : the most anterior and largest swelling is formed by the head ; then comes a concavity opposite the middle of the length of the yolk-sac ; and then another marked convexity further back. The hinder end of the embryo projects freely as a short blunt tail, TL, beyond the stalk of attachment to the wall of the vesicle, TZ, which now arises from the ventral surface of the hinder end of the embryo. The two halves of the heart, R, form prominent swellings between the head and the yolk-sac. There is as yet no trace of either visceral arches or clefts ; and the dorsal surface of the embryo is enveloped in the thin membranous amnion, AN, which now lies rather closer to it than in the case of the embryo E.

Human embryos of about the same age as the embryos E and SR have been described by Allen Thomson (Fig. 177), Keibel, V. Spee (Figs. 180 to 184), Kollmann (Fig. 185), and others. These all agree in essential respects, and leave 110 doubt that the stage must be regarded as a perfectly normal one.

V. Spee's embryo, which was studied by means of sections, is of considerable importance, as it has shown the internal struc


FIG. 180.


FIG. 180. A Human Embryo of about the thirteenth day, from the left side : the wall of the blastodermic vesicle has been in chief part removed. (After V. Spee.) x 8.

FIG. 181. The same embryo from the dorsal surface. (After V. Spee.) x 14.

AN", inner or true amnion. HD, head end of embryo. NQ-, neural groove. NT , neurenteric canal. PS, primitive streak. VM", villi of choriou. YS, yolk-sac.

ture, and the relations of the germinal layers, at the stage in question.

This embryo is represented from the left side in Fig. 180, and from the dorsal surface in Fig. 181. It is, if anything, slightly younger than the embryo E, and the constriction separating the embryo from the yolk-sac has hardly commenced to form. The head of the embryo (Fig. 181, HD) is wide and flat, and the neural groove is shallow. At the hinder end, the two neural folds diverge from each other, and embrace between them the anterior end of a well-marked primitive streak (Fig. 181, PS) ; and just in front of the primitive streak is a small but well defined neurenteric passage, NT, leading from the surface into the cavity of the yolk-sac.

The sections (Figs. 182-84) will render the relations of the parts clearer. Fig. 182, which passes across the anterior part of the head, shows the widely open neural groove, NG ; the fore-gut,



FIGS. 182-184. Sections across the Human Embryo of the thirteenth day,

represented in Figs. 180 and 181. (After v. Spec.) x 45. FIG. 182. Transverse section across the head end of the embryo. FIG. 183. Transverse section across the middle of the body. FIG. 184. Transverse section across the hinder end of the embryo and the yolk-sac, the section passing through the neurenteric canal.

AN, inner or true amnion. CH, commencing notocliord. E, epiblast of the embryo. OF, fore-gut. GT, mid-gut. IT, hypoblast. M, mesoblast. ME, somatopleuric layer of mesoblast. MH, splanchnoplemic layer of mesoblast. M"Q-, neural groove. NT, neurenteric canal. YS, cavity of yolk-sac.

GF, lined by hypoblast, and just shut off from the yolk-sac, YS ; and the mesoblast, with its contained blood-vessels. Fig. 183 passes through the body region, and shows the commencing notochord, CH, as a thickened plate of hypoblast cells in the mid-dorsal wall of the gut.

Fig. 184 passes through the neurenteric canal, NT, which places the mid-gut and yolk-sac in direct communication with the exterior. It will be noticed that the wall of the yolk-sac consists of an inner lining of hypoblast, H, and an outer wall of mesoblast, MH, in which are very numerous blood-vessels. On the right side the mesoblast was torn in the section, and is indicated by a dotted line in the figure.

The amnion, AN, in all three figures is seen to consist of both epiblastic and mesoblastic layers.

Kollmann's embryo (Fig. 185) is rather older than the others described in this section, and may be estimated as of the fourteenth day. It affords an important transitional stage


FIG. 185. Human Embryo of about the fourteenth day, from the right side. The yolk-sac and the wall of the blastodermic vesicle have been removed. (After Kollmann.) x 27.

AN", outer or true amnion. BF, fore-brain. DS, stomatodEeum. MT, mesoblastio somite or protovertebra. NGf, neural groove. NO-', point behind which the neural groove is closed to form the neural tube. H, heart. TL, tail. TZ, allantoic stalk. ~W, vitelline vein. YS, yolk-stalk, cut short.

between the embryos E and SR on the one hand, and on the other the embryos of the third week, which will be described in the next section.

Kollmann's embryo (Fig. 185) measures 2'5 mm. in length. As compared with the earlier embryos, the head is larger and more prominent ; and the embryo is much more distinctly constricted off from the yolk-sac. The neural folds have met and fused, to complete the neural canal, in the hinder part of the body, but the neural groove is still widely open in the head and anterior part of the body. The brain vesicles are becoming evident, and the flexure of the anterior end of the head is already commencing. A distinct stomatodapal depression, DS, is present on the under surface of the head. The two halves of the heart, R, have united ; and the heart, now a single tube, is already twisted in a characteristic S shape. Fourteen or fifteen pairs of mesoblastic somites, or protovertebrse, MT, are clearly visible from the surface.

There are as yet no traces of visceral arches or clefts, nor of eyes, ears, or limbs.

On comparing the embryos E and SR (Fig. 179), with the corresponding stages in the development of the rabbit, i.e. with rabbit embryos towards the end of the eighth day, before closure of the neural canal has occurred at any point, there are seen to be several points of difference.

The rabbit embryo at this stage is still on the surface of the vesicle, while the human embryo is already covered by a wellformed amnion. In connection with this separation from the surface there is a further point of difference ; the human embryo is connected at its hinder end by a short thick stalk (Fig. 179, TZ) with the wall of the vesicle; while in the rabbit the tail fold is only just commencing, and the hinder end of the embryo is still directly continuous with the wall of the blastodermic vesicle.

This stalk of connection (Fig. 179, TZ), between the embryo and the wall of the vesicle, arises from the under surface of the hinder end of the embryo, and its relations are practically identical with those of the allantois of a tenth-day rabbit embryo. As this stalk contains a tubular diverticulum of the hind-gut, and transmits the allantoic arteries and veins (cf. Fig. 198), it clearly corresponds, at any rate in great part, to the rabbit's allantois, and may consequently be spoken of as the allantoic stalk.

As regards amnion and allantois, the main difference between the human embryo and the rabbit may be briefly expressed by saying, that both amnion and allantois develop in the human embryo at an earlier stage, relatively to the embryo itself, than is the case in the rabbit. The probable explanation of this precocious development of amnion and allantois, and comparatively late appearance of the embryo itself, in the human species, as compared with the rabbit, will be considered further on.


Another difference of even greater importance between the two embryos is found in the relations of the yolk-sac in the two cases respectively, to which attention has already been directed. In the rabbit, the yolk-sac (Fig. 146, YS) is part of the blastodermic vesicle itself; while in the human embryo (cf. Fig. 188) it lies freely within this. This difference is explained by Keibel as due to a relatively early extension of the splitting of the mesoblast, in the human embryo, right round the lower half of the blastodermic vesicle. In the rabbit (Fig. 147) the mesoblast, and consequently the cavity, C, between its layers, only extends halfway round the blastodermic vesicle, stopping at the sinus terminalis, ST. If, in the rabbit, the mesoblast, and the split between its somatic and splanchnic layers, were to extend round to the lower pole of the blastodermic vesicle, then the yolk-sac would be completely split away from the wall of the vesicle, and a condition similar to that of the human embryo would be attained.

No stages intermediate between Reichert's ovum and His' embryo E, or V. Spee's embryo, have yet been described. The gap, though probably only a slight one in actual time, is of great importance ; for, while Reichert's ovum has no trace of an embryo, His' embryo E possesses neural groove and folds, heart and yolk-sac, and has both amnion and allantois well developed.

His has attempted to bridge over the interval, and has given a series of diagrams, reproduced in Figs. 186-188, showing hypothetical intermediate stages.

The figures represent diagrammatic longitudinal sections through embryos at successive stages of development, and should be compared with Fig. 174, which represents a similar section through Reichert's ovum.

In Fig. 186, which is a hypothetical stage, the commencement of the formation of the embryo is indicated. The embryonal or germinal area has become somewhat depressed, but at its anterior end, to the right in the figure, is lifted up by the commencing head fold.

In Fig. 187, also a hypothetical stage, the general depression of the embryonal area has increased, the embryo being pushed down into the blastodermic vesicle. The head fold has deepened, and the head end of the embryo is now more prominent, and is raised distinctly above the yolk-sac. At the hinder end of the embryo, to the left in the figure, the embryonal area still preserves its primitive connection with the chorion or wall of the vesicle. The head end of the embryo is covered over by the commencing head fold of the amnion.

Fig. 188 is a somewhat diagrammatic section, at a stage


FlGS. 186-188. Diagrammatic longitudinal sections through Human Embryos, representing hypothetical stages intermediate between Reichert's ovum and His' Embryos, E or SR. (From His.) x 5.

FlG. 1 86. Shows the commencement of the head fold of the embryo, and of the

amnion. FIG. 187. A rather later stage, in which the embryo is depressed into the

blastodermic vesicle, but still remains in connection with the wall of the

vesicle through the allantoic stalk. The dotted lines indicate, hypotheti cally, the further growth of the amnion. FlG. 188. A later stage, equivalent to that of His' embryos E or SR (Fig.

179). The amnion is complete, and the villi extend the whole way round

the vesicle.

Am, inner layer of the amnion, or true amnion. S.Cft, dotted line representing the future extension of the outer layer of the amniou. Ys, yolk-sac.

corresponding to that of the embryos E or SR (Fig. 179). The changes necessary to derive it from the stage shown in Fig. 187 are very slight. The hinder end of the embryonal area now forms the thick allantoic stalk, connecting the embryo with the chorion ; and a tubular diverticulum of the ventral wall of the hind gut, the allantois proper, now extends some way along the stalk. The amnion extends over the whole back of the embryo ; a change due possibly, as Professor His suggests, to backward growth of the head fold of the amnion of the earlier stages, as indicated by the dotted lines in Fig. 187 ; but more probably caused, at any rate in part, by approximation and meeting of the side folds of the amnion along the mid-dorsal line.

His' diagrams (Figs. 186 to 188) undoubtedly give an intelligible and consistent theory concerning the development of the human embryo from the stage indicated by Beichert's ovum to that of the embryos E and SR, and it is greatly to be hoped that opportunity may occur for testing their correctness by actual observation.

If the transition occurs in the way suggested by Professor His, then both the amnion and the allantois of the human embryo present features differing widely from those of the rabbit. The amnion has no tail fold, which is almost the only part developed in the rabbit ; while the allantois is, from the first, continuous with the chorion.

Even in the rabbit, however, an approach is made towards the mode of development of the allantois supposed to occur in the human embryo ; the mesoblast of the allantois in the rabbit being, from the first, continuous with the mesoblast of the tail fold of the amnion (Fig. 146), and very early fusing with the chorion as well (Fig. 147).

The precocious development of the allantois, which is one of the most striking points about the human embryo, may be connected with the precocious appearance of the vascular layer of mesoblast lining the blastodermic vesicle ; and both features, in so far as they are exceptional , may be regarded as examples of the tendency to shortening or abbreviation of the processes of development, which is so constantly encountered by the student of embryology.

The establishment of a vascular connection between the embryo and the chorion, and so indirectly with the mother, is the characteristic feature of mammalian development ; and it is not surprising to find, in the most highly developed of all Mammals, this feature thrown back to an earlier stage than that at which it originally appeared ; and hurried on prematurely, even at the expense, as it would seem, of the embryo itself, whose development is unusually retarded.

The germinal layers of the human embryo. But little can be said on this point at present. Concerning the mode of establishment and of differentiation of the germinal layers, we know nothing. In Reichert's ovum the outer wall of the vesicle consists of a single layer of epithelial cells, which from Reichert's figures appear to be flattened, or pavement cells, and which must almost certainly be epiblastic. The central mass of cells, forming the yolk-sac, is certainly hypoblastic ; and so also, in all probability, is the thick disc of granular cells forming the deeper layer of the embryonal area.

Whether mesoblast is, or is not, present in Reichert's ovum is a disputed point ; but at a stage not much later, in the embryos E and SR, layers of vascular mesoblast are present, not only in the embryo itself, but covering the outer surface of the yolk-sac, and lining the wall of the blastodermic vesicle as well (cf. Figs. 182, 183, and 184). The mode of origin, and the time of appearance, of the mesoblast are unknown ; but from the stage represented by the embryos E and SR, the history of the layer is practically the same as in the rabbit or chick.

3. The Third Week

During the third week, the embryo assumes more definite form. The neural canal is closed along its whole length ; the brain vesicles, optic vesicles, and auditory sacs are formed ; the visceral arches and clefts develop ; and the head and neck acquire their characteristic embryonic shape. The embryo increases considerably in size ; the constriction between embryo and yolk-sac becomes much more marked ; and towards the end of the week the first rudiments of the limbs appear.

During this time, development proceeds rather slowly, the changes passed through in the course of the week corresponding roughly to those effected during the second and third days in a chick embryo.

Only a limited number of embryos of the third week have been described, not much more than a dozen in all, and only a few of these were in satisfactory condition for detailed examination. Outlines of some of the more important specimens are given in Figs. 189 to 195, and more detailed drawings on a larger scale in Figs. 196, 197, and 198.


Coste's embryo (Fig. 1 96). An embryo described and figured by Coste, about the precise age of which there is some doubt, appears to belong to the commencement of the third week. The whole vesicle measures 16'2 mm. along its greater diameter, and is covered externally with short, slightly branched villi. The embryo is attached to the inner surface of the vesicle by a short thick allantoic stalk, A.s. The head end of the embryo is well developed, and raised freely above the yolk-sac ; but the body is still so closely connected with the yolk-sac that a distinct yolkstalk can hardly be said to be present. The body of the embryo is concave upwards, and the tail has a well-marked upward direction. In the neck, three visceral arches are visible as thickenings, but the grooves between them are only faintly indicated below the neck, in the angle between the embryo and the yolk-sac, is the heart, H, a large tube sharply twisted on itself. Blood-vessels are present in the wall of the yolk-sac, YS, and also in the allantoic stalk, from which latter they pass into the wall of the blastodermic vesicle, the inner layer of which is vascular throughout its whole extent, although the blood-vessels do not penetrate into the villi.



FIGS. 189 to 195. Outline figures of seven Human Embryos of the third week. (From His.) x 5.

FIG. 189. Embryo lettered by Professor His, Lg, and estimated as fifteen days old. (Cf. Fig. 197, p. 489.) FIG. 190. Embryo lettered by Professor His, Sch, and estimated as fifteen days old. FIG. 191. Embryo lettered by Professor His, M, and estimated as eighteen days old. FIG. 192. Embryo figured by Professor Allen Thomson, and probably about eighteen days old. FIG. 193. Embryo lettered by Professor His, BB, and estimated as about eighteen days old. FIG. 194. Embryo lettered by Professor His, Kin, and estimated as about twenty days old. FIG. 195. Embryo lettered by Professor His, Lr, and estimated as twenty or twenty-one days old. (Cf. Fig. 198, p. 491 )



FIG. 196. Human Embryo at the commencement of the third week. (From His, after Coste.) x 15.

A, inner or true amnion. As, allantoic stalk. If, lie-art. T", blood-vessel of yolk-.-ac. 1't, yolk-sac.


The middle portion of the embryo is clearly divided into protovertebras, but there are no traces of limbs.

Embryo lettered by Professor His, Lg (Figs. 189 and 197), and estimated as fifteen days old. The entire blastodermic vesicle in this case measures 17 by 11 mm., and is covered with villi, except at two patches at opposite poles of the vesicle. The embryo (Fig. 197) is rather closely invested by the amnion, AN, and is connected with the wall of the vesicle by a short thick allantoic stalk, TZ. The yolk-sac, YS, is nearly spherical, and about 2 mm. in diameter, and is still widely continuous with the ventral surface of the embryo.


The most marked feature in the general form of the embryo is the very sharp bend in the middle of the back, opposite the yolk-sac. A similar and equally sharp bend has been noticed in other embryos of about the same age (cf. Figs. 190, 193), but it is not yet certain whether it is to be regarded as a normal feature. His has suggested that it may possibly be caused by the embryo tending to increase in length more rapidly than the closely fitting amnion will allow it to ; the embryo consequently becoming bent at the place where its ventral wall is weakest, i.e. opposite the yolk-sac.

In the head, cranial flexure is well marked, the anterior part of the head being bent down at right angles to the hinder part, and the fore-brain being in consequence carried round to the under surface of the head. The prominent angle of the brain is formed by the mid-brain, BM, behind which comes the nearly straight hind-brain.



FIG. 197. Human Embryo lettered by Professor His, Lg, and estimated as fifteen days old. The wall of the blastodermic vesicle has been removed, except the portion with which the allantoic stalk is continuous. (From His.) x 30.

AN, inner or true amnion. BM, mid-brain. El, auditory pit. HC.l, first branchial cleft. HM, hyomandibular cleft. MN. mandibiilar arch. R, heart. TL, tail. TZ, allantoic stalk. VN", villi of chorion. YS, yolk-sac. '


At the sides of the fore-brain are lateral swellings, caused by the outgrowing optic vesicles, but there is as yet no trace of the lens. The auditory pits, El, are a pair of shallow depressions, with widely open mouths, at the sides of the hind-brain.


At each side of the neck there are a couple of slit-like depressions, HM, and HC.I, transverse to the axis of the embryo. These are the external grooves, which lie opposite the hyomandibular and first branchial gill-pouches, or diverticula from the pharynx. The epiblastic grooves and the corresponding hypoblastic pharyngeal pouches lie in close contact with one another, but do not communicate, so that there are at this stage no complete gill-clefts, or actual perforations in the wall of the neck.

The hyoid arch is the ridge or strip of the neck lying between the first branchial and hyomandibular grooves, HC.I and HM. The mandibular arch, MX, forms a much more prominent ridge, in front of the hyomandibular groove ; and, wedged in between the dorsal end of the mandibular arch and the under surface of the head, is the comparatively small maxillary arch.

The stomatodseum, or mouth depression, is a shallow pit on the under surface of the head, bounded in front by the head itself, at the sides by the maxillary arches, and behind by the mandibular arches. It does not yet open into the fore-gut (cf. Fig. 232).

The heart, R, is large, and lies immediately below the branchial region of the neck, between this and the yolk-sac ; it is a single tube, twisted so as to form a strongly curved loop, with its convexity towards the right side of the embryo.

In the body region, the outlines of the somites or protovertebrse can be seen through the skin ; about thirty-five pairs being already present. The tail, TL, forms a prominent rounded stump ; and from the under surface of its base the short thick allantoic stalk, TZ, arises, which attaches the embryo to the chorion. There is as yet no trace of the limbs.

An embryo lettered by Professor His, Lr (Figs. 195 and 198), and estimated as twenty or twenty-one days old, may be taken as typical of the condition attained by the end of the third week.

Apart from the great increase in size, best seen by comparing Figs. 189 and 195, p. 487, the chief points in which the embryo Lr differs from the embryo Lg, are : The narrowing of the yolkstalk, by which the embryo is separated more markedly from the yolk-sac ; and the almost complete disappearance of the sharp bend in the back, opposite the yolk-sac, which is so curiously characteristic of the embryo Lg, and of others of the same age.

The greatest length of the embryo Lr, measured in a straight line, from the swelling caused by the mid-brain to the rounded hinder end of the body, is 4'2 mm.


FIG. 198. Human Embryo lettered by Professor His, Lr, and estimated as twenty or twenty-one days old. The right wall of the pericardial cavity has been removed, to expose the heart ; and the arteries, the veins, and the hinder part of the alimentary canal are represented as though the embryo were transparent. (From His.) x 23.

A, dorsal aorta. A.I, first aortic arch, in the mandibular arch. A. 2, second aortic arch, in the hyoid arch. A.3, third aortic arch, in the first branchial arch. A.4, fourth aortic arch, in the second branchial arch. A.5, fifth aortic arch, in the third branchial arch. AA, allantoic artery. El, auditory vesicle. GH, hind-gut. GT, mid-gut, opening into the yolk-stalk. HM, hyomandibular cleft or groove. OF, olfactory pit. RA' right auricle. RT, truucus arteriosus. RV, ventricle. TA, diverticulum of liind-gut into allantoic stalk. TL, tail. TR, cloaca. TZ, allautoic stalk. VA, allantoic vein. VB. anterior cardinal vein. VC, posterior cardinal vein. VD, Cuvierian vein. VV, vitelline vein. YK, yolk-stalk.


In the head, the outlines of the several brain vesicles can be recognised fairly accurately through the skin ; and the proportions have not altered very greatly from those of the earlier embryo, Lg. The axes of the fore-brain and of the hind-brain are still at right angles to each other, the mid-brain forming a prominent rounded swelling at their junction. The fore-brain is wider from side to side than before, owing to the optic vesicles, which project outwards from its sides.

The olfactory pits, OF, are a pair of shallow depressions, on the under surface of the extreme anterior end of the head, above the mouth. The lens of the eye has not yet commenced to form ; and the auditory vesicles, El, are now a pair of closed sacs, embedded in the mesoblast at the sides of the hind-brain.

The gill-cleft region is a triangular patch on each side of the neck, with the apex directed backwards, and is bounded both dorsally and ventrally by shallow grooves. The hyomandibular cleft, HM, and the first, second, and third branchial clefts are all represented by pharyngeal pouches, and by external grooves corresponding to them ; but none of the clefts are actually perforated.

The stomatodasal pit is much deeper, and more clearly defined than before, owing to uprising of its lips : it now opens into the fore-gut.

The heart, RA, RV, RT, is larger than before, and its several divisions are more clearly marked off from one another by constrictions. The whole heart has shifted backwards to a certain extent, the greater part of it now lying behind the gillcleft region.

The somites, or protovertebrae, are more distinct than before. A pair of longitudinal ridges which run along the sides of the body, ventral to the somites, are spoken of as the Wolffian ridges. Each of these ridges is more prominent at two places, opposite the posterior end of the heart, and opposite the allantoic stalk respectively. These more prominent parts of the continuous Wolffian ridges are the rudiments of the arms and legs.

4. The Fourth Week

The fourth week is marked by a great increase in the size of the embryo, growth being relatively more active at this than at any other period.

In the early part of the fourth week (Figs. 199-201), the flexure of both head and body is very strongly marked, the embryo being rolled up on itself so that the head and tail touch or even overlap, and the outline of the entire embryo being approximately circular. The several parts of the head are more conspicuous than before ; the visceral clefts and arches are more clearly defined ; the nose and ear are more prominent ; the heart is very large ; the Wolffiau ridges are still present as continuous structures, but the enlargements of the ridges, which become later the arms and legs, are rapidly increasing in size. Towards the end of the fourth week (Figs. 202, 203, 204), the embryo acquires a very characteristic form, corresponding closely in shape, in size, and in internal structure with a chick embryo at the end of the fourth day, or a rabbit embryo of the eleventh day.



FIG. 199-201. Outline figures of three Human Embryos, estimated as about twenty-three days old. (From His.) x 5.

FlG. 199. Embryo figured and described by Coste. For a figure of the uterus and blastodermic vesicle from which this embryo was obtained, see Fig. 255, p. 608.

FIG. 200. Embryo lettered by Professor His a. FIG. 201. Embryo figured and described by Allen Thomson. By a mistake of the engraver's this figure has been reversed ; and it is really the right side of the embryo, and not the left, that is shown.



FIGS. 202 and 203. Outline figures of two Human Embryos, estimated as twenty-seven days old. (From His.) x 5.

FIG. 202. Embryo lettered by Professor His, B. FIG. 203. Embryo lettered by Professor His, A.


The embryo (Fig. 204), which measures 7 '5 mm. along its longest diameter, is still strongly flexed. The back is rather straighter than before, but, owing to the very sharp bend in the cervical region, at the junction of the head and trunk, the under surface of the head is still almost in contact with the tail.



FIG. 204. Human Embryo lettered by Professor His, A, and, estimated as twenty-seven days old. (From His.) x 13.

BR.l, first branchial arch. El, auditory vesicle. HC.l, first branchial cleft. HM. hyomandibular cleft. HY. iiyoid arch. LA, fore-limb or arm. LP, hind-limb or leg. MN, mandibular areli. MX, maxillary arch. OC, lens. OF, olfactory pit. O J, Jacobsou's organ. SU, sinus pracervicalis. VN, villi of choriou.


The several parts of the brain are readily distinguished through the skin, the mid-brain being especially prominent. The nerve ganglia, both cranial and spinal, are well developed, and form swellings that are clearly visible from the surface. The olfactory pits, OF, on the under surface of the fore part of the head, are larger and deeper than before, and are bordered by prominent lips, with somewhat irregular outlines. At the inner and ventral corner of each olfactory pit there is a small but deep notch, oj, with a sharply defined border : from this notch the organ of Jacobson is developed.

The eyes are very much smaller than in a chick embryo at d, corresponding stage of development. The lens is present as a small circular pit, with an open mouth, oc.

The auditory vesicles, EI, appear from the surface as a pair of rounded swellings, just above the dorsal ends of the hyoid arches, HY.

The visceral arches have undergone considerable modification. The maxillary arch, MX, lies immediately behind the eye ; it is larger than before, but is still much smaller than the arches next behind it. The mandibular arch, MN, is the largest of the series, and is partially divided by a constriction, about the middle of its length, into dorsal and ventral portions. The hyoid arch, HY, is nearly as large as the mandibular arch, and is also constricted across its middle. The first branchial arch, BR.I, lies behind the hyoid arch, and is of much smaller size than this. A still smaller second branchial arch is present, but is not visible from the surface, being overlapped and concealed by the first (cf. Fig. 239, p. 552).

With regard to the visceral clefts, it is probable that none are open at this, or indeed at any period in development ; but the point has not been determined with absolute certainty. Behind the first branchial arch there is, 011 each side of the neck, a deep pit, the sinus pragcervicalis, SU. This is a blind pocket, (cf. Fig. 239), formed by a process of telescoping, through which the hinder pairs of branchial arches are carried forwards, so as to lie between the anterior arches, instead of behind these. The sinus prascervicalis does not open into the pharynx or into any other cavity, and at a later stage it is obliterated by fusion of its anterior and posterior walls with each other (cf. Fig. 210, su).

The mouth (cf. Fig. 206) is much wider from side to side than in the earlier stages ; it is bounded in front by the fronto-nasal process, at the sides by the maxillary arches, and behind by the mandibular arches.

In the body of the embryo, thirty-five pairs of somites, or protovertebrge, are clearly visible ; of these, eight are cervical, twelve thoracic, five lumbar, five sacral, and five caudal. The tail projects freely as a short conical process.

The fore and hind limbs, LA, LP, are flattened buds, with rounded margins ; they are about as long as they are wide, and show as yet no trace of a division into segments, or into fingers and toes. The outer surface of each limb is its extensor surface ; and the inner, facing the body of the embryo, is the flexor surface. The root of attachment of the fore-limb, or arm, extends from the fifth cervical to the second thoracic somite ; and the attachment of the hind-limb, from the fourth or fifth lumbar somite to the third or fourth sacral. The Wolffian ridge connecting the arm and leg of each side is still present, but is inconspicuous.

The heart is of great size, and its several divisions can be easily recognised through the thin wall of the pericardial cavity. The liver, which is also large, forms a prominent swelling between the heart and the fore-limbs.

The yolk-sac is about the size of the head and neck of the embryo ; and the yolk-stalk is now long and slender. The inner, or true, amnion is a thin transparent membrane which invests the embryo rather closely ; and the allantoic stalk, which lies to the right of the tail, and to the left of the yolkstalk, is about 2 mm. long, and rather more than 1 mm. in diameter.

5. The Fifth Week

The fifth week is marked by great increase in size of the whole embryo, and especially of the head ; by further straightening of the back, and uplifting of the head ; by the more definite formation of the face ; and by rapid growth of the limbs.

The cervical flexure, at the junction of the head and body, is still very conspicuous, and throughout the greater part of the fifth week the greatest length of the embryo is, as in the fourth week, a line drawn from this cervical prominence to the sacral curvature. At the close of the fifth week, the head becomes lifted up more markedly, and the length of the embryo, about 15 mm., is now represented by a line drawn from the top of the midbrain to the sacral curve (c/. Fig. 211).

Throughout the fifth week, the head of the embryo grows rapidly, and by the end of the week it forms, with the neck, about half of the entire embryo. The shape of the head is still determined almost entirely by the brain, of which the several divisions are clearly recognisable from the surface. All parts of the brain increase considerably in size, and more especially the cerebral hemispheres.

During the fifth week the face is gradually acquiring definite form, and the features are becoming established.

The olfactory pits deepen considerably ; and their inner and outer borders become raised into prominent lips. The inner borders are formed by the lateral margins of the fronto-nasal process, which grow out as two rounded wings, the processus globulares (Figs. 206, 207, ro). The outer borders are formed by the lateral frontal processes, which separate the olfactory pits from the eyes.

The lower margin of each olfactory pit is incomplete, and between the processus globularis and the lateral frontal process there is a deep nasal groove (Fig. 206), leading from the olfactory pit to the stomatodasum. Towards the end of the fifth week, the maxillary arches (Fig. 207, MX) become more prominent, and growing inwards meet the processus globulares, FO, and fuse with these ; thus bridging over the nasal grooves, and converting them into short tubes, the posterior narial passages, which lead from the olfactory pits to the mouth. At the same time the apertures of the olfactory pits become narrowed, to form the external nostrils.

The bridge of the nose is formed from the median part the fronto-nasal process (Figs. 206, 207, FP). At the commencement of the fifth week this is a triangular area, slightly depressed below the level of the surrounding parts ; but towards the close of the week, a blunt process appears in the centre of the area, formed by a sagittal fold of its surface, and gradually grows forwards to form the bridge of the nose. For some time the nose is very short and inconspicuous, and the nostrils very far apart ; but towards the end of the second month (cf. Figs. 213 and 214), the nose grows forwards more prominently, and the nostrils are brought closer together. The alee nasi, forming the outer borders of the nostrils, are developed from the lateral nasal processes.


FlG. 205. Human Embryo lettered by Professor His, Rg, and estimated as thirty-two or thirty-three days old. (From His.) x 5.




FIG. 206. The under surface of the head of a Human Embryo lettered by

Professor His, Hn, and estimated as about twenty-nine days old. (From

His.) x 7|. FIG. 207. The under surface of the head of a Human Embryo lettered by

Professor His, C.I I, and estimated as about thirty- four days old. (From

His.) x 5.

BS, cerebral hemisphere. DS, stornatodaeum. 3PO, processus globularis, or lateral portion of fronto-nasal process. FP, median portion of fronto-uasal process. HM, hyomandibular cleft. MK", mandibular arch. MX, maxillary arch. OC, eye. OK, olfactory pit.

The mouth changes its shape very markedly during the fifth week. At the beginning of the week (Fig. 206, DS) it is a wide opening, extending transversely across the under surface of the head ; but before the end of the week (Fig. 207) it has become greatly reduced in size, by convergence of the maxillary arches and the processus globulares, and is now a narrow transverse slit. Between the maxillary arch and the lateral nasal process of each side is a depression, the lacrymal groove, which at first (Fig. 206) leads into the stomatoda3um, but which on the completion of the narial passage opens into this latter (Fig. 207). In the region of the visceral arches and clefts, important changes occur. The tendency of the anterior arches to grow backwards over the hinder arches, or, as it may be better expressed, the telescoping of the hinder arches within the anterior ones (cf. Fig. 239), has been already referred to. At the end of the fourth week (Fig. 204), the second branchial arch is overlapped by the first, BR.I, and is completely concealed by this in surface views. Early in the fifth week, the second branchial arch is in its turn overlapped by the hyoid arch ; and from about the thirtieth day onwards (Fig. 205) the only arches visible on the surface of the neck are the maxillary, mandibular, and hyoidean. Behind the hyoidean arch is the deep fissure caused by the sinus preecervicalis (cf. Fig. 240, su), which must not be mistaken for a visceral cleft.

During the fifth week the borders of the hyomandibular cleft become more prominent, and gradually give rise to the folds from which the external ear is developed, in the following manner.

At the end of the fourth week (Fig. 204), the hyomandibular cleft, HM, is a deep groove between the mandibular and hyoid arches, and running about halfway across the head. The mandibular arch is divided by a slight constriction, about the middle of its length, into dorsal and ventral portions : of these, the ventral portion bears at its upper and posterior border a small rounded process, well shown in the figure, and named the tuberculum tragicum ; while the dorsal portion of the arch, to which the reference line, MN, runs, is the tuberculum anterius helicis. Opposite the dorsal end of the hyomandibular cleft is a longitudinal ridge, the tuberculum intermedium helicis.

The hyoid arch is divided, by two transverse constrictions, into dorsal, middle, and ventral lobes : of these, the dorsal lobe is named the tuberculum anthelicis ; the middle lobe, to which the reference line, HY, in Fig. 204 runs, is the tuberculum antitragicum ; and the ventral lobe, which is the smallest of the three, is the tuberculum lobulare.

In the course of the fifth week, these swellings assume more definite form, and gradually give rise to the several parts of the external ear or pinna. The tuberculum anterius helicis (Fig. 208, 2), and tuberculum intermedium (3) unite together, and with a vertical ridge, the cauda (sc), which arises along the posterior border of the hyoid arch, to form the horse-shoe shaped marginal rim, or helix, of the ear. The ventral ends of the hyoid and mandibular arches fuse, and so give more definite shape to the hyomandibular cleft, which latter becomes the external auditory meatus. The tuberculum lobulare (G) fuses with the lower end of the cauda helicis (Fig. 209), and at a later stage grows ventral wards to give rise to the lobule of the ear. The tubercula anthelicis, tragicum, and antitragicum, give rise to the antihelix, tragus, and antitragus respectively of the adult ear.

The body of the embryo presents no external characters of special interest during the fifth week. Owing to the increasing thickness of the muscular and connective tissue walls, the outlines of the internal organs are less distinctly seen from the surface than in the earlier stages.


FIG. 208. The left ear of a Human Embryo lettered by Professor His, Br.2,.

and estimated as thirty-five days old. (From His.) x 20. FIG. 209. The left ear of a Human Embryo, lettered by Professor His, Dr,

and estimated as thirty-eight days old. (From His.) x 20.

1, tuberculum tragicum. 2, tuberculum anterius lielicis. 3, tuberculum intermedium helicis. 3c and c, cauda helicis. 4, tuberculum anthelicis. 5, tuberculum antitragicum. C, tnbcrculum lobulare.

The limbs undergo important changes during the week, and afford ready means of determining the age of the embryo. In the early part of the fifth week they become divided, first into two, then into three segments. By the middle of the week this division is well marked, the terminal segments, i.e. the hands and feet, forming broad flattened terminal plates, with free rounded margins. A day or two later (Fig. 205), a distinction appears in the hand, between a more swollen basal part, and a thin flattened marginal part ; and towards the close of the week the first traces of fingers appear, as small lobes at the boundary between the basal and marginal portions, which soon extend to the free edge, but do not project beyond this until the sixth week.


The hind-limb is slightly behind the fore-limb in its development, and at the end of the fifth week the toes are only just commencing to appear.

The fore- and hind-limbs of each side are still connected by a low and inconspicuous Wolffian ridge. During the fifth week the tail (Fig- 205) is more conspicuous than at any other stage



FIG. 210. A Pregnant Uterus of .about the fortieth day. The uterus has been opened from the ventral surface, and the decidua reflexa and chorion cut through by a crucial incision, and the flaps turned aside to expose the embryo. The embryo is still inclosed in the amnion, and the small yolksac, with its long stalk, are seen lying between the amnion and the chorion. At the upper part of the figure the apertures of the Fallopian tubes are seen. (From Kolliker, after Coste.) x f.

in development ; it is a thin pointed projection, usually bent either laterally, or backwards, by the pressure of adjacent parts.

6. The Sixth Week

During the sixth week the embryo increases in size, though not so rapidly as in the earlier stages. At the commencement of the week it is about 15 mm. long, and at the close about 19 or 20 mm., but the actual measurements depend rather on the degree to which the head is lifted iip, by straightening of the cervical flexure, than on any real increase in the dimensions.

The position of the embryo within the uterus, about the fortieth day, is shown in Fig. 210. The embryo is connected with the placenta by a thick allantoic stalk. The yolk-stalk is long and thin : its proximal part is bound up with the allantoic stalk in a sheath formed round both by the inner or true amnion ; while its distal portion, ending in the small yolk-sac, lies between the amnion and the chorion. The amnion is a transparent sac some distance from the embryo.

The embryo itself is rapidly assuming more definite shape, and by the end of the week is distinctly human in appearance. Owing to the thickening of the muscles and of the subcutaneous connective tissue, and the formation of skeletal elements, the shape of the embryo as a whole, and especially of the head, is much less dependent on the internal organs than in the earlier stages.

The head is still of great size. The face has made considerable progress, and the features are now well established. The nose is larger than before, but is still very broad and flat. The eyelids are commencing to form, as folds of skin, above and below the eyes. The lips appear as folds at the margins of the jaws, but only reach a small development during the sixth week : the red ridge of each lip arises independently, and not until a much later period ; about the middle of the third month.

Up to the end of the fifth week there is a distinct notch in the median plane where the two mandibular arches meet : during the sixth week this notch is gradually filled up, and the chin formed as a median projection.

The external ear makes considerable progress during the week (Figs. 209, 211), and by its close the relations and proportions of the several parts are readily comparable with those of the adult.

Apart from the external auditory meatus and the external ear, the visceral clefts and arches are no longer recognisable. The sinus prsecervicalis has closed, and the neck is becoming established as a constricted region between the head and trunk.

The limbs have increased considerably in size, the upper arm and thigh in particular being much longer than before.


The fingers project beyond the margin of the hand by the middle of the sixth week ; and the toes become clearly established, although they do not reach the margin of the foot until the early part of the seventh week. The elbow and knee project outwards at first, but towards the end of the sixth week the limbs become rotated so as to lie alongside the body, the elbow being directed backwards, and the knee forwards.



FIG. 211. - Human Embryo about the middle of the sixth week. (From His.) x 5.


The tail is less conspicuous than before, and owing to the growth of adjacent parts is gradually becoming incorporated in the body.

7. The Second Month

At the end of the second month the embryo measures from 25 to 30 mm. in length, and weighs from 12 to 20 grammes.

The cervical flexure has almost disappeared : the head is well lifted up, and is still of very large size, forming nearly half the entire embryo. The eyelids, nose, lips, and external ear have all made considerable advance ; the nose is still broad and flat, and the nostrils wide apart, though much closer than before. The median part of the upper lip is formed by the two processus globulares, which meet and fuse shortly before the end of the second month. The cheeks are now well formed.



FIG. 212. Human Embryo at the end of the second month. (From His.) x 5.

The limbs project some distance beyond the body ; and the fore-limb, which is still the larger of the two, has the characteristic shape of the huinan arm. The thumb is clearly marked off from the fingers, and the deltoid swelling at the shoulder is already prominent. The leg is smaller than the arm, and is so directed that the soles of the feet are apposed.

The neck is well marked, though short. The ventral wall of the body is completely formed. The umbilical cord, which attaches the embryo to the placenta, is about 8 or 10 mm. long : it is as a rule straight, but may be slightly twisted on itself. It is formed by the allaiitoic stalk and yolk-stalk, bound together by the amnion, and it still contains at its base a loop of the intestine.



FIG. 213. Head of a Human Embryo at the end of the seventh week. HM, external auditory meatus. (From His.) x 5.

FIG. 214. Head of a Human Embryo at the end of the second month. (From His.) x 3.


From the end of the second month, when the definite human form is established, up to the time of birth, it is customary to use the term foetus in place of embryo.

8. The Third Month

At the end of the third month the foetus measures about 7 cm. in length, or 9 to 10 cm. if the legs be included, and weighs from 100 to 125 grammes.

The head is still very large relatively to the rest of the body, but not nearly so much so as in the earlier stages : the lips and eyelids are closed, and the helix of the ear is folded down so as to almost close the meatus. The neck is longer than before. The limbs, though small, have acquired their definite shape and proportions ; and nails are present, as thin plates, on both fingers and toes. The integument is slightly firmer than before, but is still very thin, transparent, and rose-coloured.

Up to this stage a loop of the intestine has been situated in the allantoic stalk, and therefore outside the embryo ; but by the end of the third month this loop is withdrawn, and the whole alimentary canal, which has increased greatly in length, is from this time situated within the abdominal cavity.

During the third month, the external genital organs become established. The history of their development will be given in the section dealing with the organs of reproduction.

9. The Fourth Month

At the end of the fourth month the foetus measures 12 to 13 cm. in length, from the vertex of the head to the coccyx ; or from 16 to 20 cm. if the legs be included. The weight is usually from 230 to 260 grammes.

The skin is of a rosy colour, and is much firmer than before. Short whitish hairs appear on the head, and a slight down on other parts of the body. The eyelids, nostrils, and lips are all closed. The chin, which has hitherto been inconspicuous, begins to become prominent. The legs and arms are of about equal length : and the external sexual characters are usually wellmarked.

The anus is open, and the duodenum contains meconium of a light greyish-white colour. The umbilicus, or point of origin of the umbilical cord, is low down, close to the pubes. In the skull, the bones are still far from meeting one another, so that the sutures and fontanelles are very wide. The muscles are more fully developed than before, and may give rise to distinct movements of the foetus. In abortions at this period the foetus may live for some hours.

10. The Fifth Month

By the end of the fifth month the foetus measures about 20 cm. in length; or, if the legs be included, 25 to 27 cm. The average weight is about half a kilogramme.


The skin is more consistent than before, and presents on its surface at certain places small patches of sebaceous matter. Hairs are more extensively developed than before, but are still devoid of any distinct colour. The legs are now longer than the arms, and the nails are well formed. The umbilicus is further forward than in the preceding month, and is now some distance in front of the pubes.

The head is still very large in proportion to the other parts. The heart, liver, and kidneys are also disproportionately large. The small intestine contains meconium, which, owing to the secretion of bile, is now of a pale greenish-yellow colour. The gall-bladder is of some size. Ossification has commenced in the pubes, and in the os calcis.

11. The Sixth Month

The total length of the foetus at the end of the sixth month, measured from the vertex .to the heels, is from 30 to 32 cm. The weight is very variable ; its average amount is about a kilogramme.

The skin is of a dirty reddish colour, and much wrinkled ; it is covered, at any rate in the axillae and groins, with a sebaceous deposit. The hairs are more strongly developed, and of a darker colour than before. Both eyelashes and eyebrows have commenced to appear.

The umbilicus is still further forward than before, and the meconium in the intestine is darker and more viscous. The testes of the male have not yet descended into the scrotum, but are situated within the abdominal cavity, lying on the psoas muscles, immediately behind the kidneys.

The sternum is well developed, and has commenced to ossify. The nails reach to the ends of the fingers, and extend about a quarter of the way round them.

12. The Seventh Month

The total length of the foetus at the end of the seventh month, measured from the vertex to the heels, is about 35 or 36 cm., and the weight averages about 1^ kilogramme.

The skin is still of a dirty reddish colour, but not so dark as before. There is an increased deposit of fat in the cellular tissue, causing the body to appear more plump and round. The hairs are plentiful, and about 5 or 6 mm. in length.

The several bones forming the roof of the skull become strongly convex, the central portion of each, from which ossification starts, forming a very evident prominence. The eyelids, which have been closed since reaching their- full size in the fourth month, now open.

The whole of the large intestine is filled with a dark olivegreen viscous meconium. The liver is still very large relatively to the whole body, and is of a deep brownish red colour.

The testes have, as a rule, descended as far as the inguinal rings, and may even have entered the inguinal canals.

The end of the seventh month is of interest, as being perhaps the earliest period at which the foetus can be born with any reasonable chance of surviving.

13. The Eighth Month

During the eighth month the increase in bulk is more marked than that in length. At the end of the month the total length of the foetus, from the head to the coccyx, is about 28 cm. ; and from the head to the heels about 40 cm. The weight varies from. 2 to 2^ kilogrammes.

The skin is of a brighter flesh colour than before, and is covered all over with the sebaceous deposit known as ' vernix caseosa.' This substance, which usually makes its appearance about the middle of gestation, was formerly considered to be a deposit formed from the liquor anmii, but appears rather to consist of matter formed by the cutaneous glands of the foetus, mixed with dead epithelial cells. It varies much in quantity in different 'cases, and is always more abundant in certain situations, notably the head, axilla?, and groins.

The chin is now far more prominent than before, the lower jaw equalling the upper in length. One of the testes, usually the left one, has passed through the inguinal canal into the scrotum, while the other is, as a rule, still in the canal. There is no ossification in the lower epiphysis of the femur.

14. The Ninth Month

At the full time the foetus measures about 35 cm. from the head to the coccyx, and 50 cm. from the head to the heels. The weight is, on the average, from 3 to 3^ kilogrammes.

The skin is paler than before. The subcutaneous connective tissue is filled with fat, giving roundness and firmness to the body and limbs. The hair is thick, long, and fairly abundant on the head, while the down has begun to disappear from the body.

The umbilicus is almost exactly in the middle of the body, or slightly behind this point. Both testes are, as a rule, in the scrotum, which has now a corrugated surface.

Ossification has commenced in the centre of the cartilage at the lower end of the femur. This is the first epiphysial ossification to appear in the body, and is often the only one present at full time. Ossification has sometimes commenced in the proximal epiphyses of the tibia and humerus ; but while the presence of these centres is a sure sign of full time having been reached, their absence does not, without further evidence, indicate premature delivery.

Development of the Nervous System

The general history of development of the human nervous system is the same as in other Vertebrates. Certain points, especially in connection with the brain, will require detailed notice ; and, with regard to the histological development of the nervous elements, recent researches by His, and others, have shown that human embryos are well suited for the most minute investigations.

1. The Brain

a. General account

It will be convenient to give first a general account of the development of the brain, and of its condition at successive stages, and then a more detailed description of parts, such as the cerebral hemispheres, which are of special interest.

The second week. In the youngest human embryos, such as His' embryos E and SR (Figs. 176, 178, and 179), estimated as about thirteen days old, the neural groove is widely open along its whole length, but by comparison with later embryos it is possible to determine, even at this stage, the several regions of the brain.

Thus, in Fig. 179, .the dorsal concavity, opposite the reference' line AN, marks the junction of the brain and spinal cord ; the highest point of the cephalic convexity, close to the reference line HD, is the region of the mid-brain ; and the part in front of this is the fore-brain, which is already flexed ventralwards.

The third week. By the fifteenth day (Figs. 197, p. 189, and 232, p. 515) the neural canal is closed along its whole length, except at the extreme hinder end ; the several divisions of the brain fore-brain, mid-brain, and hind-brain are well established; and cranial flexure is strongly marked, a sharp bend of about 90 degrees taking place opposite the mid-brain, by which the fore-brain is brought down to the under surface of the head.

The fore-brain is of considerable length ; its most anterior part is the vesicle of the hemispheres, a short, rounded, and comparatively inconspicuous dilatation, which as yet shows no trace of division into right and left hemispheres. The thalamencephalon, or fore-brain proper (Fig. 232, BF), is long, and compressed laterally ; from its sides arise the optic vesicles, BO, which project outwards and slightly backwards, and are already constricted at their bases to form the optic stalks. The floor of the thalamencephalon is produced downwards behind the optic stalks into a shallow pit, the infundibulum.

The mid-brain, BM, is small and rounded ; it is separated by a constriction from the fore-brain in front, and by a much sharper one, the isthmus, from the hind-brain.

The hind-brain is the widest as well as the longest part of the brain ; it is widest in front, and gradually tapers posteriorly as it passes into the spinal cord. The roof of the hind-brain is very thin, except at its anterior end, where a slightly thickened transverse band, BL, marks the commencement of the cerebellum.

During the third week the brain rapidly increases in size, and by the end of the week has attained the proportions shown in Tig. 215. The several divisions of the brain are more distinctly marked off from one another, and the vesicle of the hemispheres, BS, and the cerebellum, BL, are more conspicuous than before.

The cervical flexure, by which the entire head is bent ventral wards on the body, is commencing to appear at the junction of brain and spinal cord ; it is shown in Fig. 215, at a level between the reference lines HC.3 and CH.


The fourth week. By the end of the fourth week the shape of the brain is as shown in Fig. 216. The flexure at the level of the mid-brain, or mesencephalic flexure as it may be termed, has increased greatly in extent, and now amounts to about 180,


FIG. 215. The bead and fore part of the body of a Human Embryo lettered by Professor His, Lr, and estimated as twenty or twenty-one days old. (6y. Fig. 198.) The brain is exposed from the left side ; the rest of the embryo is represented in sagittal section. (From His.) x 28.

BF, tbalamencophalon. BH, hind-brain, or medulla oblongata. BL, cerebellum. BM, mid-brain. BO, optic vesicle. BS, vesicle of the cerebral hemispheres. CH, notochord. El. auditory vesicle. HC.l, first branchial pouch. HC.3, third branchial pouch. HM, hyoniandibular pouch. MN, mandibular arch. RT, truncus arteriosus. TO, oesophagus. "W, liver.

the infundibulum and the hind-brain almost touching each other. The cervical flexure, marking the junction of the brain and spinal cord, at the level of the reference line A. 5, is also much more pronounced than before, and forms an angle of about 90. A third, or metencephalic flexure, with the concavity directed dorsal wards, is commencing to form opposite the cerebellum, at the level of the reference line PT ; at a slightly later stage this flexure becomes very strongly marked.

As regards the individual parts of the brain, the vesicle of the hemispheres has greatly increased, and is now divided by a median fold into right and left hemispheres. By, which are


FIG. 21(j. Human Embryo lettered by Professor His, Pr, and estimated as twenty-eight days old. The brain is exposed from the left side ; and the body of the embryo has been dissected to show the heart and aortic arches, and the alimentary canal. (From His.) x 9.

A, dorsal aorta. A.3. tliird aortic arch, or carotid arch. A.4, fourth aortic arch, or systemic arch. A.5, tilth aortic arch, or pulmonary arch. BL, cerebellum. BM, mid-brain. BS, cerebral hemisphere. IN", infundibulum. KD, ureter. LG, lung. LT, larynscal chamber. OC. optic cup. PT. pituitary dlverticuluin from stomatodteum. RB, If ft auricle. RV, ventricle. T A, cavity of allantoic stalk. TC, cloaca. TN, tonpuu. TR,, intestine. TS, stomach. TZ, umbilical stalk. VA, allantoic vein. VB, anterior cardinal vein. VC, posterior cardinal vein. VE, meatus venosus. VI, posterior vena cava. VO, vitc-llinu .voin. "W, liver. "WD, bile duct. YK, yolk-stalk.

already commencing to grow backwards over the thalamencephalon. This latter is very deep dorso-ventrally, but compressed laterally ; the infundibulum, IN, is of considerable size; the optic stalks are more markedly constricted than before ; and the optic vesicles, now doubled up to form the optic cups, are smaller relatively to the other parts of the brain than at the earlier stages.

The mid-brain, BM, still remains small ; it is connected with the hind-brain by a rather long and narrow neck.

The hind-brain is very wide anteriorly ; the cerebellum is much more conspicuous than before, and consists of two lateral ridges, separated by a median notch. The sides of the medulla oblongata are thick, and the roof extremely thin.

The fifth week. During the fifth week (cf. Fig. 205) the cerebral hemispheres increase rapidly, growing backwards along


Fig. '217. The brain of a Human Embryo lettered by Professor His, Zw, and estimated as about the middle of the eighth week. (From His.) x 5.

BF, thalamencephalon. BH, medulla oblongata. BL, cerebellum. BM, miilbraiu. BS, cerebral hemisphere. BY, olfactory lobe. N"S, spinal cord. OS, optic talk.

the sides of the thalamencephalon ; from the ventral surfaces of their anterior ends the olfactory lobes arise as hollow outgrowths. The infundibulum remains of great depth ; and the mid-brain is relatively smaller than before. In the hind-brain the cerebellum has increased in size ; and both the metencephalic and the cervical flexures have increased in sharpness.

The sixth to the eighth weeks. The most marked change during the latter part of the second month consists in the great increase in the sharpness of the metencephalic flexure, which amounts to nearly 180 (Fig. 217), the cerebellum and the roof of the hinder part of the medulla oblongata being in contact with each other ; while, on the ventral surface of the brain, the angle of the flexure, which marks the place at which the pons Varolii will appear, almost touches the infundibulum.

The cerebral hemispheres have increased considerably, and now overlap nearly half the sides of the thalamencephalon. Each hemisphere is somewhat reniform in outline, the notch or hilum, opposite the optic stalk, being the commencement of the Sylvian fissure.

The third month. By the end of the third month (Figs. 218221). the cerebral hemispheres are by far the largest portions of the brain, and completely cover the thalamencephalon. The Sylvian fissure forms a conspicuous notch in the ventral border of each hemisphere ; and the sulci are commencing to appear as grooves on the surface. The midbrain is still small and undivided; but the cerebellum has increased very considerably in size.



The fourth month (Fig. 222) is chiefly marked by a still further increase in size of the cerebral hemispheres, which now completely cover the thalamencephalon, and overlap part of the mid-brain as well. The cerebellum has increased considerably in size, and the transverse fibres of the pons Varolii are commencing

to form. FIG. 218. A Human Foetus three months old, dis sected from the ,, /-,-,. n oo\ , i i i i i

dorsal surface to month (rig. 223) the cerebral hemispheres over expose the brain ] a p the cerebellum and project some distance and spinal cord. , a '*. m. a i a j j

(From Kolliker.) beyond it. The Sylvian fissure is a deep and

Natural size. conspicuous depression on the outer surface re! of each hemisphere ; the mid-brain is divided into the corpora quadrigemina by two fissures, longitudinal and transverse respectively ; and the optic chiasma, pons Varolii, olivary bodies, and other parts of the adult brain are well established.


The sixth month. By the end of the sixth



FIGS. 219-221. Three views of the brain of a Human Foetus three months old. (From Kolliker.) Natural size.

FIG. 219. From the right side.

FIG. 220. From the dorsal surface ; the dorsal parts of the cerebral hemispheres and of the mid-brain have been removed to expose the internal cavities.

FIG. 221. From the ventral surface.

c, cerebellum, cm, corpus mammillare. c.st, corpus striatum. /./', hippocampus major, h, cerebral hemisphere, in, mid-brain, mo, medulla oblongata. p, pons Varolii. tfto, optic thalamus. to, optic tract.


FIG. 222. The brain and spinal cord of a Human Foetus four months old, from

the dorsal surface. (From Kolliker.) Natural size. c, cerebellum, h, cerebral hemisphere, mo, medulla oblongata. v, mid-brain.

FIG. 223. The brain of a Human Fostus six months old, from the right side. (From Kolliker.) Natural size.

c, cerebellum, /.lateral lobe or hemisphere of the cerebellum. /., Sylvian fissure . o, olivary body, ol, olfactory lobe, p, pous Varolii.


The changes during the last three months of foetal life consist chiefly in the formation of the fissures of the cerebral hemispheres and cerebellum, and in the gradually increasing complication of all parts of the brain.

b. The Cerebral Hemispheres

The unpaired vesicle of the hemispheres is already present on the fifteenth day, i.e. at the time when closure of the brain tube is effected. Towards the close of the fourth week, a median dorsal ridge appears along the roof and anterior wall of the unpaired vesicle, and then becomes folded down so as to project as a septum into the cavity of the vesicle, which it partially divides into the right and left hemispheres.

During the fifth week, a sheet of vascular connective tissue grows into the cleft between the hemispheres, and forms the basis of the falx cerebri. Before the end of the week the inner or mesial walls of the hemispheres, bordering the falx cerebri, become thrown into folds which project into the cavities of the hemispheres ; blood-vessels from the falx cerebri soon grow in between the two layers of these folds, and give rise to the choroid plexuses of the lateral ventricles.

Up to the end of the fifth week there is no marked difference in the thickness of the walls of the hemispheres at different places, but from this time growth takes place very unequally in different directions, some parts thickening very rapidly, while others become reduced to single layers of epithelial cells.

The first important thickenings to appear are those which give rise to the corpora striata. These arise, early in the fifth week, as a pair of ridge-like thickenings of the ventral walls of the hemispheres, which project into the lateral ventricles and form prominent lower lips to the foramina of Monro, through which these ventricles communicate with the third ventricle or cavity of the thalamencephalon.

The corpora striata are formed in part as actual thickenings of the walls of the hemispheres ; but their appearance is due in the first instance to folding of the whole thickness of their walls ; the depressions formed by the Sylvian fissures on the outer surfaces of the hemispheres corresponding to the inwardly projecting ridges of the corpora striata. It is sometimes stated that the corpora striata are formed by the depressions of the surface which give rise to the Sylviaii fissures ; but it is more correct to describe the corpora striata and Sylvian fissures as both alike due to relatively rapid growth of the parts of the hemispheres in connection with which they arise ; the folds taking the direction of least resistance, and projecting inwardly into the brain cavity rather than outwardly towards the skull. The corpora striata grow rapidly : by the end of the second month they are strongly arched, and have reduced very considerably the size of the foramina of Monro, which they bound ventrally.

The main lobes of the cerebral hemispheres frontal, parietal, occipital, and temporo-sphenoidal are established during the fifth and sixth months ; they are formed by subdivision of the original hemispheres, and not as separate outgrowths from these. The olfactory lobes, on the other hand, arise as hollow outgrowths from the under surfaces of the hemispheres, which first appear about the end of the fourth week or beginning of the fifth. Each olfactory lobe early becomes divided by a constriction into two portions, of which the anterior forms the bulbus and tractus olfactorius, and the trigonum olfactorium of the adult ; while the posterior portion gives rise to the anterior perforated space, and adjacent parts of the brain.

The commissures of the cerebral hemispheres require special notice.

Towards the end of the second month, as the cerebral hemispheres extend backwards over the thalamencephalon, closely embracing this latter, extensive fusion occurs between the superficial white matter of the corpora striata, and of the optic thalami which these overlap.

This tendency to fusion of originally distinct parts of the brain occurs in other regions as well. During the third month, the inner or mesial surfaces of the right and left hemispheres come in contact, and fuse, in front of the lamina terminalis, or anterior wall of the thalamencephalon ; and from this fused portion the great commissures of the hemispheres are developed. The fusion takes place round the margins of a triangular patch (Fig. 224, SP), immediately in front of the lamina terminalis. The triangular area itself remains free, as a narrow vertical chink between the two hemispheres, which becomes the fifth ventricle of the adult. Of the margins of the area, along which the right and left hemispheres are fused together, the dorsal border, c, becomes the corpus callosum, while the posterior border gives rise to the anterior commissure, and also to the longitudinal fibres which form the body of the fornix. The anterior part of the corpus callosum, or genu, is the first to be formed ; and as the hemispheres grow backwards over the hinder part of the brain, the area of fusion extends backwards also, and so causes lengthening of the corpus callosum.

The anterior pillars of the fornix develop early, as longitudinal bauds of fibres, which form the upper lips of the foramina of Monro, and then run round in the substance of the brain walls


FIG. 224. The brain of a Human Foetus of the fifth month. The brain is bisected by a median sagittal section, and the figure shows the left half from the inner surface. (From Kolliker.) Natural size.

c, corpus callosum. cc, cerebellum, cm, middle or soft commissure, cr, crus cerebri. fc, calcarine fissure, m, mid-brain, o, optic chiasma. ol, olfactory lobe, p, pons Varolii. po, parieto-occipital fissure, pr, pyramid of the medulla oblongata. r, fissura arcuata. sp, septum lucidum, forming lateral wall of fifth ventricle, u, temporo-sphenoidal lobe of cerebral hemisphere.

to its ventral surface, where they end in the corpus albicans, which is at first single and median. The posterior pillars of the fornix develop later, about the time the backward extension of the corpus callosum is taking place.

The convolutions of the cerebral hemispheres. In regard to the sulci or fissures on the surface of the hemispheres, by which the several convolutions are mapped out from one another, a distinction must be made between (i) the primary sulci, which appear at an early stage, and cause foldings of the entire thickness of the wall of the hemisphere, but which ultimately disappear wholly or in chief part : and (ii) the secondary sulci, which are mere grooves on the surface of the hemisphere, and consequently do not give rise to corresponding internal projections ; these appear late, but persist throughout life.

(i) The primary sulci appear towards the end of the second month, and occur on both the mesial and the outer walls of the hemispheres ; they attain their maximum development between the third and fourth months, and by the end of the fourth month have disappeared almost completely. It has been suggested that their formation is due to the brain increasing in size more rapidly than the skull, and consequently becoming thrown into folds ; while at a later stage, when the skull enlarges, most of the folds become flattened out and obliterated.

On the mesial wall of each hemisphere a long curved sulcus, the fissura arcuata, appears towards the close of the second month. It runs parallel to the upper border of the hemisphere, and a little distance from this ; and extends from the anterior end of the frontal lobe round to the temporo-sphenoidal lobe (cf. Fig. 224). From the fissura arcuata a series of furrows, usually six to eight in number, radiate outwards towards the margin of the hemisphere.

On the outer wall of the hemisphere the primary sulci are less regularly arranged. In a general way, they start from the margin of the hemisphere and converge towards the Sylvian fissure, but do not meet this.

The obliteration of the primary sulci is mainly a process of unfolding, progressing from, the ends of the sulcus towards its middle ; the sulcus becoming shorter and shorter, and ultimately disappearing.

It is not quite certain whether any of the primary sulci normally persist as permanent sulci ; but it appears that three or four of the most strongly marked ones do persist as a rule, or else are replaced by permanent sulci which are formed along the same lines. The hippocampal, and portions of the calcarine and parieto-occipital sulci belong to this category. The Sylvian fissure is also a permanent one, but it differs in some respects from the primary sulci, and can only doubtfully be referred to the same group as these.

(ii) The secondary snlci. During the fifth month, and the early part of the sixth month, the surface of each hemisphere is almost smooth, the primary sulci having almost completely disappeared, and the secondary having not yet appeared. During the latter part of the sixth month, and during the seventh month, most of the principal secondary or permanent sulci appear ; but the majority of the minor or accessory sulci, to which the complex appearance of the adult brain is so largely due, are not formed till after birth.

The secondary sulci vary considerably in different individuals, and on the two sides of the same brain. The purpose of their formation appears to be to maintain the proportion of the superficial grey matter, relatively to the more deeply placed mass of white matter, in the hemispheres.

c. The Thalamencephalon

The side walls of the thalamencephalon thicken very early, and form the optic thalami, the outer surfaces of which subsequently fuse extensively with the corpora striata, in the manner noticed above.

The roof of the thalamencephalon is thin, almost from the first ; it remains flat up to the end of the fourth week, when it becomes folded to form a longitudinal,' externally projecting ridge. During the third month this ridge becomes inflected into the ventricle, and vascular folds of connective tissue, growing in between its two layers, give rise to the choroid plexus of the third ventricle. The pineal body does not appear until the end of the fifth, or beginning of the sixth week. It at first projects forwards, but later on becomes directed backwards, and its cavity gradually becomes blocked up by calcareous deposits.

The floor of the thalamencephalon is separated from the Sylvian aqueduct of the mid-brain by a strong overhanging crest. The floor is at first thin along its whole length, but becomes thickened in front by the optic chiasma, and behind by the corpus mammillare. The infundibulum is a prominent, ventrally directed depression of the floor, which early comes into close relation with the pituitary diverticulum of the stomatodaeum.

d. The Mid-brain

The mid-brain of the human embryo remains small throughout the whole period of development. The roof thickens, but remains for some time undivided. Early in the fifth month a median longitudinal groove is formed along its anterior part, and shortly afterwards a pair of transverse grooves appear, dividing the roof into a larger anterior, arid a smaller posterior division. The median groove, which divides the posterior part into right and left lobes, is not completed until the seventh month.

In connection with the floor of the mid-brain the crura cerebri are formed, as a pair of thick bundles of longitudinal nerve fibres.

e. The Cerebellum

The general history of the cerebellum has already been given. The surface remains smooth until the end of the third month. During the fourth month the convolutions and sulci appear, and rapidly increase in number and in importance. From the fourth month onwards the lateral lobes grow rapidly, and at the same time the transverse fibres of the pons Varolii are developed.

f. The Medulla Oblongata

The roof of the medulla oblongata is wide and thin, almost from the first. The floor, along the actual median line, is also thin ; the sides are greatly thickened, and are divided by wellmarked grooves along their inner surfaces (cf. Fig. 228) into ventro-lateral and dorse-lateral areas.

It has been recently pointed out that a similar division may be recognised in the side walls of the more anteriorly situated portions of the brain, the ventro-lateral areas forming the ventral half of the brain as far forwards as the optic chiasma ; while the cerebellum, the optic lobes, and the whole of the cerebral hemispheres belong to the dorso-lateral areas. It is uncertain as yet whether this distinction is of any real morphological importance.

2. The Spinal Cord and Spinal Nerves

The histological development of the spinal cord and nerves in human embryos has been studied in considerable detail, more especially by Professor His ; and it is on his descriptions that the following account is mainly based.

The spinal cord, in the early stages of its development, i.s merely a specialised tract of epithelium. Some of the component epithelial cells remain throughout life in an indifferent state, and give rise to the intrinsic skeletal framework of the adult cord, while other cells become modified to form the nerve cells and nerve fibres ; the nerve fibres arising, at any rate in NZ


FIG. 225. A transverse section through a portion of the wall of the spinal cord of u Human Embryo at the beginning cf the fourth week. The entire thickness of the wall is represented. The upper border of the figure corresponds to the inner surface of the spinal cord, next to the central canal ; the lower border of the figure to the outer surface of the spinal cord. (From His.) x 760.

NT, nuclei of the spon<rioblasts. !N"K. processes of the spongioblasts which unite to form the network or inyt'lospongium. M"X, germinal cull. N"Z, neuroblast.

the first instance, as direct prolongations of the protoplasmic bodies of the nerve cells.

The spinal cord consists at first of a single layer of columnar epithelial cells, each cell extending the whole thickness of the wall. In the mid-dorsal and mid-ventral lines, where the wall is thin, the cells are comparatively short ; but at the sides they are greatly elongated. As commonly happens in columnar epithelium, the nuclei of the several cells are placed at different levels, and so cause the epithelium to appear as though two or more cells thick.

These columnar epithelial cells are spoken of as spongioblasts, and give rise to the skeletal framework of the spinal cord. At the beginning of the fourth week (Fig. 225) each spongioblast is greatly elongated, and consists of a central body, which incloses an oval nucleus, Ni, and from which two main processes arise, inner and outer. The inner process, which is directed towards the central canal of the spinal cord, is broad, and usually unbranched ; it reaches the inner surface of the cord, where it expands to form a wide foot, which unites with those of adjacent spongioblasts to form a continuous lining to the central canal, the membrana limitans interna. These inner processes vary in length in different spongioblasts, according to the position of the nuclei ; they are all striated longitudinally.

The outer processes of the spongioblasts, though retaining a generally radial direction, branch freely : towards their outer ends they form flattened expansions, which unite with one another, and with the processes of adjacent spongioblasts, to form a reticulum, the myelospongium, NK. The outer ends of the branches reach the membrana limitans externa, on the outer surface of the spinal cord.

The cells forming the mid-dorsal and mid-ventral walls of the spinal cord remain much shorter than those of the sides, but undergo similar changes.

The germinal cells. Between the inner ends of the spongioblasts, close to or in contact with the internal limiting membrane, large spherical cells (Fig. 225, NX) are formed ; these have large nuclei, and usually show mitotic figures, indicating active cell-division. These germinal cells, as they are called, appear about the beginning of the fourth week ; they are at first few, but rapidly increase in number, and by the end of the week form an almost continuous layer along the inner surface of the spinal cord. The mode of origin of these germinal cells has not been very clearly determined ; but it appears certain that they are derived from the spongioblasts, and probably by direct modification of these. It is also uncertain whether the formation of germinal cells is limited to the inner surface of the spinal cord, or whether it may occur at all parts of its thickness.

The neuroblasts (Fig. 225, xz) are pear-shaped cells, which appear in the earlier part of the fourth week ; they lie at first close to the inner wall of the spinal cord, and are believed to be formed by division of the germinal cells, though it is possible that they may also arise directly from the spongioblasts. Each neuroblast consists of a large ovoid nucleus, surrounded by a thin layer of protoplasm which is produced at one pole into a long, striated tail. The ueuroblasts become the nerve cells of the adult spinal cord, while their tails, by further elongation, become the axis cylinders of the nerves, round which at a later stage the medullary and Schwann's sheaths are formed.

Each neuroblast at first gives rise to only one process or tail, which is directed towards the outer surface of the spinal cord. After their first formation, the neuroblasts wander outwards, apparently by their own activity, to the outer layers of the cord, where they lie about the junction of the nuclear and reticular layers of the myelospongium. The bodies of the neuroblasts remain embedded in the spinal cord, but the tails, or axis-cylinder processes, grow outwards, threading their way through the meshes of the myelospongium, and ultimately reaching the outer surface of the spinal cord.

The neuroblasts increase rapidly in numbers during the fourth week ; they move outwards towards the surface of the cord, and at the end of the week (Fig. 226) form a well-marked layer, NZ, spoken of as the mantle layer, just beyond the nuclei of the spongioblasts, Ni. After the withdrawal of the neuroblasts from the inner surface of the spinal cord, the spongioblasts in this region close in, and become arranged as a layer of columnar cells, which acquire cilia at their free ends, and form the characteristic epithelial lining of the central canal of the spinal cord.

The motor roots of the spinal nerves. In the latter part of the fourth week, the neuroblasts (Fig. 226) are much more abundant in the ventro- lateral regions of the spinal cord than elsewhere. They soon become arranged more or less definitely in groups, and the axis-cylinder processes, converging to form bundles, grow out beyond the outer surface of the spinal cord and form the ventral or motor roots of the spinal nerves, NV. The first trace of these motor roots appears about the twentyfourth day, and by the end of the fourth week they are well established along the greater part of the length of the cord.


The ventral or anterior commissure of the spinal cord. The neuroblasts of the dorse-lateral areas of the cord also give off nerve processes ; but these, in place of passing out beyond the cord, run in its walls. Some of the nerve fibres take a longitudinal course, and give rise to the white columns of the cord ; while others (Fig. 226) run downwards to its ventral surface, interlacing with the fibres of the motor roots, and, on reaching the mid-ventral surface, pass across to the opposite side of the cord, and so give rise to the ventral or anterior commissure.


FlG. 226. A diagrammatic transverse section across the spinal cord of a Human Embryo of the fourth week. (After His.) x 150.

RFC, central canal of spinal cord. NT), dorsal root of spinal nerve. NI, nuclei of spongioblasts. N"V. ventral or motor roots of spinal nerve. N"W, ventral columns of white matter. NZ, neuroblast.

The dorsal or sensory nerve roots. The early origin of the spinal ganglia in the human embryo has not been made out very satisfactorily; so far as is known, it agrees in all essential respects with that already described as occurring in chick embryos.

In Kollmann's embryo, estimated as fourteen days old (Fig. 185), the ganglion rudiments are described by Lenhossek as arising before closure of the neural canal is effected, appearing in transverse sections as small heaps of rounded cells, the neural ridges, in the angles between the external epiblast and the neural plate. On closure of the neural canal, the neural ridges of the two sides become continuous with each other in the median plane, to form the neural crest. The neural crest separates from the external epiblast, but remains in close contact with the spinal cord, forming a mass of spherical cells, wedged in like a keystone between the dorsal edges of the neural plate.

As the edges of the neural plate grow in towards each other, to complete the dorsal wall of the spinal cord, the neural crest is gradually squeezed out from between them, and its median part thins away and disappears. From the lateral edges of the neural crest outgrowths arise, which form the rudiments of the spinal ganglia : these are at first exceedingly slender.

The immediately succeeding stages in the development of the ganglia have not been followed satisfactorily in human embryos. About the middle of the fourth week the ganglia have attained considerable size, and neuroblasts are present in them in large numbers. These neuroblasts differ from those of the spinal cord in being bipolar in place of unipolar, each neuroblast giving off two processes in opposite directions, inwards and outwards respectively. The inwardly directed processes grow from the ganglion into the spinal cord, and give rise to the dorsal or sensory root of the nerve (Fig. 226, ND) ; while the outwardly directed processes give rise to the sensory portion of the trunk of the nerve. It is stated that all the cells of a spinal ganglion send nerve processes into the spinal cord, but it is not yet certain whether all the fibres of a dorsal root are directly connected with ganglion cells.

The later stages of development of the spinal nerves need not be described in detail. The neuroblasts give rise directly to the nerve cells of the cord and ganglia, each neuroblast, in the later stages, giving off processes which come into close relation with those of adjacent cells, but apparently do not anastomose with these. Each nerve fibre arises in the first instance as a process of a single cell or neuroblast, but it is not quite clear in what mode its further growth is effected. His and others maintain that it is simply by a continuation of the process by which it first arose, and that the axis cylinder throughout its whole length is to be regarded as a direct prolongation of the body of the nerve cell from which it arises.


Other investigators hold that in the further elongation of the axis cylinder, after its first appearance, the cells in the neighbourhood are actively concerned, the nerve fibre being formed either by the linear fusion of originally independent cells, or as a process of secretion, by the surrounding cells. The balance of evidence at present appears to be decidedly in favour of the first-mentioned view, i.e. that a nerve fibre is to be regarded throughout its whole length as a process of a single nerve cell.

The blood-vessels of the spinal cord do not appear until the beginning of the fifth week ; they are carried into the cord by connective tissue, which grows into its substance from without.

The spinal cord steadily increases in diameter, mainly through the formation of the longitudinal bands of white matter, i.e. of nerve fibres, on its outer surface. The median fissures of the cord are formed in the same way as in other Vertebrates, the ventral fissure being a chink left between the ventral columns of the cord ; while the dorsal fissure is of entirely different origin, and is due to the absorption of the substance of the cord along the dorsal surface in the median plane.

The seat of most active nerve growth in the early stages is the neck, the cervical nerves being, both relatively and absolutely, larger than the hinder ones during the early stages.

The cervical and brachial plexuses commence to form about the twenty-seventh day ; the lumbo-sacral plexus rather later, about the thirtieth day (Fig. 227). The phrenic nerve appears about the thirtieth day as a branch of the fourth cervical nerve.

The cervical and lumbar enlargements of the spinal cord are present in the second month, and are well marked by the end of the third month (Fig. 218).

The spinal cord originally extends to the last caudal vertebra ; and up to the end of the third month the growth of the spinal cord keeps pace with that of the vertebral column. From the fourth month onwards the vertebral column grows more rapidly. By the sixth month the spinal cord only extends to the sacral vertebrae ; at birth it stops at the third lumbar vertebra, while in the adult its lower end is opposite the lower border of the first lumbar vertebra. This shortening of the spinal cord relatively to the vertebral column is the cause of the obliquity of the roots of the hinder spinal nerves, which have to run back some distance along the vertebral canal before reaching their foramina of exit.

3. The Cranial Nerves

The structure of the brain in the early stages of development, and the sequence of changes which it undergoes, are similar to those of the spinal cord in all essential respects.

In the latter part of the third week a myelospongium, or epithelial framework is formed : in this an outer or mantle layer, containing neuroblasts, can early be distinguished from a thicker inner plate in which lie the nuclei of the spongioblasts. The neuroblasts give rise to axis-cylinder processes, which either collect in bundles and grow out from the brain as the motor roots of the cranial nerves, or else run in the substance of the brain, longitudinally, obliquely, or transversely, to form the tracts of white matter, or nerve fibres, which connect the brain with the cord, and the several parts of the brain with one another. Other bundles of nerve fibres enter the brain by growing into it from the ganglia of the sensory cranial nerves.

Histological differentiation is established in the medulla oblongata even earlier than in the spinal cord. In the cerebral hemispheres it does not appear until a comparatively late stage of development. All the cranial nerves are definitely formed by the end of the fourth week (Fig. 227).

The cranial nerves are more difficult to deal with than the spinal nerves, on account of their want of uniformity in arrangement, and the great differences in size and in relations which they present among themselves.

With the possible exception of the optic nerve, however, it appears that the cranial, like the spinal nerves, may be divided into two categories :

(i) Centrifugal or motor nerves, which are formed by outgrowth of axis-cylinder processes from groups of neuroblasts situated in the brain itself.

(ii) Centripetal or sensory nerves, which are formed by outgrowth of axis-cylinder processes from groups of neuroblasts situated, not in the brain, but in the sensory ganglia outside the brain ; the processes growing in two directions, inwards into the substance of the brain, and outwards to the peripheral distribution of the nerve.



FiG. 227. Diagrammatic figure of a Human Embryo, lettered by Professor His, Ko, and estimated as thirty-one days old. The brain and spinal cord, and the cranial and spinal nerves are shown, and certain of the other organs are represented in outline. The bases of the fore and hind limbs are indicated by the dotted outlines. In all cases in which the full length of the nerve is not shown, the end is represented as though cut across. (After His.) x 10.

BF, thalamencephalon. BM, mid-brain. BS, cerebral hemisphere. B Y, olfactory lobe. El, auditory vesicle. FG-, Froriep's ganglion. GC, ciliary ganglion. TTTVf , hyomandibular cleft, or external auditory meatus. N.I, ganglion of first cervical nerve. IsT .9, ganglion of first thoracic nerve. M".21, ganglion of first lumbar nerve. M".26, ganglion of first sacral nerve. W.31, ganglion of first coccygeal nerve. If H, phrenic nerve. OC, optic cup. OF, olfactory pit. RB, left auricle. RV, ventricle. STJ. siim.s prascervicalis. TI, vitelline loop of intestine. TL, tail. ~W, liver. Ill, third cniuial nerve. IV, fourth cranial nerve. V, Gasserian ganglion. V, ophthalmic branch of fifth, or trigeminal nerve. V&, maxillary branch of fifth, or trigeminal nerve. Vc, mandibular branch of fifth, or trigeminal nerve. VII, ganglion of the seventh, or facial nerve. VIII, ganglion of the eighth, or auditory nerve. IX, ninth, or glossopharyngcal nerve. X, ganglion of the root of the tenth, or pneumogastric nerve. XI. roots of the eleventh, or spinal accessory nerve. XII, roots of the twelfth, or hypoglossal nerve.

The nerves of the first set, i.e. the motor nerves, have localised centres of origin in the brain : the nerves of the second set, or sensory nerves, are not definitely localised in the brain, except by the points at which the fibres enter the brain. The two groups of nerves arise independently, as in the case of the spinal nerves. They may retain their independence, forming purely motor, or purely sensory nerves ; or they may become more or less closely associated with one another to form nerves of mixed, motor and sensory, function.

The course of the cranial nerves in the early stages of their development is curiously straight (Fig. 227) ; their main direction, like that of the spinal nerves, being at right angles to the axis of the head, or brain, at their points of origin. This initial course is liable to disturbance through shifting relations of the parts with which the nerves are in connection, or through growth of the skeletal or other neighbouring parts. Thus the facial nerve is at first straight, but, owing to the telescoping of the hinder visceral arches within the anterior ones, its course becomes much modified (Fig. 227, vn).

In many instances some further explanation is required : thus the glossopharyngeal nerve extends forwards in front of its proper territory, in order to reach the circumvallate papilla3 of the tongue ; while the facial nerve extends forwards to the forehead. An interesting case is the extension of the pneumogastric nerve to the heart, lungs, and stomach. The posterior limit of the head may be taken as indicated by the hinder border of the second branchial arch ; or, in the adult, by the boundary line between the thyroid and cricoid cartilages, if Callender and His are right in regarding the thyroid cartilage as developed from the cartilage of the second branchial arch. In any case, the heart, lungs, and stomach are, in the adult, far behind the head region. It must be remembered, however, that the heart originally lies between the ventral ends of the visceral arches, and that the lungs arise from the floor of the pharynx, so that both heart and lungs really lie within the proper area of distribution of the pneumogastric nerve. The stomach, however, does not do so, and in order to reach it the pneumogastric nerve must pass beyond the limits of its own territory.

In describing the cranial nerves individually it will be convenient to arrange them in two groups, in accordance with the distinction laid down above, and to describe the nerves of each group in order, from behind forwards.

Group A. Nerves arising from groups of neuroblasts in the substance of the brain, in the same way as the motor or ventral roots of the spinal nerves.

To this group belong the third, fourth, and sixth nerves ; the motor root of the trigeminal nerve ; the facial nerve ; the motor roots of the glossopharyngeal and pneumogastric nerves ; and the spinal accessory and hypoglossal nerves.

Along the spinal cord, the motor roots all leave the cord at the same horizontal level, the sole exception being at the anterior end of the cervical region, where the hinder roots of the spinal accessory nerve arise at a level dorsal to that of the motor spinal roots. In the brain there are two series of motor roots, a ventral series and a lateral series ; the ventral series including the hypoglossal , the sixth, and perhaps the fourth and third nerves as well ; and the lateral series including the anterior roots of the spinal accessory, and the motor roots of the pneumogastric, glossopharyngeal, facial, and trigeminal nerves.

The hypoglossal, or twelfth cranial nerve (Fig. 227, xn), arises by a long series of roots, each formed by a bundle of axis cylinders which arise as outgrowths from a group of neuroblasts in the ventro-lateral wall of the medulla oblongata (Fig. 228, xn). The roots commence just in front of the motor root of the first spinal nerve, and in line with this, and extend forwards to the level of the glossopharyngeal nerve and the posterior border of the auditory vesicle.

The mode of origin, and the position and relations of these roots, strongly suggest a comparison with the ventral or motor spinal roots.

In sheep embryos Froriep describes a dorsal ganglionic root of the hypoglossal nerve, in addition to the ventral roots, so the comparison with a spinal nerve or nerves seems quite legitimate. In human embryos at the end of the fourth week, and beginning of the fifth week, His has described a small ganglion, which he names Froriep's ganglion (Fig. 227, FG), lying immediately in front of the first cervical ganglion, N.i, and in line with this. Froriep's ganglion is small and gives off no nerves at all, and at a slightly later stage it disappears altogether ; it appears, however, to correspond to the ganglion described by Froriep in sheep embryos as forming a true dorsal root to the hypoglossal nerve.

It is probable, therefore, that the hypoglossal nerve is to be regarded as formed by the ventral roots of one or more nerves equivalent to spinal nerves; and of which the dorsal root, or



FIG. 228. Transverse section across the medulla oblongata of a Human Embryo, lettered by Professor His, Ko, and estimated as thirty-one days old. The embryo is the same one as that represented in Fig. 227, and the section passes through one of the roots of the hypoglossal nerve, and through both the motor and sensory roots of the pneumogastric nerve. (From His.) x 40.

X.M, motor root of pneumogastric nerve. X.S, season- root of pneumogastric nerve. XII, root of hypoglossal nerve.

roots, are represented in man by the rudimentary Froriep's ganglion alone.

The spinal accessory, or eleventh cranial nerve (Fig. 227, xi), arises by a number of roots, formed by outgrowths from groups of neuroblasts in the side of the medulla oblongata ; the roots lying at a level dorsal to that of the hypoglossal roots, and at the junction of the ventro-lateral and dorso-lateral regions of the medulla (cf. Fig. 228).

The roots of the spinal accessory nerve are very numerous. At the beginning of the fifth week (Fig. 227) the most posterior root lies in close relation with Froriep's ganglion, FG, and a very little distance in front of the first cervical nerve ; while the most anterior root lies just behind the pneumogastric nerve. The cervical roots of the spinal accessory nerve do not appear until a later stage, a probable indication that the spinal accessory is to be regarded as a cranial rather than as a spinal nerve.

The motor roots of the pneumogastric, or tenth cranial nerve (Figs. 227 and 228, XM). These lie immediately in front of the anterior roots of the spinal accessory nerve, and in line with them. They arise from groups of neuroblasts in the walls of the medulla oblongata (Fig. 228), in a manner precisely similar to that in which the ventral spinal roots are formed. The nerve fibres converge to form small bundles which leave the medulla immediately ventral to the much larger and more conspicuous sensory root (Fig. 228, x.s), by which they are covered and more or less completely concealed.

The motor roots of the glossopharyngeal, or ninth cranial nerve, are exactly similar to those of the pneumogasti'ic ; they lie immediately in front of these, and in line with them, and in transverse sections present an appearance practically identical with that shown for the pneumogastric nerve in Fig. 228, X.M.

The facial, or seventh cranial nerve (Fig. 227, vn), arises from a group of neuroblasts in the side wall of the medulla oblongata, opposite the auditory vesicle. The bundle of axis cylinders, formed as outgrowths of the neuroblasts, does not at once pass out from the medulla, but runs forwards a short distance in its substance, and emerges immediately below the auditory nerve and in very close relation with this. The root of the facial nerve lies in line with the motor roots of the glossopharyngeal and pneumogastric nerves, i.e. it belongs to the lateral series of motor roots.

The chorda tympani is present early in the fifth week as an anterior branch of the facial nerve, which runs in the tympanic membrane, but does not yet reach the trigeminal nerve.

The sixth cranial nerve belongs to the ventral series of motor roots. It arises from several groups of neuroblasts which lie in the ventro-lateral area of the medulla oblongata, in line with the hypoglossal roots, and vertically below the root of the auditory nerve, i.e. a short distance anterior to the root of the facial nerve. The sixth nerve, after emerging from the brain, runs almost directly forwards, lying to the inner side of the (iasserian ganglion, and reaches the external rectus muscle early in the fifth week. The nerve is drawn, though not named, in Fig. 2 2 7, as a thin band emerging from the ventral surface of tin- brain immediately below the ganglion of the facial and auditory nerves, and running horizontally forwards towards the hinder border of the eye.

The motor root of the trigeminal, or fifth cranial nerve, lies at a level slightly ventral to the motor roots of the facial, glossopharyngeal, and pneumogastric nerves, but clearly belongs to the lateral rather than to the ventral series of roots. It lies to the inner side of the Gasserian ganglion, and in close relation with this, and is, in its early stages, slightly anterior to this in position.

The fourth cranial nerve (Fig. 227, iv), though leaving the brain on the mid-dorsal surface, is stated by His to arise from a group of neuroblasts on the ventral surface of the isthmus, or constricted neck between the hind- and mid-brains. These roots lie close to the mid-ventral plane, and clearly belong to the ventral series. From this origin the fibres of the fourth nerve run up, in the sides of the brain, to its dorsal surface, cross those of the opposite side in the mid-dorsal plane, and finally emerge from the brain as the definite nerves.

Though very slender, the fourth nerves are of considerable length by the early part of the fifth week (Fig. 227), already reaching to the level of the eye.

The fourth nerve has long been a source of trouble to morphologists. Professor His' observations on its development in human embryos will, if confirmed and extended to other Vertebrates, throw a very welcome light on the problem, showing that, in spite of the peculiar position at which it leaves the brain, the fourth nerve really belongs to the category of ventral or motor roots.

The third cranial nerve (Fig. 227, m) arises from a group of neuroblasts in the floor of the mid-brain, rather further apart than the other ventral roots, but belonging to the same series.

Group B. The nerves included in this series arise from groups of neuroblasts, not in the brain, but in the ganglia, i.e. they are developed in the same manner as the dorsal or sensory roots of the spinal nerves.


To this group belong the sensory roots of the pneumogastric and glossopharyngeal nerves ; the auditory nerve ; the sensory root of the trigeminal nerve ; and probably the olfactory nerve as well.

In the head there are four primary ganglion masses, those of the fifth, eighth, ninth, and tenth cranial nerves. Whether these are connected in the early stages, to form a continuous neural ridge along each side, has not yet been ascertained ; neither has the precise mode in which the permanent connection of these ganglia with the brain is acquired been determined.

The four ganglionic masses are clearly visible at the end of the third week. During the fourth week they become gradually divided up, each giving rise to two or more ganglia, which, by further elongation of the connecting nerve strands, move apart to a greater or less distance from one another.

The sensory root of the pneumogastric, or tenth cranial nerve, is from the first in close relation with the motor root, being attached to the brain immediately dorsal to this latter (Figs. 227 and 228, x.s). The ganglion is at first single, but by the end of the fourth week it becomes divided into a proximal and smaller part, the ganglion of the root ; and a distal, larger, and fusiform part, the ganglion of the trunk (Fig. 227). In the later stages these two ganglia move some distance apart, owing to lengthening of the nerve trunk between them. The ganglion of the root is connected with the distal, or petrous, ganglion of the glossopharyngeal nerve by an oblique commissural band, well seen in Fig. 227 : whether this is a persistent remnant of an originally continuous neural ridge has not been determined.

By the end of the fourth week, the superior and inferior laryngeal nerves are present, and also a large branch extending down the oesophagus towards the stomach.

The sensory root of the glossopharyngeal, or ninth cranial nerve, is very similar to that of the pneumogastric, but of smaller size. The ganglion early divides into a proximal 'jugular ' portion, and a distal ' petrous ' portion (Fig. 227). The nerve itself is straight in the early stages, but becomes curved forwards at its ventral end (Fig. 227) as the first branchial arch, with which it is specially associated, is carried forwards along the inner side of the hyoid arch (cf. Fig. 240).

The auditory nerve. In the case of the auditory ganglion of


536 THE HUMAN EMBRYO.

the human embryo, nerve fibres have been traced growing out from the nerve cells of the ganglion into the brain, the attachment taking place immediately dorsal to the point of emergence of the facial nerve. Beyond its root of attachment, the ganglion of the auditory nerve divides into two main portions, the cochlear and vestibular ganglia : these diverge from each other, and between them the root of the facial nerve is wedged. The auditory ganglia very early acquire connection with the wall of the auditory vesicle, and the several ganglia of the adult ear are formed by further division of the two ganglia of the embryo.

It is stated that the geniculate ganglion of the facial nerve is derived from the same ganglionic mass from which the auditory ganglion is formed.

The sensory root of the trigeminal, or fifth cranial nerve. The ganglion of the trigeminal nerve is, from the first, of great size (Fig. 227). The three principal branches of the nerveophthalmic, maxillary, and mandibular (Fig. 227, V, a b c) are already present, and of large size, before the end of the fourth week ; as these nerves lengthen, the originally single ganglion gradually breaks up, small portions becoming detached, and moving out along the growing nerve stems. In this way, early in the fifth week, the ciliary, sphenopalatine, and otic ganglia are established ; the submaxillary ganglion is not separated until a rather later stage. The main ganglion persists as the Gasserian ganglion of the adult, and the motor root of the trigeminal nerve lies along its inner side, and in close contact with it (Fig. 227, v).

The optic nerve. The optic vesicle and optic stalk are parts of the brain, and cannot be compared with nerves, either sensory or motor. There is, however, strong reason to think that the actual optic nerve fibres do not arise in the optic stalk, but are formed independently, as outgrowths from the retinal cells which grow inwards to the brain, following the line of the optic stalk, but being fundamentally independent of this. Even then, however, inasmuch as the retina is developmentally part of the brain, the optic nerve would rather resemble the intra-cerebral fibres of the brain than the ordinary sensory nerves ; and for the present the relations of the optic nerves to the other nerves must be left undecided.


THE CRANIAL NERVES. 537

The olfactory nerve. According to the observations of Professor His, the mode of development of the olfactory nerve in the human embryo is as follows. The olfactory lobe is formed as an outgrowth of the cerebral hemisphere towards the end of the fourth week, and very early becomes divided by a transverse constriction into anterior or distal, and posterior or proximal portions.

At this stage, in embryos of from twenty-seven to twentyeight days, although the olfactory pit is well developed (Figs. 204 and 227), there is, according to His, no trace of either the olfactory ganglion or olfactory nerve. A day or two later, the olfactory epithelium begins to undergo changes similar to those which occur in the wall of the brain, or spinal cord, preparatory to the appearance of nerves. Neuroblasts are formed near its inner or deeper surface : these soon become pyriform, and give off processes which grow into the mesoblast, and towards the brain. There is thus, early in the fifth week, a mass of neuroblasts forming a ganglion in direct connection with the olfactory epithelium : from the ganglion, nerve fibres grow out towards the brain, but do not yet reach this. By the end of the fifth week the nerve fibres reach the olfactory lobe, meeting it at the constriction separating the proximal and distal portions, and thus placing the olfactory epithelium in connection with the brain.

During the second month the distal portion, or bulb of the olfactory lobe, which at first lies entirely in front of the nerve, becomes bent down so as to lie in contact with this ; and by the end of the second month the olfactory nerve arises by a number of fibres from the olfactory bulb, instead of by a single stem from the olfactory lobe behind the bulb, as in the earlier stages.

The roots of the adult olfactory nerve are formed by bundles of ascending or centripetal nerve fibres, which grow from the ganglion into the brain ; they are already present at, or shortly after, the end of the second month.

At first sight this account appears to differ widely from the descriptions given above of the development of the other sensory nerves ; but the differences are not really so great as they seem. In the case of all sensory nerves the connection with the brain, or spinal cord, is acquired by growth of nerve processes centri


538 .THE HUMAN EMBRYO.

petally, from the ganglia into the brain or cord : the ganglia themselves, though developed in close relation with the brain and cord, are not really parts of these, but are independent structures. The formation of neuroblasts in the olfactory epithelium presents no difficulty, when it is remembered that the wall of the brain or spinal cord is itself merely a specialised portion of the surface epithelium ; while, finally, it has been shown in earlier parts of this book that in other Vertebrates, such as the frog and chick for example, the surface epithelium may, in the hinder cranial nerves, take a direct share in the formation of the nerve ganglia.

It is probable indeed that the mode of development of the olfactory nerve described above, as observed in human embryos, represents a more primitive type of nerve development, from which that of the other sensory nerves has been derived.

4. The Sympathetic Nervous System.

The mode of development of the sympathetic nervous system has not been accurately determined in human embryos. From the close resemblance in the mode of development of other parts of the nervous system, it is probable that it takes place in essentially the same manner as described above in the case of the chick and rabbit.


THE DEVELOPMENT OF THE SENSE ORGANS.

1 . The Nose.

All the essential points in the development of the olfactory organ have been already described. The formation of the olfactory pit, the organ of Jacobson, the external nostrils, the bridge and alee of the nose, and the posterior narial passages, are dealt with on pp. 494 to 498 ; and the development of the olfactory lobe and olfactory nerve on pp. 517 and 537.

During the third month the olfactory pits, which are at first simple, become greatly complicated by folding of their walls ; and in this way the nasal labyrinth, supported by the turbinal scrolls, is established. The accessory cavities communicating with the nose, i.e. the antrum, and the frontal, sphenoidal, and ethmoidal sinuses, are not established until a later stage.


THE NOSE AND EYE. 539


2. The Eye.


The mode of development of the human eye is so closely similar to that of the rabbit that it will be needless to describe it in detail.

The optic vesicles appear as lateral outgrowths of the forebrain as early as the fifteenth day (Fig. 232, BO). They soon become constricted at their bases, and then doubled up to form the optic cups, in the same manner as in other Vertebrates. Owing to the mode in which this doubling up is effected, a choroidal fissure is left, leading into the cavity of the cup ; and, as in the rabbit, the choroidal fissure extends a little way along the optic stalk, towards the brain. Throughout all the earlier stages of development (Fig. 204), the eye is very small, as in Mammals generally, standing in this respect in marked contrast to the eye of the chick embryo at a corresponding stage of development (cf. Fig. 115).

The inner wall of the optic cup (cf. Fig. 155) is from the first the thicker of the two ; and by the end of the fourth week is at least four times as thick as the outer wall.

The lens develops late. In embryos three weeks old it is still an open pit : at four weeks the mouth of the pit has closed (Fig. 204), and from this time the cavity of the lens vesicle becomes rapidly filled up by elongation of the cells forming its inner or deeper wall. During the whole period of its development, the lens is enveloped in a vascular capsule which serves for its nourishment. New cells are constantly added on round the equator of the lens, and growth continues until the time of birth, when the vascular capsule atrophies and disappears.

The vitreous body is formed of mesoblast, which makes its way into the cavity of the optic cup through the choroidal fissure : it is very vascular during the earlier stages of its formation.

The cornea is formed from a layer of mesoblast which grows across the front of the eye, between the outer conjunctival epithelium and the lens, towards the end of the second month. In the deeper part of this layer a cavity appears, which becomes the anterior chamber of the eye. The thick layer of mesoblast in front of this chamber becomes the cornea, while the much thinner layer between the chamber and the lens forms the


540 THE HUMAN EMBRYO.

anterior wall of the lens capsule. The cornea becomes transparent during the fourth month, at which time it is strongly convex, more so than in the adult. The cornea is very thick from the first, and much thicker than the sclerotic ; and at the time of birth it is said to be absolutely thicker than in the adult.

The choroid is a very vascular layer, in which pigment begins to appear towards the end of the second month. The occasional occurrence in the adult human eye of a non-pigmented streak along the under surface of the eyeball, or even of complete fissure of the iris, or coloboma iridis, is commonly attributed to incomplete closure of the choroidal fissure. Inasmuch, however, as the choroidal fissure concerns primarily the optic cup alone, and not the choroid coat, it is probable that coloboma iridis should not be regarded as a simple case of arrested development, but as involving some further pathological process as well. The choroidal fissure normally closes about the seventh week.

The retina is formed, as in other Vertebrates, from the inner and thicker layer of the optic cup. After its first establishment, it grows for a time more rapidly than the outer coats of the eyeball, and consequently becomes thrown into folds which during the second month project freely into the cavity of the eye. In the later stages it becomes flattened out again. The rods and cones are formed as outgrowths from the outer surface of the thickened inner layer of the optic cup : they do not appear until very late ; shortly before the time of birth.

The mode of origin of the fibres of the optic nerve of the human eye has not been determined with exactness. It appears certain, however, that the nerve fibres are not formed out of the walls of the optic stalk, and it is probable that they arise as outgrowths from neuroblasts in the retina itself, which grow inwards towards the brain along the path afforded by the optic stalk ; and on reaching the base of the thalamencephalon pass across to the opposite side of the brain, thus forming the optic chiasma, and continue their course up the sides of the brain as the optic tracts, until they finally reach the corpora quadrigemina.

The eyelids appear, towards the close of the second month, as folds of skin above and below the eyeball (Fig. 213) ; they


THE EYE AND EAE. 541

unite with each other, thus closing the eye, about the third or fourth month, and separate again shortly before birth.

The lacrymal duct is formed along the line of the lacrymal groove, as a linear depression running from the eye to the nose, along the line of meeting of the external nasal process and the maxillary arch (Fig. 207). The duct itself arises as a solid rod of epithelial cells split off from the floor of the groove : it becomes sinuous at an early stage, and from its sides the lacrymal glands arise as solid branched outgrowths, with dilated, hollow, bulb-like ends. At a later stage the solid cords become hollow along their axes, and converted into the lacrymal ducts. At the inner canthus of each eye the duct bifurcates, while still a solid rod, to form the rudiments of the upper and lower lacrymal canals.

The third eyelid, or plica semilunaris, which is rudimentary in man, arises as a small fold of the conjunctiva at the inner canthus of the eye, within the upper and lower eyelids.

3. The Ear.

The ears appear as a pair of open pits at the sides of the hind-brain on the fifteenth day (Fig. 197, EI). Almost directly afterwards the mouths of the pits close, and the vesicles, thus formed, separate from the skin. The original mouth of each pit lengthens out into an elongated neck, the recessus labyrinthi (Fig. 229, ER), while the vesicle itself forms a flattened sac, EV, somewhat oval in outline, and lying embedded in the connective tissue at the side of the hind-brain.

At the commencement of the fifth week the auditory vesicle becomes more irregular in shape. Its ventral and anterior end (Fig. 227, EI) grows forwards as a short blunt process, which forms the rudiment of the cochlea ; while, near its dorsal end, three flattened projections appear on its outer surface, which are the first stages in the formation of the three semicircular canals.

By the end of the fifth week (Fig. 230), the auditory vesicle has increased considerably in size, and its main divisions are well established. The body of the vesicle is divided by a fold into two main portions : a dorsal division, or utriculus, UT ; and a ventral division, or sacculus, s. From the utriculus the three


semicircular canals arise, the two vertical canals, EA, EP, being


THE HUMAN EMBRYO.

already separated from the vesicle, except at their ends, while the horizontal canal, EH, is still a wide, flattened, pouch-like outgrowth from the vesicle. From the sacculus, the cochlea, EL, arises as a short, blunt, anteriorly directed outgrowth. The recessus labyrinthi, ER, is much larger than before, and is divided at its lower part by a partition into two tubes, of which one opens into the sacculus and the other into the utriculus ; these tubes affording the sole communication between the two chambers, sacculus and utriculus, of the auditory vesicle.



FIG. 229.


FIG. 230.


FIG. 229. The left auditory vesicle of a Human Embryo four weeks old, seen from the outer surface (cf. Fig. 227). From W. His, jun. x ;5.~).

FIG. 230. The left auditory vesicle of a Human Embryo live weeks old, seen from the outer surface. From W. His, jun. x 35.

EA, anterior vertical semicircular canal. ED, common stem of the two vertical semicircular canals. EH, horizontal or external semicircular canal. EL, cochlea. EP, posterior vertical semicircular canal. ER, recessus labyrinthi. EV, auditory vesicle. S, sacculus. TTT, utriculus.

By the eighth week, the shape and proportions of the internal ear are as shown in Fig. 231. The sacculus, s, and utriculus, UT, are now comparatively small parts of the internal ear. The semicircular canals have greatly increased in length ; and the cochlea, EL, has grown enormously, and is rolled up spirally on itself.

The auditory nerve, as already noticed (p. 536), very early becomes continuous with tue auditory epithelium : it soon


THE EAE. 543

divides into two main portions, the vestibular and coclilear ganglia, and from these, by further division, the several nerve endings of the adult ear are derived.

The epithelial cells of the auditory vesicle, which, it will be remembered, are derived directly from the surface epidermis of the head, become variously modified in different parts of the vesicle. Over the greater part of its surface they remain flat pavement cells, while opposite the nerve endings they become altered into the hair-cells, rods of Corti, sense cells of the ampullas, and other specialised structures.

The mesoblast of the side of the head, in which the auditory EA ^^

^K

EH



FIG. 231. The left auditory vesicle, or internal ear, of a Human Embryo of the eighth week, seen from the left side. From W. His, jun. x 17.

EA. anterior vertical semicircular canal. EA', ampulla of anterior vertical semicircular canal. ED, common stem of the two vertical semicircular canals. EH, horizontal semicircular canal. EH', ampulla of horizontal semicircular canal. EL, cochlea. EP, jwsterior vertical semicircular canal. EP', ampulla of posterior vertical semicircular canal. EB, recessus labyrinth!. S, sacculus. UT, utriculus.

vesicle is embedded, undergoes important changes. The layer in immediate contact with the epithelial vesicle becomes closely connected with this, and forms the connective tissue wall of the labyrinth ; at a little distance from the labyrinth the mesoblast becomes converted into cartilage, which forms the periotic capsule. Between the cartilaginous capsule and the labyrinth itself, the intervening mesoblast breaks down to form the perilymphatic spaces surrounding the vestibule and the semicircular canals, and the two lymphatic canals scala tympani and scala vestibuli which lie above and below the scala media, or cochlear outgrowth from the labyrinth.


5i4 THE HUMAN EMBRYO.

In the later stages the cartilaginous periotic capsule becomes replaced by bone. This is in chief part spongy bone, but is lined on the surface towards the labyrinth by layers of compact bone, formed by the periosteal membrane. The modiolus and septa of the cochlea, as well as the osseous spiral lamina, are formed wholly in connective tissue, without any preformation in cartilage.

The Accessory Auditory Organs.

The Eustachian tube and tympanic cavity are formed from the hyomandibular pouch, or diverticulum from the pharynx. The pouch does not quite reach the surface at any stage of development : the membrane closing it at its outer end becomes the tympanic membrane ; and the groove or depression on the surface of the head, opposite the hyomandibular pouch, becomes, as already noticed, the external auditory meatus ; the external ear, or pinna, being formed from a series of processes developed round its margin (cide pp. 499, 500).

Up to the time of birth, the Eustachian tube, and the tympanic cavity itself in great part, are practically obliterated ; their walls being brought into contact with each other by the development of very abundant, gelatinous, connective tissue in their substance. This is absorbed about the time of birth, and the tympanic cavity and Eustachian tube opened out.


DEVELOPMENT OF THE DIGESTIVE SYSTEM.

1 . General Account.

The alimentary canal of the human embryo, like that of the chick, is at first merely the part of the cavity of the yolk-sac which is included within the embryo, as this is constricted off by the head-, side-, and tail-folds (cf. Fig. 188).

As the constriction deepens, to form the yolk-stalk, the part of the cavity within the embryo, or mesenteron, gradually becomes more and more sharply separated from the cavity of the yolk-sac proper ; the two cavities still, however, communicating freely through the yolk-stalk.

The mesenteron soon acquires a definite tubular form, and on the fifteenth day has the shape and relations shown in Fig. 232.


THE ALIMENTARY CANAL.


545


It consists of three parts, fore-gut, mid-gut, and hind-gut, which are. approximately equal in length.

The fore-gut is widened transversely at its anterior end to form the pharynx, TP, which is separated in front by a thin, obliquely placed septum, DU, from the bottom of the stomatodseal, or mouth invagination, DS. Behind the pharynx, the fore-gut narrows to form a short tubular portion, the oesophagus, which lies immediately above the heart. Behind the oesophagus is a fusiform dilatation, the stomach, TS, beyond which the fore-gut


TR


W


BL


VA


3M



FlG. 232. Human Embryo, lettered by Professor His, Lg, and estimated as fifteen days old (cf. Fig. 197). The brain and heart are exposed from the right side ; the alimentary canal and the yolk-stalk are represented in median sagittal section. (From His.) x 30. .

AA, allantoic artery. BF.thalamencephalon. BL, cerebellum. BM, mid-brain. BO, optic vesicle. DS, stomatoda?um. DTT, septum between stomatodaeum and pharynx. El, auditory pit. GH, hind-gut. Q-T, mid-gut and yolk-stalk. B.T, truncns arteriosus. B.V, ventricular portion of heart. TA, allantoic diverticulum. TP, pharyngealregion of fore-gut. TR, cloacal dilatation of hind-gut. TS, stomach. TZ, allantoic stalk. "VA, allantoic vein. ~W, liver.

passes into the mid-gut, GT, which latter opens through the wide yolk-stalk into the yolk-sac. The hind-gut, GH, is at first narrow and tubular ; but at its hinder end it dilates to form the large cloacal chamber, Til, from the ventral surface of which the allantois, TA, arises as a narrow tubular diverticulum. There is as yet no trace of a proctodaeal, or anal invagination.

In embryos about a day older than the one represented in Fig. 232, i.e. of about the sixteenth day, the stomatodagal septum is perforated, and the mouth opening established

N N


546


THE HUMAN EMBRYO.


(cf. Fig. 215). The proctodaeal opening is not formed until a much later stage, about the end of the fifth week, and is a perforation of the integument rather than a distinct pit.

During the fourth week the alimentary canal rapidly assumes more definite form. The pharynx (Figs. 216 and 233) remains of great width from side to side, and in connection with it the gillpouches, lungs, and other important structures are formed. The oesophagus rapidly increases in length as the neck elongates ; and



FIG. 233. Outline figure of the alimentary canal of a Human Embryo, lettered by Professor His, Pr, and estimated as twenty-eight days old (cf. Fig. 216). The figure is drawn from the right side, and the cavity of the alimentary canal is alone represented, not the thickness of its walls. The curved line bounding the figure on the left is the notochord. (From His.) y 15.

All. allantois. S, cloaca. Ds, yolk-stalk. Ep, epiglottis. Kt, laryngeal chamber. Lbg, bile-duct. Lg, lunp. Mg, stomach. A', ureter. P, pancreas. RT, pituitary body. Uk, mandibular arch. W, Wolffian duct. Zg, tongue.

the stomach becomes a more conspicuous dilatation. The intestine is long, narrow, and tubular ; it forms a prominent, ventrally directed vitelline loop, from the apex of which the narrow yolkstalk arises, connecting the intestine with the yolk-sac.

The alimentary canal is at first (Fig. 232) closely attached


THE ALIMENTARY CANAL.


547


to the dorsal wall of the body along its whole length, lying immediately ventral to the notochord ; and is hence equal in length to the part of the body in which it lies. During the fourth week, the intestine grows much more rapidly than the body of the embryo, and becomes thrown into loops which project ventral wards (Fig. 233). A small duodenal loop is formed immediately beyond the stomach, and opposite the bile-duct,



FIG. 234. Outline figure of the alimentary canal of a Human Embryo, lettered by Professor His, Sch, and estimated as thirty-five days old. The figure is drawn from the right side, and the cavity of the alimentary canal is alone represented, not the thickness of its walls. The curved line bounding the figure on the left is the notochord. (From His.) x 10. An, position at which the anus will be formed. Cc, caecum. CA, notochord. Cl, rectum.

Ep, epiglottis. HI), basal portion of allantois, which becomes the bladder. Kk, larynx.

Lbff, bile-duct. Lg, lung. Alg, stomach. N, rudiment of permanent kidney ormetanephros.

/", pancreas. RT, pituitary body. Sy, clitoro-peiiis. SI, tail. Tr, trachea. Ut, mandible,

or lower jaw. Zg, tongue.

Lbg ; and a much larger vitelline loop is formed lower down, from the apex of which the yolk-stalk, Ds, arises. As the intestine lengthens, its attachment to the dorsal wall of the body becomes drawn out into a thin vertical sheet of mesoblast, the mesentery, between the layers of which the blood-vessels of the alimentary canal run,

N N 2


THE HUMAN EMBRYO.


During the fifth week (Figs. 234, 235, and 236), the oesophagus lengthens very greatly ; the stomach in consequence shifts backwards, and at the same time acquires its characteristic shape (Fig. 236, Mg\ and becomes placed across the body instead of along it. The vitelline loop of the intestine (Fig. 234) passes out some distance beyond the body ; it lengthens



FIG. 235.


FIG. 230.


FIG. 235. Outline figure of the alimentary canal of a Human Embryo, estimated as thirty-two days old. The figure is drawn from the ventral ' surface, and the cavity of the alimentary canal is alone represented, not the thickness of its walls. (From His.) x 12.

FIG. 236. Outline figure of the alimentary canal of a Human Embryo, estimated as thirty-five days old. The figure is drawn from the ventral .surface, and the cavity of the alimentary canal is alone represented, not the thickness of its walls. (From His.) x 10.

Cc, caecum. C'J, rectum. Dd, duodenum. Ds, yolk-stalk, yb, gall bladder. Lb'j, bile-duct. Ly, lung. Mg, stomach. P, pancreas.

considerably, and becomes at the same time twisted on itself. Before the end of the week, the tubular yolk-stalk (Fig. 235, Ds) separates from the intestine, although detached portions of the tube may persist along the yolk-stalk for some time longer. The cascuni, Cc, arises during the fifth week as a diverticulum from the distal limb of the vitelline loop, not far from the point of attachment of the yolk-stalk.


THE ALIMENTAEY CANAL. 549

During the fifth week the cloaca, which up to this time has been a single dilated chamber (Fig. 233), becomes divided, by the growth backwards of a septum from the angle between the allaiitoic stalk and the intestine, into two separate tubes ; of these, the dorsal one (Fig. 234, C) is continuous with the intestine and forms the rectum ; while the ventral one, H&, receives the allantoic stalk, and the Wolffian ducts and ureters, and forms the urino-genital passage.

The septum which thus divides the cloaca into rectal and urino-genital chambers is formed by the union in the median plane of two lateral folds or ridges, which arise from its sides ; it reaches the surface of the body just below the root of the tail, about the end of the fifth week (Fig. 234). The proctodssal opening is formed about the same time, but it is not certain whether this takes place before or after the completion of the septum : in the former case there would be for a short time a single cloacal aperture ; in the latter case the rectal and urinogenital apertures would be distinct from the first.

The later stages in the development of the part of the alimentary canal from the oesophagus to the rectum present few features of special interest. The epithelium lining the oesophagus is ciliated during the fifth and sixth months, and perhaps for a longer period.

The mucous membrane of the stomach is smooth up to the end of the second month ; during the third month it becomes much folded, especially at the pyloric end, and in the course of the fourth month the glands commence to develop. In the intestine the villi appear towards the end of the second month, and the glands of Lieberkiihn about the beginning of the fourth month. The large intestine is at first closely similar to the small intestine, and contains numerous villi, which about the fourth or fifth month become united by folds of the mucous membrane to form a honeycomb pattern. Peyer's patches appear about the sixth month.

2. The Pharynx.

The pharynx requires special notice on account of the importance of the structures developed in connection with it.

From the first the pharynx is distinguished from the rest of the length of the alimentary canal by its great width.


550


THE HUMAN EMBRYO.



At its first formation (Fig. 237) the pharynx is of approximately uniform width along its whole length ; but at an early stage the anterior part widens very greatly and the whole pharynx becomes funnel-shaped, with the apex directed backwards (Figs. 238 and 239).

The condition of the pharynx on the fifteenth day is shown in horizontal section in Fig. 237, which should be compared

with Figs. 197 and 232, -A. 1 which represent the same embryo in surface view, and in sagittal section. The visceral arches are seen to form prominent ridges projecting into the pharynx, and separated from one another by grooves, the visceral pouches. Of the visceral arches, the mandibular, MN, and hyoidean, HY, are well developed; and behind these the first and second branchial arches, BR 1 and BR 2 , are recognisable, though less clearly defined.

The hyomandibular and first branchial pouches are well formed ; and corresponding to them on the outer surface of the pharynx are well-marked external visceral grooves, clearly seen in surface views of the embryo (Fig. 197, HM, HC 1 ). The corresponding visceral pouches and grooves, on the inner and outer surfaces of the pharynx respectively, do not quite meet, but are separated by thin membranous partitions, of which the most anterior one, EB, between the mandibular and hyoidean arches, becomes ultimately the tympanic membrane.

Further back, there are less strongly marked second branchial, and third branchial pouches or grooves on the inner surface of the pharynx, with slight indications of corresponding visceral grooves on the outer surface.

Towards the end of the third week, and in the early part of


FIG. 237. The floor of the pharynx of a Human Embryo fifteen days old, seen from above. \Cf. Figs. 197 and 232.) (From His.) x 50.

Al, first aortic arcb, in the maudibular arch. A2, second aortic arch, in the liyoid arch. BR1. first branchial arch. BB2, second branchial arch. C, body cavity, or coelom. EB, membrane closing the hyomandibular cleft, which becomes afterwards the tympanic membrane. FL, furcula. HY, hyoid arch. MN, mandibular arch. TIT, tuberculum impar.


THE PHARYNX.


551


the fourth week, the hinder visceral arches, and the pouches separating them from one another, become much more clearly defined : the pharynx also changes its shape, becoming much wider in front, and narrowing posteriorly towards the oesophagus (Fig. 238).

The mandibular, hyoid, and first and second branchial arches are well defined (Fig. 238, MN, HY, BR 1 , and BR 2 ), the hyoid arch being especially large. Both the internal visceral pouches, and the external visceral grooves between the successive arches are well marked. There is some doubt as to whether any of the gill-clefts are actually open in the human embryo ; such evidence



FIG. 238. The floor of the pharynx of a Human Embryo, twenty-three days old, seen from above. Cf. Fig. 243, which represents the same embryo. (From His.) x 30.

A2, second aortic arch, in the hyoidean arch. A3, third aortic arch, in the first branchial arch. A4, fourth aortic arch, in the second branchial arch. A5, fifth aortic arch, in the third branchial arch. BR1, first branchial arch. BR2, second branchial arch. BR3, third branchial arch. EB, membrane closing the hyornandibular cleft, which afterwards becomes the tympanic membrane. FL, furcula. HY, hyoid arch. LGr, lung. MN, maudibular arch. TU, tuberculum iinpar.

as has been obtained points to the conclusion that none of the clefts are really completed either at this or any other stage in development ; the visceral pouches and the corresponding visceral grooves being always separated by thin partitions, as at EB in Fig. 238.

The second branchial arch, BR 2 , is bounded posteriorly by the conspicuous and deep third branchial pouch ; immediately behind this is a ridge, BR 3 , projecting into the cavity of the pharynx, and bounding laterally the entrance to the oesophagus. Although there is no external ridge on the surface of the embryo corresponding to this internal ridge, yet its relations to other


552


THE HUMAN EMBRYO.


organs, and more especially the fact that in it, as in the anterior arc-lies, an aortic arch, or branch of the truncus arteriosus, A 5 , is present, show that the ridge in question, BR 3 , is really a third branchial arch.

In Fig. 238 it is seen that the second branchial arches, BR 2 , not only lie nearer the middle line than the first branchial arches, BR 1 , but are also in part overlapped by these. During the latter part of the fourth Aveek, this overlapping becomes much more marked, the posterior visceral arches shifting forwards, and being telescoped within the arches in front of them.

In Fig. 239 the condition at the end of the fourth week is shown, at which time the first branchial arches have com


TU



su


Km. 239. The floor of the pharynx of a Human Embryo twenty-eight days old, seen from above. Cf. Fig. 216, which represents the same embryo (From His.) x 30.

A.3. third aortic arch, in the first branchial arch. A.4, fourth aortic arch, in the second branchial arch. A. 5. fifth aortic arch, in the.third branchial arch. BR.1, first branchial arch. BB..2, second branchial arch. EB, membrane closing the hyomandibular cleft, which afterwards becomes the tympanic membrane. IPK, foramen caecum. TTV,' byoid arch. MM", mandibular arch. SU, sinus praecervicalis. TH, median thyroid' rudiment. TU. tuberculum impar. V.3. innndibular branch of trigeminal nerve. VII, hyoidean branch of facial nerve. IX, glosso-pharyngeal nerve. X, branchial" branches of pnenmogMtlic nerve.

pletely overlapped the second branchial arches, BR 2 , so as to conceal them in surface views of the embryo.

During the fifth week the first branchial arches are in their turn overlapped and concealed by the hyoid arches (Fig. 2 10). so that in surface views of embryos of this age none of the arches behind the hyoid can be seen (cf. Fig. 205).


THE PHAEYNX.


553


By this telescoping of the visceral arches a deep cleft is formed at each side of the neck, extending round to its ventral surface, and dividing the pharyngeal region from the trunk. This cleft, which presents a certain resemblance to the opercular cavity of a tadpole, is the sinus prascervicalis (Fig. 240, su) ; it ultimately becomes obliterated by fusion of its anterior and posterior walls.

3. The Upper Lip and the Palate.

The fronto-nasal process consists, as already described, of a median area (Fig. 240, FP), and two lateral lobes, the processus


OD



BR.2.


FIG. 240. The head and neck of a Human Embryo thirty-two days old, seen from the ventral surface. The floor of the mouth and pharynx has been removed. Cf. Fig. 205, which is an outline figure of the same embryo. (From His.) x 12.

BR.l, first branchial arch. BR.2, second branchial arch. EB, membrane closing the hyoinandibular cleft, which afterwards becomes the tympanic membrane. FC, processus globularis. FP, median part of fronto-nasal process. HM, hyomandibular poncli. HY, hyoid arch. LG, lung. LR, larynx. MN", mandibular arch. MX. maxillary arch. OD, eye. OK, mouth of olfactory pit, or external nostril. PT, pituitary body. SU, sinus prascervicalis.

globulares, FC. The processus globulares form the inner lips of the nasal grooves, which connect the olfactory pits with the mouth, and of which the outer lips are formed by the inner edges of the maxillary arches, MX. By fusion of their inner and outer lips, the nasal grooves become converted into the posterior


554 THE HUMAN EMBRYO.

nasal passages, a pair of short tubes leading from the olfactory pits to the fore part of the roof of the mouth, into which they open in much the same position as the posterior nares in an adult frog.

At a later stage, after the outgrowth of the median part or bridge of the nose, the two processus globulares meet each other in the median plane, and fuse to form the median part of the upper lip (</. Figs. 207 and 241).

There are, thus, in the upper lip three sutural lines : a median one, where the inner borders of the two processus globulares meet and fuse with each other ; and a pair of lateral ones, where

FO

FO'

MX'


MX



FIG. 241. The roof of the mouth of a Human Embryo about two and a half months old, showing the mode of formation of the palate. (From His.) xlO.

FO, processus jrlobularis. 3TO', palatal process of prooessus globularis. MX, maxillary arch MX', palatal process of maxillary arch. OB, mouth cavity. OD, eye. OK, aperture of olfactory pit, or nostril. OL, lens.

the outer borders of the processus globulares meet and fuse with the inner ends of the maxillary arches.

The median cleft is the one which persists throughout life in the hare or rabbit, but it is doubtful whether it ever remains open in man ; what is called hare- lip in man being due to imperfect closure of one or other of the lateral clefts.

The palate is formed, as regards its most anterior portion, by a pair of horizontal shelf-like outgrowths from the processus globulares (Fig. 241, FO'), which meet and fuse in the median plane. The rest of the palate, comprising the greater part of its length, is formed by two similar outgrowths, MX', from


THE PALATE AND TONGUE. 555

the inner surfaces of the maxillary arches. The palatal processes grow rapidly, and by the beginning of the third month the anterior ends of the maxillary processes, MX', have met and fused with each other in the median plane, immediately behind the premaxillary processes, or outgrowths from the processus globulares, FO'. A small aperture is left in the median plane between the four palatal processes, and persists as the foramen incisivum. The completion of the palate is effected by the extension backwards of the fusion of the inner edges of the maxillary processes, towards their hinder ends. Occasionally the union fails to take place properly, and the malformation known as cleft palate results.

By the formation of the palate, the anterior part of the mouth cavity becomes divided into dorsal or nasal, and ventral or buccal portions, and the communication between the posterior nostrils and the buccal cavity is shifted backwards to the level of the hinder edge of the palate.

The septum narium is formed in the first instance by upgrowths from the inner edges of the palatal processes, which fuse together in the median plane, and grow dorsalwards as a partition, dividing the nasal chamber into right and left halves.

4. The Tongue.

The tongue arises from the floor of the fore-gut, so that its epithelial covering is entirely of hypoblastic origin. It is formed from two rudiments, which are at first completely separate from each other ; an anterior median swelling, the tuberculum impar, from which the body and tip of the tongue are developed ; and a posterior V-shaped ridge, which gives rise to the root of the tongue.

On the fifteenth day (Fig. 237) the ventral ends of the mandibular arches, MN. almost meet each other in the median plane ; the ventral ends of the hyoid arches, HY, are some little distance from each other ; and the ventral ends of the first and second branchial arches, BR.I, BE.2, are still further apart. There is thus left in the floor of the pharynx, between the ventral ends of the visceral arches, a triangular, mesobranchial area, the apex of which is directed forwards. From the dorsal surface of this area the tongue is developed ; while the heart (Fig. 232) lies immediately beneath it.


556 THE II UMAX EMI5RY".

At the anterior end of the mesobranchial area, between the ventral ends of the uiandibular and hyoid arches, is a small rounded elevation, the tuberculum impar (Fig. 237, TU). Behind this, and between the ventral ends of the first and second branchial arches, there is a ranch larger elevation, with prominent rounded margins and a median longitudinal furrow. This is the furcula (Fig. 237, FL) ; and from it the epiglottis will be developed at a later stage, while the median groove will become the glottis.

The furcula lies at first immediately behind the tuberculum impar ; but in the early part of the fourth week (Fig. 238) the two become separated by a transverse ridge, formed from the ventral ends of the hyoid and first branchial arches, which unite together and extend across the floor of the mouth. This ridge



HY

BR


FIG. 242. The tongue and floor of the mouth of a Human Embryo at the end of the second month. (From His.)

BR.l, first branchial arch. FK, foramen caecum. H Y, hyoid arch. LT, glottis. MN, iiiainlibular arch. TU, body of the tongue, formed from the tuberculum iuipar.

soon grows forwards at the sides of the tuberculum impar. embracing it like a V. At the angle of the V, between the ridge and the tuberculum, a small backwardly directed pit is formed, the mouth of which becomes the foramen caecum (Fig. 239, FK), while the pit itself becomes the median portion of the thyroid body, TH.

The median part of the transverse ridge soon becomes marked off by lateral grooves, and fusing with the tuberculum impar gives rise to the root of the tongue (Fig. 242). The V-shaped groove, marking the boundary between the two originally separate elements of which the tongue consists, is very conspicuous throughout development, and is often well


THE TONGUE AND THE THYROID BODY. 557

marked in the adult : it is always indicated, in the median plane, by the foramen caecum (Fig- 242, FK). The line of circumvallate papillge, which appears during the third month, lies immediately in front of this groove, and therefore in the part of the tongue formed from the tuberculum impar : immediately in front of the foramen cascum, and sometimes surrounding it, is a single, very deeply depressed circumvallate papilla.

The double origin of the tongue is indicated by its nerve supply ; the body and tip of the tongue, developed from the tuberculum impar, are supplied by the gustatory branch of the trigeminal nerve ; while the root and sides of the tongue, developed from the transverse ridge, are supplied by the glossopharyngeal. It must be noted, however, that in order to reach the circumvallate papillae the branches of the glosso-pharyngeal nerve have to overstep the boundary between the two parts of the tongue, and invade the part formed from the tuberculum impar.

5. The Thyroid Body.

The thyroid body is formed from three independently arising rudiments, which remain distinct until a rather late stage in development : (i) a middle thyroid rudiment (Fig. 239, THJ, which is a deep pit commencing at the foramen caecum, at the junction of the body and root of the tongue, and extending downwards and backwards in the floor of the mouth ; and (ii) a pair of lateral thyroid rudiments, which are outgrowths of epithelium from the floor of the mouth at the sides of the larynx, in close relation with the third branchial pouches.

The middle thyroid rudiment, w r hich appears about the middle of the fourth week, consists at first of a short tubular duct, which divides at its blind end into right and left lobes (Fig. 239, TH). During the fifth week the median duct, or thyro-glossal duct, elongates rapidly, growing downwards and backwards until its bifurcated distal end lies opposite the larynx, or upper end of the trachea. During this rapid growth the duct usually loses its lumen, and becomes a solid rod of epithelial cells extending, in the median plane, from the foramen caecum to the trachea.

Towards the end of the fifth week, this epithelial cord usually becomes broken up in the middle part of its course into a number of detached fragments ; and a little later it becomes


558 THE HUMAN EMBRYO.

still further interrupted by the formation of the cartilaginous body of the hyoid, which lies exactly in its path.

The paired lateral thyroid rudiments early separate from the epithelium, and form a pair of lobed masses lying at the sides of the larynx, and of considerably larger size than the bifurcated median rudiment. At a later stage they shift still further back, so as to lie alongside the trachea, and then fuse with the median rudiment to form the definite thyroid body. The median rudiment gives rise to the isthmus of the adult thyroid, and probably to parts of the lateral lobes as well ; the greater part of the lateral lobes, however, are formed from the much larger lateral rudiments.

At an early stage the lobes are excavated by a number of detached cavities, which become the vesicles of the adult thyroid From the history of their development it follows that the epithelial walls of these vesicles are of hypoblastic origin.

The duct or stalk of the middle thyroid rudiment usually disappears in great part ; detached portions of it not uncommonly persist as accessory suprahyoid or epihyoid bodies, or as cysts.

Occasionally the upper part of the stalk persists as a tube, the lingual duct, extending from the foramen caecum, on the dorsum of the tongue, backwai'ds aud downwards towards the body of the hyoid, or actually reaching this in some cases.

The lower or posterior part of the stalk may also occasionally persist, forming the so-called pyramid of the thyroid, a somewhat pyriform body, enlarged and saccular at its lower or posterior end, and tapering upwards to a fibrous cord which is attached to the dorsal surface of the hyoid bone. The pyramid is apparently formed by persistence and enlargement of one of the two branches into which the stalk bifurcates at its lower end. When present, it is usually single, but cases have occurred in which two pyramids were found, due apparently to persistence of both branches of the bifurcation.

6. The Thymus.

The thymus is a paired organ of epithelial origin, developed in connection with the second and third branchial clefts, and perhaps the first branchial cleft as well.

It appears about the middle of the fifth week ; but as to the precise mode of its formation there is still some doubt. Born


THE THYEOID, THYMUS, AND THE TEETH. 559

maintains that the thymus of man, like that of other Vertebrates, is developed from the hypoblastic lining of the pharynx ; His' observations, on the other hand, support an epiblastic origin ; the thymus, according to him, being formed from the epiblastic walls of the sinus prsecervicalis, the deep fissure at the side of the neck caused by the overlapping of the hinder visceral arches by the more anterior ones (cf. Fig. 240, su).

The thymus gradually shifts backwards towards the root of the neck, extending along the pneumogastric nerve and carotid artery almost as far as the heart. It attains a great size in later fcetal life, and continues to increase after birth up to about the end of the second year, when it measures two inches or more in length.

7. The Salivary Glands.

The salivary glands commence to form early in the second month, and by the end of the month have attained a considerable size. The ducts arise as grooves of the buccal epithelium, which by fusion of their lips become tubes ; the glands themselves are, at first, solid outgrowths of epithelial cells, which later become hollowed out by extension of the cavities of the ducts into their substance. The submaxillary glands appear first, then the parotid, and lastly the sublingual glands.

8. The Teeth.

The teeth are developed in man in very much the same way as in the rabbit. In embryos about seven weeks old the epithelium becomes thickened along the border of each jaw, and the deeper or Malpighian layer of the epithelium grows down into the substance of the jaw as a continuous keel-like ridge, the common enamel germ. This soon becomes enlarged at intervals to form the enamel organs of the milk or deciduous teeth, while between the enamel organs the ridge becomes less conspicuous, and ultimately disappears.

Each enamel organ is flask-shaped, consisting of a terminal enlarged portion, buried deeply in the jaw, and a narrow neck or stalk which connects the enlarged part with the surface epithelium of the jaw. Opposite each enamel organ the connective tissue of the jaw becomes more compactly arranged to form the dental papilla (cf. Fig. 156, TM). The dental papilla soon becomes moulded into the shape of the future tooth, and


560 THE HUMAN EMBRYO.

the enamel organ becomes closely fitted, like a cap, over the surface of the papilla, which acquires the form of the crown of the future tooth.

From the dental papilla the main substance of the tooth, or dentine, is formed in the following manner. On the surface of the papilla next the enamel organ a layer of special cells, the odontoblasts, appear. These form, by excretion on their outer surfaces, a dense matrix in which fine filamentous processes of the odontoblasts are embedded ; by .calcification of the matrix the dentinal substance is formed, the dentinal tubules being the narrow channels in the matrix occupied by the processes of the odontoblasts. The first formed part of the dentine is the outermost layer of the crown of the tooth, and this layer thickens by further formation of dentine on its inner surface, the odontoblasts gradually withdrawing further and further from the surface, as the dentine increases in thickness.

The enamel is formed from the layer of epithelial cells of the enamel organ which lies in immediate contact with the dental papilla. This layer consists of closely set, columnar or prismatic cells, and it is by direct calcification of these cells that the enamel is produced. The rest of the enamel organ is merely nutritive in function, and does not give rise directly to any part of the tooth.

The crown is thus the first part of the tooth to be formed. After it is completed, the tooth increases in length by the further formation of dentine round the lower part of the papilla. The aperture at the base of the tooth is at first a widely open one ; but, as the tooth approaches its full size, the aperture becomes gradually narrowed to form the root or fang of the tooth. In the case of the grinding teeth, the aperture becomes divided by bridges of dentine into two or three separate openings, and by elongation of the margins of these openings the double or triple fangs of the adult tooth are produced. At the apex of each fang a minute hole remains, through which the blood-vessels and nerves gain admittance to the pulp of the tooth, which latter is the part of the dental papilla that remains encapsnled in the middle of the tooth after its completion.

The cement, or outermost layer of the fully formed tooth, is a bony deposit developed from the connective-tissue sheath which surrounds it.


THE TEETH. 561

On the first appearance of the bony jaws, the teeth lie in continuous grooves extending all round their free borders. Partitions are soon formed, dividing these grooves into separate compartments or alveoli, which grow round the teeth so as to closely embrace them.

The order of appearance of the milk teeth takes place in regular sequence ; but the actual dates at which the several teeth emerge, or are ' cut,' vary within certain limits. The cutting of the milk teeth usually commences about seven months after birth, and is completed by the end of the second year. The central lower incisors appear first, about the seventh month ; the upper incisors two or three months later ; a few months later still the lower lateral incisors, and the first premolars ; four or five months later the canines ; and about the end of the second year, the second premolars.

The permanent teeth are developed in the same manner as the milk teeth. From the stalk or neck of the enamel organ of each milk tooth, a small outgrowth arises at a very early stage, about the sixteenth week, which becomes the enamel organ of the corresponding permanent tooth. A dental papilla is formed opposite each enamel organ, and the permanent teeth are formed in the jaw, a little way behind and below the corresponding milk teeth, and in precisely similar fashion.

The three hinder grinding, or molar teeth, which have no milk predecessors, are formed by extension backwards of the original common enamel germ, from which the milk teeth are developed. The enamel organ for the first permanent molar appears about the fifteenth week of embryonic life ; that for the second permanent molar about seven months after birth ; and that for the third permanent molar, or ' wisdom tooth,' not until the third year.

The eruption, or cutting, of the permanent teeth of the lower jaw takes place at the following dates, the teeth of the upper jaw usually appearing a little later :

Molars, first . . 6 years.

Incisors, central .


lateral

Bicuspids, anterior posterior Canines Molars, second

third (or wisdom)


. 9 . 10

11 to 12

12 13 17 25




5 62 THE HUMAN EMBRYO.

9. The Lungs.

On the fifteenth day (Fig. 237), a swelling is present on the floor of the pharynx, opposite the first, second, and third branchial arches ; and along the middle of this swelling, or furcula, FL, there runs a longitudinal groove.

By the sixteenth day this groove is much more pronounced, and its posterior end leads into a short blind pocket

By the end of the third week the pocket has become much deeper, extending backwards, ventral to the oesophagus and independently of this ; and its hinder end is split into right and left lobes (cf. Fig. 238, LG). These lobes are the rudiments of the lungs ; the tube leading to them is the trachea ; and the slit-like opening, or groove, on the floor of the pharynx is the future glottis.

During the fourth week the lungs grow rapidly, extending back alongside the oesophagus, and dorsal to the heart (Figs. 216 and 233, Lg~) ; their distal ends are enlarged and are commencing to divide into lobes. The right lung has three terminal buds or lobes, and the left lung two ; these buds forming the rudiments of the five lobes of the adult lungs.

During the fifth week the lungs continue to increase rapidly ; the main lobes elongate greatly, and give rise to secondary and tertiary buds, which end in slightly expanded ampullee (Figs. 235 and 236, Lg).

The further development of the lungs consists in a continuation of the process of budding, by which new tubules and ampullas arise from the older ones, either by dichotomous division, or, as in the later stages, by lateral branching. The air cells appear, as closely set pouchings of the walls of the ampullge, which are not recognisable until the time of birth.

The trachea is at first short, but rapidly elongates during the fifth and following weeks. The larynx first becomes evident, as a dilatation of the anterior part of the trachea, towards the end of the fifth week (Fig. 234). The vocal cords, and the ventricles of the larynx, are not formed until about the fourth month.

The anterior, median part of the furcula becomes the epiglottis (Fig. 233, Ep) ; while the lateral ridges give rise to the ary-epiglottic folds and the arytenoid cartilages. The thyroid cartilage is said by Callender, and by His, to be formed from the


THE LUNGS AND THE LIVER. 568

cartilages of the second branchial arches, though Kolliker believes it to arise independently.

The lungs, as they grow backwards, project into the dorsal part of the body cavity, pushing before them the peritoneal lining of the cavity, which forms their pleural covering. At a later stage, the portions of the body cavity into which the lungs hang are shut off, by the diaphragm and pericardium, from the rest of the cavity, and become the definite pleural sacs.

10. The Liver.

The liver is present on the fifteenth day (Fig. 232, w) as

a short, hollow diverticulum, with a compact mass of cells at its blind end, arising from the ventral wall of the fore-gut and the anterior wall of the yolk-stalk, immediately behind the heart.

By the end of the third week (Fig. 215, w) the liver is of larger size, and the bile-duct, or wide tubular passage connecting the liver with the gut, is longer than before, but otherwise the relations are much the same as in the earlier stage.

During the fourth week the liver enlarges very rapidly (Figs. 216, 243, w). It consists of a close network of anastomosing epithelial cylinders, the development of which has not been followed accurately : the meshes of the network are chiefly occupied by blood-vessels, which are present in large numbers, and of great size. The development and relations of these blood- vessels of the liver will be described in the next section of this chapter.

The rapid growth of the liver continues during the succeeding weeks. In the second half of gestation it is rather less marked in proportion to the other viscera, but even at the end of pregnancy the weight of the liver is to that of the whole embryo as 1 to 18, while in the adult it is only 1 to 36. After birth the liver diminishes rapidly, both in size and weight, owing to the cutting off of the blood supply previously brought to it by the allantoic veins.

, The bile-duct rapidly lengthens during the fourth week ; and the gall-bladder appears, as a diverticulum of the bile-duct, before the end of the fifth week (Fig. 236, g.b}.

The large size of the liver during almost the whole period of gestation, and its abundant vascular supply, indicate that it must be of great physiological importance. It probably serves to modify in some way the nutrient material brought from the

o o 2


564 THE HUMAN EMBKYO.

placenta by the allantoic veins ; and it almost certainly acts as an important excretory organ during embryonic and foetal life.

The brown, or greenish-brown, mass known as meconium, which occurs in the small intestine from the third to the fifth month, and lower down, in the large intestine and rectum, during the later months of pregnancy, contains bile in considerable quantity, as well as mucus, and epithelial and other debris.

11. The Pancreas.

The pancreas arises, towards the close of the fourth week, as a dorsally directed diverticulum from the duodenum, almost opposite the opening of the bile-duct (Figs. 233 and 234, P), and lying in the thickness of the mesentery which attaches the duodenum to the dorsal body-wall. The pancreas grows rapidly, giving off lobed offshoots, from which the acini and their ducts are formed. The original diverticulum from the duodenum persists as the pancreatic duct ; it at first opens a little distance from the bile-duct, but ultimately the two ducts lie close alongside each other, and open into the duodenum by a single orifice.

12. The Mesentery.

The mesentery is the thin vertical sheet of mesoblast which slings the stomach and intestine to the body- wall. The relations of the mesentery are at first extremely simple, but as the intestine lengthens, and especially as it becomes thrown into convolutions, they become greatly complicated. The attachment of the dorsal border of the mesentery to the body-wall remains comparatively unmodified throughout life, though at certain places oblique or transverse lines of attachment are acquired in addition to, or in place of, the original simple longitudinal attachment.

The part of the mesentery which attaches the stomach to the body-wall, commonly spoken of as the mesogaster, undergoes special modification. The stomach originally lies lengthways along the body (Fig. 233, My, and 243), the mesogaster being attached along the border which will afterwards become the greater curvature of the stomach, and the pancreas (Fig. 233, P) lying in the thickness of the mesogaster near its posterior limit. When the stomach shifts its position, and becomes placed transversely across the body (Fig. 236), its original left side becomes ventral, and its right side dorsal ; while the mesogaster remains attached along what is now the posterior border of the stomach.


THE PANCKEAS AND THE MESENTEEY. 565

This part of the mesogaster, along the posterior border, or greater curvature of the stomach, becomes produced into a double fold or sac, the great omentum, which hangs down, like a curtain, over the coiled mass of the intestine, close to the ventral wall of the abdomen.

Shortly after birth, the two layers of the omental sac coalesce, so that the omentum becomes a single membranous layer, in which fat early tends to accumulate.

The dorsal part of the mesogaster, which is attached to the dorsal body- wall, and in the thickness of which the pancreas is contained, comes into close contact with the layer of mesentery suspending the transverse colon, and ultimately fuses completely with this ; a change which causes the pancreas to appear to lie altogether dorsal to the mesentery, instead of in its substance.

The Development of the Circulatory System

The general history of the development of the blood-vessels in man, their relations at the different periods of embryonic and of fo3tal life, and the changes by which at the time of birth the adult circulation is established, are closely similar to those already described in the rabbit. Certain differences have been noticed in the mode of formation of the valves of the heart, and in the development of the great veins, more especially of those in relation with the liver ; these are, however, of comparatively small importance, and are possibly, in some cases, due to the difficulty of obtaining human embryos in satisfactory histological condition, and of the particular age desired.

In the following account, which is based mainly on the descriptions of Professor His, the development of the heart will be dealt with first, then that of the arteries and the veins, and finally a brief description will be given of the course of the circulation in the embryo and fetus, and of the changes which occur at birth.

1. Development of the Heart

General Account. The early stages in the development of the heart in the human embryo are known very imperfectly, and only as regards the external form of the organ.

In the youngest human embryos, as in the corresponding stages of the rabbit, the heart consists of two symmetrical and perfectly distinct halves. On the thirteenth day (Fig. 179, R) the heart is present as a pair of straight tubes lying along the sides of the anterior end of the embryo, between the neural folds and the yolk-sac, and in connection at their hinder ends with the vessels which return the blood from the yolk-sac.

At a slightly later stage (Fig. 185, R), the two halves of the heart have united to form a single tube, which is already twisted on itself.

By the fifteenth day (Figs. 197 and 232), the heart has. advanced considerably in development, and forms a prominent swelling on the under surface of the embryo, between the head and the yolk-sac. It is a single tube, of considerable size ;. attached, both in front and behind, to the floor of the fore-gut,, but free along the middle portion of its length, which is twisted into a prominent S-shaped loop. The dorsal and posterior end of the loop is the auricular portion of the heart ; and is separated by a slight constriction, the canalis auricularis, from the succeeding or ventricular portion (Fig. 232, RV). This forms the widest and most prominent part of the loop ; it runs almost transversely across the body from the left to the right side, and then turns forwards rather sharply, and passes into the truncus arteriosus, RT, or terminal limb of the loop. The anterior end of the truncus arteriosus (Fig. 232) is attached to the floor of the fore-gut, very far forwards, opposite the mandibular arches.

The wall of the heart (Fig. 232) is double along its entire length, consisting of an outer mesoblastic tube, in which musclecells are already present on the fifteenth day, and an inner endothelial tube, the origin of which has not been determined. The endothelial tube is very much smaller than the muscular tube, and the space between the two is occupied by a gelatinous substance traversed by fine radial fibres, apparently of the nature of connective tissue (cf. Fig. 215).

During the third week the heart continues to increase rapidly in size, and its several divisions become more clearly marked off from one another by constrictions. By the end of the third week it has reached the condition shown in Figs. 198 and 215. The auricular portion (Fig. 198, RA) is much larger than before ; it is very wide from side to side, and is produced into conspicuous ear-like appendages. A marked constriction, the canalis auricularis, separates it from the ventricular portion. This latter, RV, is shaped something like the adult stomach, and lies almost directly across the body ; its right-hand or distal end bends sharply forwards, and passes into the truncus arteriosus, RT, which is attached to the floor of the fore-gut rather further back than before, opposite the hyoidean and first branchial arches (Fig. 215) . The structure of the heart is the same as in the earlier stages, except that the muscular elements have increased considerably. The wide space between the muscular and the endothelial walls is well shown in Fig. 215, as is also the fibrous network connecting the two walls.



FIG. 243. Human Embryo, lettered by Professor His, Bl, and estimated as twenty-three days old. The brain and spinal cord are exposed from the right side ; and the body is dissected to show the heart, the blood-vessels, and the alimentary canal. (B^rom His.) x 20.


In describing the further development of the heart it will be convenient to take the several divisions one by one.

The sinus venosus. The blood is returned to the heart by three main veins on each side : the Cuvierian vein (Fig. 243, VD), from the body of the embryo ; the vitelline vein, from the yolk-sac ; and the allantoic vein, from the placenta. These three pairs of veins form by their union a single large vessel, the sinus venosus, which runs transversely across the body, immediately in front of the liver, and opens through a median aperture into the auricular portion of the heart.

The sinus venosus is at first situated behind the diaphragm, but during the fourth week it gradually extends over, and in front of this, and so comes to lie in the pericardial cavity, immediately behind the auricle (Fig. 243).

Towards the end of the fourth week the sinus venosus becomes placed somewhat obliquely, in place of transversely, across the body ; at the same time its right side becomes larger than the left, and the opening into the auricular cavity, which was at first median, shifts so as to lead distinctly into the right side of the auricle (Fig. 244, RS). During the fifth week, the opening from the sinus venosus into the auricle widens out very considerably, so that the sinus becomes part of the auricle itself, and ceases to exist as a separate cavity. The left horn of the sinus venosus, which now only receives the left Cuvierian vein, retains its independence more completely, and persists as the coronary sinus.

The auricles. The auricular chamber is at first single, but towards the end of the fourth week it becomes imperfectly divided into the right and left auricles (Fig. 244). The division is indicated externally by a slight constriction, and more markedly by the outgrowth of the auricular appendices, which very early show characteristic crenations along their margins.


Seen from within, the auricular portion of the heart has, at the end of the fourth week, the appearance shown in Fig. 244. Opposite the external constriction, a fold, SK, the septum superius, projects into the cavity from its anterior end and ventral wall, and reduces the communication between the two auricles to a rather small circular aperture, nearer the dorsal than the ventral surface.



FIG. 244. The dorsal half of the heart of a Human Embryo, twenty-eight days old, seen from within. The heart has been bisected lengthways, and the ventral half removed. (From His.) x 32.

BA, right auricle. BB, left auricle. BF, auriculo-ventricular aperture. BK, canalis auricularis. BS, opening of sinus venosus into right auricle. RV, right ventricle. BY, left ventricle. SB, septum spurium. SD, septum inferius. SK, septum superius. SZ, spina vestibuli. V D, right vena cava anterior. VU, Eustachian valve.


The conspicuous projection into the dorsal part of the right auricle, shown in the figure, is caused by the sinus venosus. The aperture from the sinus venosus into the auricle is an obliquely placed slit, RS, of which the outer lip is thickened, and forms the Eustachian valve, vu ; while the opposite, or inner lip, is a thin fold, which at the lower end of the slit passes into a triangular thickening of connective tissue, the spina vestibuli, SZ, projecting into, and partially blocking up, the aperture between the right and left auricles. This spina vestibuli, according to Professor His, plays an important part in the formation of both the interauricular and interventricular septa.

An additional fold, the septum spurium, SB, projects into the cavity of the right auricle, opposite the upper end of the slit-like opening of the sinus venosus ; it is a transient structure, and ultimately disappears completely.

Of the two auricles, the right one, RA, is at first (Fig. 244) much the larger. The walls of the auricles, like the rest of the heart, consist of two layers, muscular and endothelial ; these are at first some distance apart, but about the twenty-third day they come in contact, and unite firmly to form the definite auricular wall. The connective-tissue elements of the wall are derived apparently from the gelatinous matter which originally separates the muscular and endothelial walls from each other.

The interauricular septum is formed, according to His, by coalescence of the septum superius (Fig. 244, SK) with the spina vestibuli, SZ ; the latter growing downwards towards the ventricle as a thickened plug, which before the end of the fifth week divides the originally single auriculo-ventricular aperture into separate right and left openings.

It is not quite clear whether the foramen ovale in the human embryo is merely due to the interauricular septum remaining incomplete dorsally ; or whether it is a new aperture formed in the dorsal part of the septum, as described by Born in the case of the rabbit.

The canalis auricularis, at the beginning of the fourth week (Fig. 243), is a short, straight, and rather narrow tube, connecting the auricular and ventricular portions of the heart. In the latter part of the fourth week this portion of the heart shortens somewhat, the auricular and ventricular portions approach each other, and the canalis auricularis becomes telescoped within them (Fig. 244, RK), projecting partly into the auricular and partly into the ventricular cavity, and being no longer visible from the surface except as a sharply marked anmilar constriction.

The lumen of the canalis auricularis becomes at the same time reduced to a narrow transverse slit, its dorsal and ventral walls thickening to form a pair of endothelial cushions, which fuse with the lower border of the spina vestibuli to complete the interauricular septum, and from which also the auriculoventricular valves are derived.

The ventricles. The ventricular cavity becomes partially divided towards the close of the fourth week by a fold, the septum inferius (Fig. 244, so), which arises from its dorsal and posterior wall, and the position of which is indicated externally by a slight groove on the surface of the heart. The completion of the interventricular septum is a somewhat complicated process, and will be described after the truncus arteriosus has been dealt with.

The ventricular wall consists at first of an outer muscular tube, and an inner and much smaller endothelial tube, the two tubes being separated by a considerable quantity of gelatinous connective tissue (cf. Fig. 215). The thickening of the ventricular wall is effected, in the first instance, by the outgrowth of bands from the muscular tube into the gelatinous tissue : these bands interlace and unite with one another to form a spongework of muscular trabeculae. The gelatinous tissue now becomes greatly reduced in amount, so that the endothelial and muscular walls are brought much closer together, and the endothelium becomes moulded to the surface of the muscular wall, covering the trabeculae, and lining the depressions of the spongework. The wall of the ventricle is now in much the same condition as it remains in throughout life in the frog. In the later stages of development, the outer, compact muscular wall thickens very considerably, and the spongework becomes less conspicuous, forming ultimately the columnae carnese.

The walls of the two ventricles are of equal thickness throughout almost the whole of foetal life, as the resistance to be overcome by the two is approximately equal until the time of birth.

The truncus arteriosus. In the truncus arteriosus the most important change is the formation of the aortic septum, by which the single tube becomes divided into two, lying side by side, which become the systemic and pulmonary trunks respectively ; or, in the adult, the ascending aorta and the pulmonary artery.

This division of the truncus arteriosus is effected by two longitudinal ridge-like thickenings of the endothelial lining, which, arising from opposite sides, encroach on the lumen, reducing it to a slit, dumb-bell shaped in section; by further growth, the two ridges meet each other and fuse, so as to divide the lumen into two completely separate passages.

The endothelial ridges, and consequently the septum itself, appear first at the distal end of the truncus arteriosus, between the origins of the systemic and pulmonary aortic arches, and gradually extend backwards towards the ventricle. The septum first appears towards the end of the fourth week, and is completed before the end of the fifth week ; it has a slightly spiral course, so that the two tubes, into which it divides the truncus arteriosus, are respectively dorsal and ventral at the proximal end, next to the ventricle, and right and left at the distal end of the truncus.

Of the two tubes, the one (Fig. 245, EX) which lies dorsally at its proximal end, and on the right side distally, is the systemic trunk ; the other, R\v, which is ventral proximally, and on the left side distally, is the pulmonary trunk ; and the same relations are retained throughout life by the ascending aorta and the root of the pulmonary artery, into which the trunks develop respectively.

The separation of the systemic and pulmonary trunks at first concerns their internal cavities alone ; but it is soon followed by the appearance of external grooves, which deepen until they completely separate the two trunks from each other.

The interventricular septum. The truncus arteriosus originally arises from the right-hand corner of the ventricular cavity, and the two trunks into which it becomes split retain for a time the same relations. In other words, at a time when the interventricular septum is already partially formed (Fig. 244, SD), both the systemic and pulmonary trunks arise from the right ventricle, and the left ventricle has for a time no outlet, except through the right ventricle.

The completion of the interventricular septum has to be effected in such a way that while the pulmonary trunk is left in connection with the right ventricle, the systemic trunk shall be cut off from the right ventricle and placed in communication with the left ventricle.

The formation of the interventricular septum is consequently somewhat complicated. The greater part of the septum is formed from the septum inferius (Fig. 244, SD), but it is completed above, partly by the lower edge of the interauricular septum, and partly by a prolongation of the aortic septum, which divides the truncus arteriosus into systemic and pulmonary trunks.

The aortic septum grows back beyond the truncus arteriosus, so as to project a certain distance into the ventricular cavity ; it then fuses with the free lower edge of the interauricular septum, in such a way as to cut off the systemic trunk from the right ventricle, and to place it in communication with the left ventricle ; while finally the septum inferius extends so as to meet and fuse with the interauricular septum, and so completes the separation of the ventricles from each other.

The valves of the heart. The outer flaps of the auriculoventricular valves, both mitral and tricuspid, are formed from the lower lips of the canalis auricularis, which hang down into the ventricular cavity (Fig. 244) ; the inner flaps of the valves are derived from the lower edge of the interauricular septum. The valves are at first very thick and soft, and only later become thin and membranous.

The semilunar valves are formed, about the end of the fifth week, as cushion-like thickenings of the endothelium, which soon become hollowed out into pockets.

2. The Arteries

The general plan of arrangement of the arteries in the human embryo is the same as in other Vertebrates ; and has already been described, in previous chapters, in the case of the rabbit, the chick, and the frog.

From the anterior end of the truncus arteriosus a series of pairs of aortic arches arise, which run round the sides of the pharynx, lying in the visceral arches (Fig. 243). On reaching the dorsal surface of the pharynx, the aortic arches of each side open into a longitudinal vessel, the aorta. The two aortae run backwards along the body, ventral to the notochord ; they are at first separate along their whole length, but early fuse together in the hinder part of their course to form the definite dorsal aorta. From the dorsal aorta, vitelline arteries are given off to the yolksac ; and at the posterior end of the embryo the aorta divides into the two large allantoic arteries, which carry blood from the embryo to the placenta.

The aortic arches of man, as of other Vertebrates, are developed in order from before backwards.

At the fifteenth day (Figs. 197 and 232) there are two pairs of aortic arches present, lying in the mandibular and hyoidean, arches, and corresponding, therefore, to the most anterior pairs in rabbit, chick, or frog embryos. By the sixteenth day three additional pairs have appeared, in the first, second, and third branchial arches ; and up to the end of the third week all five pairs are still present (Fig. 198, A.1-A.5).

The point of attachment of the truncus arteriosus to the floor of the mouth shifts backwards during development, as already noticed, and at the end of the third week is opposite the hyoidean and first branchial arches. The truncus arteriosus, at this stage, immediately on entering the floor of the mouth, divides into two branches on each side (Fig. 198). Of these, the anterior branch runs forwards, and divides into the mandibular, A.I, and hyoidean, A. 2, aortic arches ; while the posterior branch runs backwards, and divides into the three hinder aortic arches, A. 3, A.4, A. 5.

The aortic arches diminish in size from before backwards (Fig. 198) ; and, owing to the funnel-like shape of the pharynx {c/. Fig. 238), the hinder arches lie much nearer the median plane than do those further forward.

All five pairs of arches are complete, opening at their dorsal nds into the aortas (Fig. 198). In front of the first, or mandibular arch, each aorta is continued forwards as the internal carotid artery, which runs along the side of the brain, and gives off branches supplying this.

During the fourth week important changes occur in the aortic arches, closely comparable with those already described in other Vertebrates, and leading to the establishment of the adult scheme of circulation.

Early in the fourth week (Fig. 243) the middle portion of the first, or mandibular, aortic arch of each side becomes obliterated, and disappears ; and very shortly afterwards the cor. responding portion of the second, or hyoidean, aortic arch disappears in the same fashion.

By the end of the fourth week the condition of the aortic arches is as shown in Fig. 216. The mandibular and hyoidean aortic arches have lost their connection with the aortas. Their ventral or proximal ends persist as the external carotid arteries and their various branches ; the mandibular arch, according to His, giving rise to the external and internal maxillary arteries, and the temporal artery ; while from the second, or hyoidean arch, the lingual and ascending pharyngeal arteries arise, and perhaps also the occipital and posterior auricular arteries.

The third aortic arch, A. 3, in the first branchial arch, remains complete. As seen from the side (Fig. 216), it is somewhat S-shaped, its curvature being such that the direction of flow of the blood in it is naturally forwards, along the internal carotid artery, towards the head.

The fourth and fifth aortic arches, A.4 and A.5, are both complete, opening at their dorsal ends into the aortae. From the fifth arches, near their ventral ends, the pulmonary arteries arise, early in the fourth week, as small branches which run backwards to the lungs (Fig. 243, AP).

During the fifth week further changes of importance occur. The division of the truncus arteriosus, by formation of the aortic septum, is completed, and the systemic and pulmonary trunks are now entirely independent of each other ; the systemic trunk (Figs. 245, 246, RX) remaining in connection with the fourth and third aortic arches, and with the persisting remnants of the second and first arches as well ; while the pulmonary trunk, RW, communicates with the fifth pair of aortic arches alone.

The portion of the aorta between the dorsal ends of the third and fourth, or, as we may now call them, the carotid and systemic arches, disappears (Fig. 245).

The third, or carotid arch, becomes more directly continuous with the anterior prolongation of the aorta, the two vessels together forming the internal carotid artery, Ai ; while the common carotid artery (Figs. 245, 246) is formed by lengthening of the arch at its origin from the systemic trunk.

Towards the end of the fifth week the heart travels rapidly backwards, as the neck elongates ; this causes great lengthening of the common carotid artery (Fig. 246, AE), and straightening of the course of the internal carotid artery. It further leads, among other changes, to the pulling out of the laryngeal branch of the pneumogastric nerve, to form its recurrent loop.


In the early part of the fifth week, the left fourth, or systemic arch, becomes distinctly larger than the corresponding arch of the right side ; and this difference soon becomes more pronounced. By the end of the fifth week the fourth right arch is not only markedly smaller than the left arch, but has lost its connection with the aorta, and now forms only the vertebral and subclavian arteries of the right side.

The fifth aortic arch of the right side disappears, beyond the origin of the right pulmonary artery. The fifth left arch, however, remains of large size up to the close of foetal life ; the portion of the arch between the root of the left pulmonary artery and the dorsal aorta being known as the ductus arteriosus (Figs. 245 and 246, A.S).



FIG. 245. The aortic arches of a Human Embryo thirty-two dnys old, from the left side. (From His.) x 18.

A, dorsal aorta. A.4, fourth, or systemic aortic arch. A.5, fifth, or pulmonary aortic arch. AE, external carotid artery. AT, internal carotid artery. AP, pulmonary artery. AV, vert dn-al artery. A~W, inter vertebral or segmental arteries. LQ.lunp. T-jR, trachea. MM", mandible, or lower jaw. PT, pituitary rliverticulam from mouth. R"W, pulmonary trunk. RX, systemic trunk. TN, tongue. TO, fflsophagus. ^.^


The dorsal aorta and its branches. The point at which the two aortas unite, to form the single dorsal aorta, is about the junction of the cervical and dorsal regions, in embryos at the end of the fourth week, but the exact position varies considerably in different specimens. As the union proceeds backwards, the dorsal aorta increases considerably in size, and its diameter in the lumbar region is more than double that in the anterior thoracic region. At the hinder end of the lumbar region the aorta divides into the right and left allantoic arteries, which run along the allantoic stalk to the placenta, and which, at any rate in the early stages, appear as direct continuations of the aorta rather than as branches of it.

The proximal ends, or roots, of the allantoic arteries persist throughout life as the common iliac arteries, from which the external iliac arteries arise as branches, on the formation of the hind limbs. The hypogastric arteries are the abdominal, or intra-foetal, portions of the allantoic arteries, beyond the origin of the internal iliac arteries ; their cavities become obliterated after birth, but their walls persist as solid cords, crossing the sides of the bladder obliquely, and running forwards and upwards to the umbilicus.



FIG. 246. The aortic arches of a Human Embryo thirty-five days old, from the left side. (From His.) x 30.

A.4, fourth, or systemic aortic arch. A.5, fifth, or pulmonary aortic arch. AE, common carotid artery, at its point of division into internal and external carotid arteries. AI, internal carotid artery. AP, pulmonary artery. AS, subclavian artery. AV, vertebral artery. CH, notochord. LR, trachea. B,"W, pulmonary trunk. B.X, systemic trunk. TH, thyroid body. TN", tongue. TO, oesophagus.


The vertebral arteries appear, about the twenty-fourth day, as a pair of longitudinal trunks running along the sides of the brain, and extending from the level of the ears to the commencement of -the cervical region. They have at first no communication with the other vessels, but towards the end of the fourth week their anterior ends unite to form the median basilar artery, which becomes connected with the internal carotid arteries to form the circle of Willis. About the same time, a series of paired segmental or intervertebral arteries arise, as branches from the dorsal wall of the aorta, along the cervical and thoracic regions (Fig. 245, AW), and supply the spinal cord. One, or more, of the anterior pairs of these intervertebral arteries become continuous with the hinder ends of the vertebral arteries (Fig. 245, AV), which thus acquire their connection with the aortae. In the later stages, some of the intervertebral arteries further back become connected in similar fashion with one another, and with the vertebral artery ; and by the acquisition of these posterior connections, with simultaneous loss of the older and more anterior ones, the point of origin of the vertebral artery from the aorta is gradually shifted backwards to the root of the neck.

The subclavian arteries arise as branches of the vertebral arteries (Fig. 246, AS) ; but, as the fore limbs increase in size, the relative proportions of the two vessels soon become reversed, and the vertebral arteries appear as branches of the subclavians.

From the sides of the dorsal aorta a series of pairs of arteries arise, which supply the Wolffian bodies. The coeliac axis is from the first a median artery ; it arises from the ventral wall of the aorta, in the anterior thoracic region, and gradually shifts backwards, until its adult point of origin, opposite the last thoracic vertebra, is attained.

In the development of the aorta, and in that of all the other arteries as well, the wall of the vessel consists at first of a single layer of endothelial cells. Outside this, the layer of circular muscle-fibres is formed from the surrounding mesoblast, early in the third week. At a later stage a layer of connective tissue is formed between the muscular and the endothelial layers, but it is not clear from what source this connective tissue is derived. His suggests that it is formed directly from the blood in the blood-vessel itself.

3. The Veins

The general arrangement, and mode of development, of the veins in man is the same as in the rabbit. The most important differences consist in the disappearance of the left anterior vena cava, and in certain modifications in connection with the veins of the liver.

In the latter part of the third week (Fig. 198), the blood is returned to the heart by three pairs of veins, of approximately equal size : the Cuvierian, vitelline, and allantoic veins.

Of these, the Cuvierian veins, VD, return blood from the embryo itself, and are formed on each side, as in the rabbit and the chick, by the union of an anterior cardinal or jugular vein, VB, from the head, with a posterior cardinal vein, vc, from the trunk.

The vitelline veins, vv, return blood from the yolk-sac, and enter the embryo by the yolk-stalk.

The allantoic veins, VA, return blood from the placenta ; they enter the embryo along the allantoic stalk, and run forwards in the side walls of the body to the heart.

The veins are at first of equal size on the two sides of the body, and by the union of the six veins the transversely placed sinus venosus is formed. In following their further development it will be convenient to take the several veins separately.

The vitelline veins are comparatively small, as in Mammals generally, owing to the small size of the yolk-sac. They lie in the splanchnopleuric mesoblast, and, after entering the embryo at the umbilicus, run forwards along the sides of the alimentary canal to the sinus venosus (Fig. 243, vv). The vitelline veins are closely associated with the liver, and they become surrounded by this as it is developed ; furthermore, the principal changes which they undergo are in connection with the vascular supply of the liver.

Early in the fourth week, about the twenty-third day (Fig. 243), the vitelline veins become interrupted as they pass through the liver, breaking up into a set of afferent hepatic vessels supplying the liver, and a set of efferent hepatic vessels conveying the blood from the liver to the heart. The afferent and efferent hepatic vessels are connected by capillaries only, so that all the blood entering the liver by the vitelline veins must traverse the substance of the liver in order to reach the heart.

About the same time, the right and left vitelline veins become connected together, immediately before they enter the liver, by three transverse commissural vessels. Two of these commissural vessels pass ventral to the duodenum, while the third, or middle one, is dorsal to it ; and the three together form two vascular rings, or sinus annulares, encircling the duodenum (Fig. 243). From the anterior ring, afferent vessels arise which cany blood into the liver.

At a slightly later stage, during the latter part of the fourth week, the right and left vitelline veins unite to form a single vein, which is joined, before it reaches the liver, by veins returning blood from the intestine, and which may from this time be spoken of as the hepatic portal vein.


FIG. 247. The liver and the veins in connection with it of a Human Embryo twenty-four or twenty-five days old, seen from the ventral surface. (From His.)

PA, pancreas. TI, intestine. TS, stomach. VA, left allantoic vein. VA', right allantoic vein. VA", anterior detached portions of the allantoic veins. VE, ductus venosus, or vena Arantii. VH, efferent hepatic vessel. VL, afferent hepatic vessel. VO, hepatic portal vein. VV, vitelline vein. VV', portions of the sinus annulares which disappear. "W, liver. "W"D, bile duct.


Of the two sinus annulares, the left half of the anterior one r and the right half of the posterior one, disappear ; the persistent portions form a single vessel (Fig. 247, vo), which becomes the anterior part of the hepatic portal vein, and which, from the mode of its development, runs round the duodenum with the spiral course characteristic of the vein in the adult.

The allantoic veins are at first paired, but they soon fuse together at their hinder ends, within the allantoic stalk, to form a single vessel ; further forwards, within the embryo itself, they remain separate, running in the side walls of the body, close to the base of the amnion folds (Fig. 198).

During the fourth week, both allantoic veins lose their connection with the sinus venosus. The right allantoic vein (Fig. 247, VA'), which is now much the smaller of the two, breaks up into two sets of vessels : an anterior set, VA", which run in the body wall, and join the efferent hepatic vessels as these leave the liver ; and a posterior set, VA', which disappear at a slightly later stage.

The left allantoic vein, VA, which is much larger than the right, also divides into two sets of vessels : an anterior set, VA", which resemble those of the right side ; and a large posterior vessel, VA, which joins the anterior sinus annularis, or hepatic portal vein, just as this enters the liver substance.

The ductus venosus. At about the twenty-third day, both the vitelline and the allantoic vessels have lost their direct connections with the sinus venosus, and in order to reach the heart the blood in these vessels must traverse the liver capillaries. A direct communicating passage is now established between the portal vein, just before it enters the liver, and the right hepatic vein just before this reaches the sinus venosus. This communication (Fig. 247, VE) is the ductus venosus, sometimes called the vena ascendens or vena Arantii ; it enlarges very rapidly, and affords a wide and direct path by which the blood from the placenta can reach the heart without passing through the liver capillaries.

In rabbit and chick embryos the ductus venosus is the persistent anterior part of the fused vitelline veins ; in man, according to Professor His, whose descriptions have been followed above, it is, as just described, an entirely new vessel.

The posterior vena cava is a very insignificant vein in the earlier stages. It is formed by the junction of the iliac veins, and does not appear until the hind-limbs have begun to become prominent. It joins the ductus venosus as this emerges from the liver.

The Cuvierian veins. Each Cuvierian vein (Fig. 198, vo) is formed by the junction of an anterior and a posterior cardinal vein. The anterior cardinal vein persists as the external jugular vein, and is joined later on by the internal jugular and subclavian veins.

The posterior cardinal veins disappear, in the middle part of their course, on the replacement of the Wolffian bodies, with which they are specially related, by the permanent kidneys. The hinder ends of the veins become the internal iliac veins, and acquire connections with the allantoic veins. The anterior portion of the right posterior cardinal vein gives rise to the azygos vein.

The Cuvierian veins themselves run at first transversely ; but, as the heart shifts backwards, their direction becomes at first oblique, and finally longitudinal.

The right Cuvierian vein persists as the anterior vena cava. The left Cuvierian vein undergoes important changes : up to the end of the second month it is as large as the right vein ; but during the third month a communicating vessel is formed between the left and right Cuvierian veins, just behind the junction of the jugular and subclavian veins. Through this communicating branch, which is very large and has a somewhat oblique course, the blood from the left jugular and subclavian veins is carried across to the right Cuvierian vein, instead of returning to the heart as before by the left Cuvierian vein. The left Cuvierian vein, having no longer any function to perform, shrinks up and becomes obliterated more or less completely. Portions may persist, either as fibrous cords, or as venous channels of greater or less size ; and the posterior end, where it opens into the sinus venosus, is said to give rise to the coronary sinus.

The pulmonary veins appear late, about the end of the fifth week : they open into the left auricle, close to the interauricular septum. At first there is only a single opening into the auricle, but at a later stage, about the fourth month, there are two openings, and in slightly older foetuses all four openings are present ; the change being apparently due to the opening out of the originally single orifice, and the absorption of the vein, as far as its first branches, into the wall of the auricle ; much in the same way as the sinus venosus is opened out, and made part of the wall of the right auricle.

4. The Course of the Circulation during the first Four Months of Gestation

In the early stages, up to the end of the first month, the blood brought back to the heart whether from the body of the embryo itself, from the placenta, or from the yolk-sac is poured into the sinus venosus, and thence, through a median slit-like aperture, into the single auricular cavity. Complete mixture of the streams from the several sources must necessarily occur, in both the sinus venosus and the auricle, and the blood driven out through the truncus arteriosus by the ventricle will be of a mixed character.

After the sinus venosus is taken into the heart, in the early part of the second month, there are for a time three separate openings into the right auricle : those of the right and left Cuvierian veins, and of the posterior vena cava respectively. The auricular septum is now partially formed, but there is still free communication between the two auricles through the foramen ovale. Of the three veins, the opening of the posterior vena cava lies nearest to the foramen ovale ; and the Eustachian valve, a fold of the wall of the auricle along the right-hand side of the opening, tends to direct the blood from the posterior vena cava through the foramen ovale into the left auricle. The foramen ovale is at this stage a mere aperture in the auricular septum, not guarded by valves, so that a certain amount of direct mixture of the blood returned to the auricle by the different veins must of necessity take place.

During the third month, the transverse communication from the left to the right Cuvierian vein is being established ; and by the end of the fourth month the left Cuvierian vein has practically disappeared, the whole of the blood from both sides of the head, and from both fore limbs, being returned by the right Cuvierian vein, or anterior vena cava as it may now be called. Neglecting the coronary sinus, which is comparatively insignificant, there are at this stage only two vessels returning blood to the right auricle : the anterior vena cava, which returns venous blood from both sides of the head, and from both fore limbs ; and the posterior vena cava, which brings back blood, mainly arterial in character, from the placenta, and also from the hinder part of the body of the embryo, and from the yolk-sac.

During the fourth month the foramen ovale, which has hitherto been a free opening, becomes partially blocked up by a fold, which acts as a valve, allowing blood to pass from the right to the left auricle, but obstructing its return in the opposite direction.

The Eustachian valve becomes larger at the same time ; and partly owing to its increased size, and partly to slight changes in the position and direction of the opening of the posterior vena cava, the whole of the blood returned by this latter vessel is now discharged through the foramen ovale into the left auricle.

5. The Course of the Circulation during the Latter Half of Gestation

During the latter four months or so of gestation the course of the circulation is as follows IThe right auricle receives blood from three sources

  1. From the anterior vena cava.
  2. From the coronary sinus.
  3. From the posterior vena cava.

The anterior vena cava returns venous blood from both sides of the head, and from both fore-limbs.

The coronary sinus, which is the persistent terminal portion of the original left anterior vena cava, returns venous blood from the walls of the heart itself.

The posterior vena cava, which is much the largest of the three, returns blood : (a) from the hinder part of the body, and especially the kidneys and the hind limbs ; and (b) from the placenta, the intestine and the liver. The latter of these two streams requires further consideration.

Of the two allantoic veins, by which the blood was returned from the placenta in the earlier stages, the right one has disappeared. The left allantoic vein, which is very large, enters the body at the umbilicus, and runs forwards to the hinder border of the liver ; here it is joined by the hepatic portal vein, which returns blood from the intestine, and is formed in part from the vitelline veins of the earlier stages.

On reaching the liver, the blood has two alternative routes open to it, by either of which it can reach the posterior vena cava. Part of the blood is conveyed by the afferent hepatic vessels into the substance of the liver, from which it is returned by the efferent hepatic vessels, or hepatic veins, to the posterior vena cava ; the greater part, however, continues straight onwards through the wide ductus venosus, and so reaches the posterior vena cava without having traversed the liver.

The blood brought back to the heart by the posterior vena cava is thus derived very largely from the allantoic vein, and in part from the renal veins ; it is therefore purer as regards gaseous constituents, and freedom from nitrogenous excretory matters, and is richer in nutrient matters, than the blood returned by the anterior vena cava ; and the blood in the .anterior and in the posterior vena? cavae may consequently be contrasted as venous and arterial respectively.

The venous blood brought to the right auricle by the anterior vena cava passes, on the auricular contraction, into the right ventricle. From the ventricle it is driven along the pulmonary trunk (Fig. 246, RW) ; a small portion passes along the pulmonary arteries, AP, to the lungs, but as the lungs are in an unexpanded condition there is considerable resistance to the entrance of blood into the pulmonary vessels, and only an insignificant portion of the stream takes this path. Nearly the whole of the venous blood in the pulmonary trunk passes along the ductus arteriosus (Fig. 246, A.o) to the dorsal aorta, down which it courses to the bifurcation of the aorta into the two common iliac arteries ; then down these latter, and partly along the external iliac arteries to the hind limbs, but mainly along the allantoic arteries to the placenta, where it gains nutrient matter and oxygen, and from which it is returned to the foetus by the allantoic vein.

The arterial blood brought to the right auricle by the posterior vena cava does not really enter the cavity of the right .auricle, but is directed at once, by the Eustachian valve, through the foramen ovale into the left auricle, which also receives the very small quantity of blood returned from the lungs by the pulmonary veins. From the left auricle the blood passes into the left ventricle, and is thence driven along the systemic trunk (Fig. 246, RX), and through the carotid and subclavian arteriesto the head and fore-limbs.

It is probable that very little, if any, blood from the left ventricle gets into the dorsal aorta, for this is already filled r through the ductus arteriosus, from the right ventricle ; and as the two ventricles have at this stage walls of about equal thickness, and presumably of equal strength, there will be as strong a tendency for the blood of the right ventricle to pass forwards along the arch of the aorta, as for the blood from the left ventricle to pass backwards along the dorsal aorta.

Theoretically, the aorta might be ligatured just in front of the point at which the ductus arteriosus joins it, without in any way disturbing the foetal circulation ; and instances of malformation have occurred, in which the aorta was completely obliterated at this place, and yet development in other respects proceeded normally. Such a malformation, though causing nodisturbance of the circulation so long as the foetus is receiving nourishment through the placenta, is fatal at the time of birth, as the arterial supply of the whole body behind the arms is then cut off.

6. The Changes in the Circulation at the Time of Birth

At birth, the placental circulation is arrested, and the allantoic and vitelline vessels are interrupted ; and, as the lungs become inflated, the pulmonary circulation is fully established.

In connection with this shifting of the seat of respiration, from the placenta to the lungs, important changes are effected in the circulation, the principal of which are :

  1. Shrinking and obliteration of the ductus arteriosus, and of the hypogastric, or allantoic, arteries.
  2. Obliteration of the ductus venosus, and of the part of the allantoic vein within the body of the child.
  3. Closure of the foramen ovale.


By these changes it is brought about that the blood in the posterior vena cava, which is now entirely venous, is no longer able to get into the left auricle, owing to closure of the foramen ovale, but passes, with that of the anterior vena cava, from the right auricle to the right ventricle. From the right ventricle, owing to the obliteration of the ductus arteriosus, it can no longer reach the aorta, but passes entirely along the pulmonary arteries to the lungs. From the lungs it is returned by the pulmonary veins, which are now greatly enlarged, to the left auricle, and so to the left ventricle, which drives it not only to the head and upper limbs, but also along the dorsal aorta to the hinder part of the body.

By obliteration of the ductus venosus, all the blood in the hepatic portal vein is compelled to pass through the capillaries of the liver in order to reach the posterior vena cava. In other words, by these three changes obliteration of the ductus arteriosus, obliteration of the ductus venosus, and closure of the foramen ovale the foetal circulation has been converted into that of the adult.

These changes do not occur immediately on birth, nor are they effected simultaneously.

Obliteration of the allantoic or hypogastric arteries occurs first ; it is effected partly by contraction of the entire vessels, but chiefly by thickening of their inner coats, and is usually completed by the third or fourth day after birth.

The allantoic veins and the ductus venosus remain open rather longer, but are generally obliterated by the sixth or seventh day.

The ductus arteriosus, according to Allen Thomson, ' is rarely found open after the eighth or tenth day, and by three weeks it has, in almost all instances, become completely impervious.'

Closure of the foramen ovale is the last of the changes to be completed. The closure is at first effected merely by the valve, which projects into the left auricle, being kept closely applied to the margin of the aperture by pressure of the increased quantity of blood now returning by the pulmonary veins. At a later stage the edge of the valve gradually coalesces with the margin of the opening, but the union often remains incomplete for some months ; and it not unfrequently happens that an oblique valvular aperture, large enough to admit a probe, persists for the first year of infancy, and may even be permanent throughout life, in which case a direct passage of venous blood into the left auricle is liable to occur, especially on overexertion.


Development of the Urinary Organs

The general history of development of the urinary organs in man is the same as in the rabbit. Paired Wolffian ducts and Wolffian bodies appear first ; these form the excretory organs of the early stages, and attain a considerable size during the second month, after which time they gradually shrink, ultimately losing their excretory function, and becoming modified to form accessory parts of the reproductive system.

The permanent or adult kidneys arise, as in the rabbit, as outgrowths from the hinder ends of the Wolffian ducts : from the third month onwards they replace the Wolffian bodies as the functional excretory organs.

A pair of Mtillerian ducts is formed, independently of the Wolffian ducts, and in the female becomes modified to form the oviducts, uterus, and vagina. The head-kidney, if present at all, is in a very rudimentary and evanescent condition.

1. The Wolffian Duct and Wolffian Body

According to Kollmann, the Wolffian ducts appear, about the fourteenth day, as a pair of longitudinal grooves of the external epiblast, just below the level of the myotomes (Fig. 248, KG). By the middle of the third week the ducts are tubular, and lie embedded in the mesoblast of the intermediate cell mass. It is not yet certain, however, whether the tubular duct is formed by closure of the lips of the groove, or by splitting off of a rod of cells from the thickened floor of the groove, which subsequently acquires a lumen, and becomes tubular : while the observations recorded in the case of rabbit embryos render it possible that the origin of the Wolffian duct from the epiblast may prove to be apparent rather than real (cf. p. 421).

The Wolffian ducts at first end blindly behind, but about the end of the third week or beginning of the fourth week they grow back to the cloaca, and open into its sides (Fig. 243, EC).

The Wolffian bodies appear about the eighteenth day as a pair of longitudinal ridge-like thickenings of the dorsal wall of the body cavity, one on each side of the mesentery. These soon become more prominent, and by the beginning of the fourth week extend from about the sixth cervical to the last lumbar somite.


Each Wolffian body consists at first of rods of cells, which appear to arise independently of the Wolffian duct. The rods soon become S-shaped : early in the fourth week they acquire axial cavities, and so become tubes ; and by the end of the week the tubes, or Wolffian tubules as they may now be termed, grow towards the Wolffian duct and open into it. The opposite, or



FIG. 248. Transverse section across the body of a Human Embryo, estimated as fourteen days old. For a figure of the whole embryo, see Fig. 185, p. 481. The embryo had thirteen pairs of mesoblastic somites, and thesection figured passes through the tenth pair. (From Kollmann.) x 240.

A, aorta. C, coelom. CH, notochord. Q-T, mid-gut. KG, Wolfflan duct. ME,somatopleuric layer of mesoblast. MH, splanchnopleuric layer of mesoblast. MT, myotome or mesoblastic somite. M"C, central canal of spinal cord. M"S, spinal cord.

closed ends of the tubules become dilated, and then invaginated to form Malpighian bodies, the glomeruli being derived from branches of the aorta which penetrate into the Wolffian body along its whole length ; while the veins open into the largeposterior cardinal veins, which are intimately associated with the Wolffian bodies from their first appearance.

The Malpighian bodies are more abundant along the inner side of each Wolffian body, while the duct lies along its outer border, except at the hinder end, where it crosses to the inner side.

During the second month the Wolffian bodies grow rapidly the Malpighian bodies increase greatly, both in number and in size; and new Wolffian'tubules are formed, apparently by budding from the old ones. In each tubule the part next the Malpighian body, which is probably the secreting portion, has thicker walls, formed of larger epithelial cells, than the more distal part which opens into the Wolffian duct.

The Wolffian body reaches its greatest development about the eighth week, from which time it slowly diminishes in size. Degeneration commences, and proceeds more actively at the anterior end of the Wolffian body, which from the first has lagged behind the rest of the organ in development. Ultimately the whole structure becomes affected ; by the fifth month the Malpighian bodies have almost entirely disappeared, and in the end the Wolffian body becomes reduced to an accessory part of the reproductive apparatus.

2. The Kidney and Ureter

The ureter arises on each side as a diverticulum from the hinder end of the Wolffian duct, in the early part of the fourth week (Fig. 243, KD). This soon acquires an independent opening into the cloaca, a little way behind that of the Wolffian duct (Figs. 216, KD, and 233, N). At its opposite or blind end the ureter grows forwards, between the hinder end of the Wolffian body and the vertebrae. It dilates to form a somewhat elongated sac, which is the pelvis of the future kidney ; and from this sac branching tubular diverticula grow out (Fig. 234, N), and become the urinary tubules. These rapidly increase in number and in length ; Malpighian bodies are formed in connection with their distal ends, and the kidney structure is definitely acquired by the end of the second month, at which time the degeneration of the Wolffian body commences.

The bladder is formed by dilatation of the basal or proximal part of the allantois. Beyond the bladder the allantoic stalk loses its cavity and becomes a solid rod, the urachus, leading from the bladder to the umbilicus. The lumen usually disappears early in the fifth week, but it may persist for a much longer time, or even be present in the adult.

3. The Mullerian Duct

About the end of the fourth week, a longitudinal ridge-like thickening of the peritoneum appears along the outer side of each of the Wolffian bodies. The ridge lies close to the Wolffian duct, and extends along its whole length, but is quite independent of this.

Early in the fifth week, the Miillerian duct is formed in this ridge ; it is a narrow straight tube, lying along the outer side of the Wolffian duct, but distinct from this. Its anterior end opens into the body cavity by an elongated slit-like mouth, situated in a patch of thickened peritoneal epithelium, a little way in front of the anterior end of the Wolffian body. Posteriorly, the Miillerian duct ends blindly.

By the eighth week the Miillerian duct has undergone some changes. It commences in front with a wide funnel-like mouth, the margins of which are already slightly fimbriated. Behind this mouth, the duct runs straight backwards for some distance, along the outer side of the Wolffian body, then turns sharply inwards, crosses ventral to the Wolffian duct, and continues backwards in close contact with the Miillerian duct of the opposite side ; it still ends blindly behind.

In the male, the Miillerian ducts begin to atrophy shortly after reaching this stage. In the female, they undergo further development, and give rise to the oviducts, uterus, and vagina, as will be described in the section dealing with the accessory organs of reproduction.

4. The Head-kidney

Janosik has described, in an embryo eighteen to nineteen days old, what he thinks may prove to be a rudimentary pronephros, in the form of a couple of peritoneal funnels just in front of the anterior end of the Wolffian duct ;. the anterior funnel having close to it a structure not unlike an external glomerulus. The early development and subsequent fate of these structures have not yet been determined.

The Development of the Reproductive Organs

1. The Essential Eeproductive Organs

These have already been described, in the introductory portion of this chapter (pp. 449 to 457) ; but a few further details may conveniently be added here.

In embryos thirty-two days old (cf. Fig. 205), the genital ridges are present as a pair of bands of epithelium, many cells thick, and lying along the inner sides of the Wolffian bodies. Primitive ova are already present, and, according to Nagel, are found not only in the genital ridges themselves, but also beyond their limits, and especially in the thickened epithelium in the neighbourhood of the Miillerian ducts. This may perhaps be taken as an indication that the genital epithelium was originally less sharply circumscribed than at present.

Nagel has shown that distinct differences may be detected in the genital ridges of the two sexes from as early a period as thirty-two days ; and he is inclined to doubt whether there is absolute identity at any time, even in the earliest stages.

In the male, the genital ridge, at thirty-three days, shows a fairly definite arrangement of the cells in strings ; these form a network of tortuous anastomosing cords, arranged somewhat regularly, and bound together by connective tissue. Embedded in the cellular cords are the primitive sperm cells. These arecomparatively few in number ; their formation ceases at an early stage, in embryos of about six or seven weeks, on the completion of the tunica albuginea ; but in the later stages, although no new primitive sperm cells are formed from the germinal epithelium, those which are already present increase freely by division. The cellular cords themselves become converted into the seminal canals, which are thus derived directly from thegerminal epithelium.

In the female, the primitive ova, in embryos of thirty-three days, are much more numerous than the primitive sperm cells of the male. They are found in various phases of development, and the formation of new primitive ova continues until about the close of gestation. It is very doubtful whether any new primitive ova are formed after birth, and by some authorities their formation is believed to stop about the seventh month. The tendency for the smaller cells to become grouped around the primitive ova, and so form follicles, is evident even in the fifth week, and affords a good clue by which a young ovary may be distinguished from a young testis, and the sex of the embryo thus determined.

2. The Accessory Reproductive Organs

As in the rabbit, the chick, and indeed the great majority of Vertebrates, the genital ducts of the human embryo are formed from tubes which originally belong to the excretory system ; the oviducts being formed from the Miillerian ducts, and the vasa deferentia of the male from the Wolffian ducts ; while other portions of the embryonic excretory apparatus persist in a modified or vestigial form, as accessory organs in relation with the reproductive system.

a. In the Male

The Mullerian ducts begin to atrophy about the middle of the third month, and ultimately disappear completely along the greater part of their length. The anterior end of the Miillerian duct may persist, and in connection with it the hydatids of Morgagni are believed to be formed ; this name being given to one or more small pedunculated bodies, lying between the testis and the head of the epididymis. One of these bodies is of larger size, and more constant occurrence, than the others.

It is stated that the posterior ends of the Miillerian ducts unite together, and give rise to the uterus masculinus, a small pocket-like diverticulum from the dorsal wall of the prostatic portion of the urethra, a quarter to half an inch in depth, and bearing on its margins the slit-like openings of the vasa deferentia. The statement, however, needs confirmation.

The Wolffian body and Wolffian duct. The greater part of the Wolffian body disappears, but the anterior end becomes intimately connected with the testis, and persists throughout life. From the Wolffian tubules of this anterior end tubular outgrowths arise, which during the fourth month grow into the substance of the testis, and give rise to the vasa efferentia ; these soon become connected with the seminal tubes, which latter, according to Nagel, are formed directly from the germinal epithelium. The anterior Wolffian tubules become the coni vasculosi ; and the Wolffian duct is converted, in front, into the extremely tortuous epididymis, and further back into the vas deferens.

The structures known as the vasa aberrantia, a series of tortuous tubular diverticula from the lower end of the epididymis ; and the parepididymis, or organ of Giraldes, are probably persistent portions of some of the hinder Wolffian tubules.

b. In the Female

The Mtillerian ducts, at the beginning of the third month, are still quite distinct from each other. Their anterior ends, with the abdominal openings, are widely separate ; their posterior portions lie side by side, between and slightly dorsal to the Wolffian ducts, and bound up with these by connective tissue, to form what is spoken of as the genital cord. The Miillerian ducts still end blindly behind.

Towards the end of the third month, the two Miillerian ducts fuse together, opposite the middle third of the genital cord ; and from this point the fusion extends rather rapidly forwards, and much more slowly backwards. The fused portion, or uterovaginal canal, enlarges steadily, especially in its transverse diameter. By the beginning of the fourth month, a distinction appears between the uterine and vaginal portions of the canal ; the proximal portion, or uterus, being lined by a columnar epithelium, and the distal portion, or vagina, by a squamous epithelium.

During the fourth mouth, the boundary line between the uterus and vagina becomes a much sharper one. The uterus becomes considerably dilated : the vagina, on the other hand, is flattened dorso-ventrally ; and, by proliferation of its epithelial cells, its lumen becomes completely blocked up for a time, reappearing in the course of the fifth month.

The two Miillerian ducts thus give rise to the whole length of the female passages ; the anterior or proximal ends of the ducts remaining distinct from each other, and forming the oviducts or Fallopian tubes ; while the posterior or distal portions fuse together, and give rise to the uterus and vagina.

The fusion of the two halves of the uterus is not completed until the latter part of the fourth month ; and the occasional retention of a more or less complete uterine septum, even in the adult, is due to imperfect fusion of the two originally distinct ducts.

The cervix uteri is established during the fifth month, at the time when the lumen of the vagina is reappearing. The folds of the wall of the cervix, spoken of as the arbor vitse, appear during the fourth month; while the differentiation of the muscular walls, and of the enormously developed muscularis mucosae commences in the sixth month. The uterine epithelium is devoid of cilia during the whole of foetal life ; and up to the time of birth there are no glands in the body of the uterus. Glands are, however, present in the cervix, and apparently secrete the plug of mucus which commonly occupies the os uteri at the time of birth.

The Wolffian body. In the female, outgrowths from the anterior Wolffian tubules into the ovary occur, similar to those which in the male give rise to the vasa efferentia ; but they do not give rise to any adult structure.

A number of the Wolffian tubules of the anterior end of the Wolffian body persist throughout life, forming the structure known as the parovarium (Fig. 249, a), sometimes called the



FIG. 249. The adult Ovary, Parovarium, and Fallopian tube. From Quain ' Anatomy.' (After Kobelt.)

a, a, parovarium, epoophoron, or organ of Rosenmliller ; formed from the anterior end of the Wolffian body. 6, remains of some of the anterior Wolffian tubules, sometimes forming hydatids. e, the longitudinal duct of the parovarinm, formed from the anterior end of the Wolffian duct, rf, rudimentary Wolffian tubules, e, atrophied remains of the Wolfflan duct, or duct of Gaertner. /, the terminal bulb or hydatid. h, Fallopian tube, t, hydatid attached to the end of the Fallopian tube. /, ovary.

epoophoron or organ of Rosenmiiller; a series of transverse tubes which run, with a somewhat tortuous course, in the fold of peritoneum between the ovary and the Fallopian tube, and are connected with the anterior end of the ovary.

A small portion of the hinder part of the Wolffian body may persist as a rudimentary structure, the paroophoron, lying in the peritoneum opposite the hinder end of the ovary.

The Wolffian duct persists, in front, as the longitudinal duct of the parovarium (Fig. 249, c), into which the transverse tubules, a, open, and which corresponds to the epididymis of the male. The hinder part of the Wolffian duct usually disappears, but it may persist along a greater or less portion of its length as the duct of Gaertner, running alongside the Fallopian tube (Fig. 249, e), and sometimes extending along the walls of the uterus, or even as far as the vagina.

3. The External Genital Organs

These are practically identical in the two sexes, in the early stages of their development ; the distinction between male and female, as regards the external genital organs, not being evident until about the ninth or tenth week.

At the end of the fifth week (Fig. 234), the septum dividing the rectum from the urino-genital passage has almost reached the surface, but the two passages apparently open by a single cloacal aperture. Immediately in front of this aperture is a


FlG. 250. The external genitalia of a Human Embryo of about the ninth week (probably rather younger). (From Kolliker, after Ecker.) x 2.

c, genital tubercle, or clitoro-penis. /, groove, continuous with urine-genital passage. hi, labio-scrotal folds, n, umbilical cord, s, coccygeal region.

FIG. 251. The external genitalia of a Human Embryo of about the tenth

week. (From Kolliker, after Ecker.) x 2.

rt, anus, e, genital tubercle, or clitoro-penis. /, genital groove, continuous with urino-geuital aperture. M, labio-scrotal folds, s, coccygeal region.

small conical projection, sg, the genital tubercle or clitoro-penis. The posterior surface of this tubercle is marked by a longitudinal groove, which leads, through the cloacal aperture, into the urino-genital passage ; the lips of the groove are slightly swollen, and are continuous with the lips of the cloacal opening, which form the inner sexual folds. The tip of the genital tubercle is expanded into a small knob, the glans.

A little later, towards the end of the second month, the septum between the urino-genital passage and the rectum reaches the surface, dividing the cloacal aperture into two separate openings, an anterior or urino-genital (Fig. 25 1,/), and a posterior or anal (Fig. 251, a).

Up to this time the course of development is practically the same in all embryos, but from about the tenth week differences become apparent between the two sexes.

In the male, the genital tubercle elongates, and forms the penis. The lips of the groove, along the posterior surface of the tubercle, meet and fuse to form the canal of the penis, or penial urethra ; and, by a similar fusion of the lips of the urino-genital opening, the penial urethra and urino-genital passage become directly continuous with each other. The glans penis is at first solid, but towards the end of the third month the groove extends forwards along it, and, gradually closing from behind forwards, carries the opening of the urethral canal to the apex of the glans. The prepuce appears, towards the end of the third



FIG. 252. The external genitalia of a male Human Embryo towards the end of the third month. (From Kolliker, after Ecker.)

a, anus, f, penis. /, raphe formed by union of lips of genital groove, hi, scrotum. r, vaphe formed by union of the two halves of the scrotum, s, coccyx.

FIG. 253. The external genitalia of a female Human Embryo towards the end of the third month. (From Kolliker, after Ecker.)

a, anus. >, clitoris. /, genital groove, ug, urino-genital aperture. M, labia majora. n, labia minora. or inner genital folds, s, coccygeal region.

month, as a fold of skin round the base of the glans, and is at first interrupted ventrally by the urethral groove.

The scrotum is formed from a pair of folds of skin, the labioscrotal or outer genital folds, which arise at the sides of the urino-genital opening, and ultimately unite with each other in the median plane behind the penis (Fig. 252, hi).

In the female the genital tubercle remains small and becomes the clitoris (Fig. 253, e) ; and the genital groove remains open. The inner genital folds (Fig. 253, w), at the sides of the urinogenital opening, become the labia minora or nymphas ; while the outer genital, or labio-scrotal folds, lit, become the labia majora, and, in front, the mons Veneris.


The urino-genital canal shortens considerably in the female, so as to bring the aperture of the urethra close to the surface.

The above changes are usually completed in both sexes by the end of the third month ; but they may be delayed until a much later date.


The Foetal Membranes and the Placenta

1. The Amnion

The amnion is the thin transparent membrane which invests the embryo like a sac, covering its dorsal surface and sides (</. Fig. 197, AN).

The mode of formation of the amnion in the human embryo has not yet been determined. In Reichert's ovum, estimated to be twelve or thirteen days old, there was no trace of an amnion present (Figs. 172 and 173); while in the embryos E and SR (Figs. 176, 178, 179), which are believed to be of the thirteenth day, the amnion is already fully formed. Figs. 18G to 188 show the mode in which the development of the amnion is believed to occur, by growth backwards of a fold of the wall of the blastodermic vesicle over the embryo ; but the figures are purely hypothetical, and the intermediate stages which they represent have not been seen.

Of the two layers of which the amnion consists, the outer one (cf. Fig. 188) is simply a part of the wall of the blastodermic vesicle, and it is usual to limit the term amnion to the inner layer, which more immediately invests the embryo. The space between this inner layer, or amnion, and the embryo is spoken of as the amnionic cavity, and is filled with fluid.

The rate of growth of the amnion, as compared with that of the embryo itself, varies considerably at different periods of development. On its first formation, about the thirteenth day, the amnion invests the embryo fairly closely (Fig. 179). During the third week the amnion grows rather more rapidly, so that the space between it and the embryo enlarges somewhat (Fig. 196). During the fourth week, as at the corresponding stages in the rabbit or chick, the embryo grows considerably, and at the end of the week the amnion invests it very closely.

During the second month the amnion enlarges much more rapidly, and the amnionic cavity becomes a space of considerable size, filled by the liquor amnii (Fig. 25i). Owing to this increase in its dimensions, the amnioii forms a sheath around the umbilical cord, and also comes into close contact with the wall of the blastodermic vesicle over the whole extent of its inner surface.

The liquor amnii, which occupies the amnionic cavity, between the amnion and the embryo, varies much in quantity at different periods of gestation. It is apparently most abundant about the fifth or sixth month. Its actual quantity is difficult to fix, as it varies greatly in different cases ; wherein excess, i.e. more than about 1^ litre, it constitutes the affection known as hydrops amnii.

The liquor amnii contains urea, especially during the later months of gestation ; this appears to be a true excretory product, separated by the kidneys of the foetus, and discharged through the urino-genital aperture into the amnionic cavity.

Structurally, the human amnion consists, like that of the rabbit or chick, of a single layer of epiblast cells, supported on a thin layer of mesoblast (cf. Figs. 182 to 184). The mesoblast consists of a homogeneous matrix, with embedded cells ; while the epiblastic epithelium is, according to Minot, noteworthy on account of the distinctness with which the intercellular bridges of protoplasm, connecting the several cells with one another, can be made out; the boundaries between adjacent cells being formed, not by divisional planes, but by lines of vacuoles, between which the protoplasmic bodies of the cells are directly continuous with one another.

2. The Umbilical Cord

The umbilical cord, which connects the embryo with the placenta (Fig. 254), is formed in the first instance by the allantoic stalk (Fig. 179, 197, TZ). This stalk, in the human embryo, is directly continuous, from the first, with both the embryo and the wall of the blastodermic vesicle (Figs. 18G to 188), and has the appearance of a direct prolongation backwards of the hinder end of the embryo. On the formation of the tail, the allantoic stalk is gradually driven down to the ventral surface of the embryo (cf. Figs. 179, 197, 198, TL and TZ), and acquires the position and relations characteristic of it in rabbit or chick embryos at corresponding periods.

The chief purpose of the allantoic stalk is to afford a path, along which the allantoic vessels can pass from the embryo to the placenta (Fig. 198, AA, VA) ; and the explanation of the pre


FIG. 254. A diagrammatic section of the pregnant Human Uterus at the seventh or eighth week. (From Quain's 'Anatomy,' after Allen Thomson.)

al, allantoic stalk, am, true amnion ; the part shaded horizontally, between the amnion and the embryo, is the amnionic cavity, c, cavity of the uterus, c', plug of mucus in the cervix uteri, ch, chorion. dr, decidua reflexa. ds, decidua serotina. dv, decidua vera. f, intestine of embryo. , allantoic arteries, y, yolk-sac, y', yolk-stalk.

cocious appearance of the allantoic stalk, and' probably also of the peculiarities of its development, in human embryos, is to be found in the importance of establishing vascular relations between the embryo and the mother at as early a period as possible.

In the later stages of development, the yolk-stalk, or pedicle of the yolk-sac (Fig. 254, y') : becomes closely applied to the allantoic stalk, and bound up with it in a sheath formed by the spreading amnion, am ; and it is to the compound structure formed of these elements that the name umbilical cord is given.

The umbilical cord increases considerably in length during development. About the middle of gestation it is usually from 13 to 21 cm. long, and from 9 to 11 mm. thick. At the time of birth, its average length is from 48 to 60 cm., and its thickness 11 to 13 mm. ; but it is liable to very great individual variations. It may be as short as 12 cm. ; or, on the other hand, may attain a length of 167 cm.

The umbilical cord is almost invariably twisted spirally on itself, and the cause of this twisting, which commences about the middle of the second month, has been the subject of much discussion. If examined more closely, it is found that all the constituents of the cord are not twisted to the same extent ; the spirals described by the allantoic arteries being always more numerous, and closer together, than those of the whole cord, or than those of the veins round which the arteries appear to twist. The twisting appears to be due to the allantoic arteries increasing in length more rapidly than the other constituents of the cord, and so being compelled to adopt a tortuous instead of a straight course. The allantoic arteries may describe as many as thirty or forty complete turns in passing from the foetus to the placenta.

As the spiral growth involves the whole umbilical cord, and this cord is fixed at its placental end, it is clear that, as the cord twists, the embryo must rotate in the liquor amnii. The cord may become twisted round the neck of the foetus, and may even be tied in knots : these knots being produced by the cord, at an early stage of development, becoming thrown into a loop, and the embryo then floating through the loop.

Structure of the umbilical cord. The fully developed umbilical cord consists of the following structures (Fig. 254) :

1 . The sheath formed around it by the amnion. This invests the cord very closely, except at its insertion into the placenta.

2. The right and left allantoic arteries, u. These are usually quite distinct from each other along the greater part of the length of the cord, but just before reaching the placenta are almost invariably united by an anastomotic branch.

3. The allantoic vein. This has thinner walls than the arteries, and has also, according to Kolliker, rudimentary valves. There are at first two allantoic veins, but the right one is, almost from the first, smaller than the left, and disappears completely about the fourth week.

4. The epithelial lining of the allantoic cavity. During the first and early part of the second month, the allantoic stalk is hollow, its cavity extending from the cloaca of the foetus along the whole length of the cord, as far as the wall of the uterus. Later on, in the third or fourth month, the cavity becomes constricted, or altogether obliterated. Isolated portions of it may, however, persist, especially at the proximal or foetal end of the cord, up to the time of birth.

5. The yolk-stalk and its vessels, the vitelliiie arteries and veins. These usually disappear during development, and are seldom to be distinguished in the cord at full time. The yolkstalk at first lies in a groove in the allantoic stalk ; but it soon becomes completely surrounded by this latter, and then ceases to be distinguishable.

6. The Whartonian jelly : this forms the matrix of the cord, in which are embedded the various structures described above. It consists of a complex network of branching connective-tissue cells, embedded in a clear gelatinous matrix. Immediately beneath the surface epithelium, and around the blood-vessels and the allantoic cavity, the connective-tissue meshwork is rather denser than elsewhere; and, in the matrix, fibres are developed, especially during the later months of gestation.

7. Up to the close of the third month, the end of the umbilical cord next the embryo contains, as already noticed, a loop of the intestine (Fig. 254, ), but after this date the alimentary canal is, as a rule, completely withdrawn into the body of the foetus.

3. The Chorion

The term chorion has been used in very different senses by writers on embryology. It is convenient to employ it for that part of the blastoderm, or blastcdermic vesicle, which is not directly concerned in the formation of the embryo. It is usual to exclude the amnion from the definition ; but in the case of the human embryo the outer layer of the amnion, or ' false amnion ' as it is commonly called in other Vertebrates, is so directly continuous with the wall of the vesicle that it is better to include it under the same name.

Thus in Reichert's ovum (Fig. 174) the chorion is the whole wall of the vesicle, except the embryonic area, a. In His' embryo E (Fig. 188), the chorion forms the entire wall of the vesicle, the embryo being now depressed within its cavity.

It is the chorion which comes in contact with the walls of the uterus (Figs. 254, 255), and it is from the chorion that the foetal part of the placenta is developed.

The human chorion is remarkable for its veiy early and ' complete separation from the yolk-sac (Figs. 186 to 188); and I a also for the very early period at which villi are developed from its outer surface.

Structurally, the chorion consists of an outer layer of epiblast, which from the first is two cells thick ; and an inner and thicker layer of mesoblast, which very early becomes vascular, the bloodvessels being derived from the allantoic arteries and veins, which reach the chorion along the allantoic stalk, and which are, of course, directly continuous with the blood-vessels of the embryo.

In Reichert's ovum (Figs. 172, 173, 174), the villi are confined to a broad marginal zone round the equator, the centres of the two flattened surfaces forming bare patches. At a very slightly later period, in His' embryo E, and in others of about the thirteenth day (Figs. 175, 188), the villi cover the entire surface of the chorion.

The chorionic villi consist at first entirely of epiblast. They arise as solid buds of epiblast, which become hollow as they increase in size ; and at a later stage the mesoblast grows into them along their axes, carrying the blood-vessels with it. During the fourth week the villi grow actively ; they branch freely, and in a very irregular manner. They penetrate the decidua, or modified mucous membrane of the uterus, to a slight depth ; but do not, as was formerly believed to be the case, grow into the uterine glands. They become attached to the decidua at their tips, but remain free along the rest of their length. As in their first appearance, so also during the later stages of their growth, the epithelial layer, or epiblast, is always in advance of the mesoblastic connective-tissue core ; the villi presenting lateral processes, or knobs, caused by local thickenings of the epithelium, into which at a later stage the vascular connective tissue penetrates.

The villi are at first of uniform size over the whole surface of the chorion (Fig. 188) ; but towards the end of the second month, or early in the third, they begin to develop unequally. Opposite the decidua serotina, or part of the uterine wall to which the ovum is directly attached (Fig- 254, cZs), the villi increase greatly in size and in complexity, forming ultimately the foetal part of the placenta. Over the rest of the surface of the chorion, opposite the decidua reflexa, cZr, the villi, on the contrary, begin to shrink ; the blood-vessels which supply them undergoing at the same time a gradual diminution in size.

In this way a distinction is established between the chorion frondosum, opposite the decidua serotina, which is very vascular, and beset with closely placed and richly branched villi ; and the chorion laeve, opposite the decidua reflexa, which is a thin transparent membrane, with no blood-vessels, and connected with the decidua reflexa merely by a few scattered, slightly branched, and inconspicuous villi. By the end of the fourth month, the villi of the chorion lasve have almost completely disappeared, except from a narrow fringe round the margin of the placenta, where they persist until the close of gestation.

Up to the end of the third month, the villi can be fairly readily withdrawn from the crypts of the decidua in which they are lodged, and the foetal and maternal structures thus separated from each other ; but, after the placenta is definitely established, the connection between the fcetal and maternal elements becomes so intimate that complete separation is no longer practicable.

The epithelium of the chorion frondosum undergoes important changes during the later months of gestation. Of the two layers of cells of which it consists from the first, the inner or deeper layer becomes thickened in irregular patches, very variable in number and in size ; the individual cells are also very irregular, and show signs of degenerative changes. The outer, or surface layer of epithelium undergoes more extensive changes. The cell boundaries become lost, and the cell bodies run together to form a dense stratum, in which the nuclei remain visible for a time ; ultimately the nuclei disappear, and the whole layer becomes modified into a hyaline, very refractive substance, permeated by numerous channels, so as to present a reticular appearance, and absorbing staining reagents very readily. This substance, formed by degeneration of the surface epithelial cells of the chorion, has been described, before its epithelial origin was known, as canalised fibrin.

Over the villi, the deeper or cellular layer of the epithelium disappears in great part, persisting only in isolated patches. The surface cells become converted in great part into a fibrin layer, similar to that of the chorion frondosum itself.

In the chorion laeve the epithelium retains its cellular character, and no fibrin layer is formed.

4. The Decidua

The decidua is the mucous membrane of the pregnant uterus. The early stages in its formation are, so far as they are known, identical with those by which the catamenial or menstrual decidua is formed. The mucous membrane becomes thicker and more pulpy than in its quiescent condition ; the blood-vessels enlarge ; the glands elongate, and their deeper ends become tortuous and dilated ; the deeper part of the mucosa becomes crowded with modified, and apparently proliferating connective-tissue cells; and the surface epithelium, lining the uterus, together with the immediately underlying connective tissue, show a tendency to disintegrate.

Up to this point, the formation of the catamenial decidua and of the decidua of pregnancy appear to be identical; the sole difference between the two is that, in the former, the processes having reached a certain point, stop and then become retrogressive, the decidua being broken up and discharged, together with a certain amount of blood, as the menstrual fluid ; while, on the other hand, in the case of the decidua of pregnancy, development, after reaching the point mentioned, does not stop, but proceeds to further stages of elaboration.

The difference between the two courses seems to depend solely on the presence of a fertilised ovum within the uterus in the latter case, and on the absence of such an ovum in the former; so that the catamenial decidua may be viewed as a preparation on the part of the uterus for an ovum which never reaches it ; the decidua, after waiting a certain time, becoming broken up and discharged. If, however, impregnation is effected, and a fertilised ovum reaches the uterus, a new stimulus is set up, and the developmental processes, instead of stopping, go on to further stages, and so give rise to the decidua of pregnancy.

Prior to the arrival of the ovum in the uterus, the decidua forms a complete lining to the uterus. It does not cover the orifices of the Fallopian tubes (Fig. 254), which remain open throughout the greater part or the whole of pregnancy ; neither does it extend into the cervix uteri, but stops abruptly at the os internum. With these exceptions, the decidua forms a layer, of approximately uniform thickness and structure, covering all parts of the uterine wall.

It seems to be, to a great extent, a matter of chance with what part of the uterus the ovum will come in contact, on entering its cavity ; and it is therefore important that all parts of the surface should be equally prepared to receive it. In the great majority of cases, the attachment of the ovum is in the neighbourhood of the fundus, usually rather to one side of the median line, and more frequently on the dorsal than the ventral surface. It may, however, be situated in almost any part of the uterus ; and its position may become a point of much practical importance. Ercolani has suggested that the ovum, on entering the uterus, is prevented from at once sinking to the cervix, by the fluid secreted by the utricular glands of the uterus, and that it floats on the surface of this fluid, until it comes in contact with, and adheres to, the wall of the uterus ; the actual place of contact would in this case vary considerably, according to the amount of fluid in the uterus at the time.

The youngest ovum yet found in situ within the human uterus, that described by Reichert, was not simply attached to the decidua, but completely embedded in this (cf. Fig. 175); a relation which is retained throughout the whole period of gestation (Fig. 254).

There has been some discussion as to the mode in which this encapsuling of the ovum is brought about ; there are no direct observations on the point in the case of human embryos, but the fact that the opening? of the uterine glands occur on both surfaces of the encapsuling layer of the decidua, together with the known facts in regard to other Mammals, render it practically certain that the view first advanced by Sharpey is correct, and that, immediately after the ovum has attached itself to the uterine wall, the decidua grows up as a fold around and over it, so as to encapsule it ; the object being, partly to maintain the ovum in contact with the uterine wall ; and partly, perhaps mainly, to provide an increased extent of vascular surface from which the embryonic villi can draw nutriment.

The fold of the decidua which incloses, or encapsules, the ovum is spoken of as the decidua reflexa; it is at first very thin (Fig. 175, DX, and Fig. 254, dr), but it has the same structure as the other parts of the decidua. In its early stages it is exceedingly vascular, the vessels converging from its margin to a small patch of a cicatricial appearance on its most prominent part, which probably indicates the point of meeting, and fusion of the folds by which it is formed.

The part of the decidua to which the ovum is directly attached, and from which the decidua reflexa is developed, is called the decidua serotina (Fig. 175, DW, and Fig. 254, ds) ; while the term decidua vera is given to the whole of the rest of the decidua, which lines the cavity of the uterus, but has no direct relation with the embryo (Fig. 175, DV, and Fig. 254, dv).

The decidua vera plays no part in the nourishment of the embryo, and during the latter half of pregnancy becomes greatly reduced in thickness, and undergoes degenerative changes ; that it should be formed at all is due, as already noticed, to the fact that, as it is quite uncertain with which particular part of the uterine wall the ovum will come in contact, all parts must be ready in the first instance to receive it.

The fact that the decidua vera, though lining the greater part of the uterus, takes no share in the nutrition of the embryo, and after attaining a certain stage, first stops, and then undergoes retrograde development, renders the comparison between the menstrual decidua and the decidua of pregnancy a still closer one ; it further helps to render intelligible the not very uncommon cases in which menstruation takes place at least once after conception has occurred ; and also those much rarer cases in which it has been stated to occur regularly throughout the greater part or even the whole of pregnancy.

The decidua reflexa and decidua serotina are at first of very small extent (Fig. 175). However, as the chorionic vesicle with its contained embryo increases in size, the decidua reflexa necessarily grows with it. This growth is at first more rapid than



FIG. 255. A pregnant Human Uterus of about the twenty-fifth day. The uterus has been cut open longitudinally from the ventral surface; the decidua reflexa has been cut open, and the flap turned down to expose the chorionic vesicle ; and the right ovary has been bisected to show the large corpus luteum. The embryo that was taken from this chorionic vesicle is shown in Fig. 199. (From Quain's ' Anatomy,' after Coste.)

dr, decidua reflexa. dv, decidua vera. o, cavity of the decidua reflexa, in which the clioriouic vesicle is lying, u, uterus.


that of the uterus as a whole, and in consequence the decidua reflexa ultimately comes in contact with the decidua vera, and so completely obliterates the cavity of the uterus (cf. Fig. 254). This usually occurs about the sixth month : the two layers, decidua reflexa and decidua vera, are generally described as not only coming in contact, but as fusing more or less completely together, so as to form a single membrane ; but according to Minot's observations, the decidua reflexa, which early undergoes degenerative changes, is entirely absorbed by the sixth month, so that the chorion comes into contact with the decidua vera.

So long as the uterine cavity remains an actual one, i.e. up to the time when the chorion meets with the decidua vera, there remains an open passage from the vagina, through the uterus and along the Fallopian tube, to the ovary ; and it is, at least theoretically, possible for spermatozoa to reach the ovary, and for what is termed super-fostation to occur.

The decidua serotina is simply the part of the decidua with which the impregnated ovum comes in contact, on entering the uterus, and to which it adheres ; and it is at first, therefore, identical in structure with the decidua vera. It very early, however, acquires special characters, owing to the chorionic villi of the ovum becoming intimately connected with it. For some time longer, the decidua serotina and decidua reflexa still remain closely similar to each other ; but towards the end of the second month (Fig. 254), the chorionic villi opposite the decidua reflexa begin to diminish in size and importance, and to show signs of degenerative changes ; while those in connection with the decidua serotina become much larger and more complicated. The relations between the foetal villi and the maternal tissues become still more intricate, and gradually the complex and elaborate structure of the fully formed placenta is acquired.

With regard to the detailed changes that take place in the different parts of the decidua during pregnancy, our knowledge is still imperfect in many important respects.

In the region lined by the decidua vera, the mucous membrane becomes greatly thickened ; and the uterine glands become dilated and elongated, and acquire exceedingly tortuous courses. By the end of the fifth month the mucous membrane is nearly half an inch thick. The surface layer, about one-fourth of the entire thickness, is spoken of as the stratum compactum : in it the gland tubes remain comparatively straight and narrow, while in the interglandular tissue numbers of large, epitheliumlike, decidual cells appear, apparently formed by modification of connective-tissue corpuscles. The deeper three-fourths of the thickness of the mucous membrane, or stratum spongiosum, has a different structure, the gland tubes being greatly dilated and very irregular in shape, and their lining epithelium consisting of flattened or cubical, in place of columnar cells.

After the fifth month, by which time the chorion has met with the decidua vera so as to obliterate the cavity of the uterus, the decidua vera gradually becomes thinner and less vascular, and undergoes degenerative changes, leading ultimately to the almost complete disappearance of the glands, with the exception of their deepest or outermost ends.

The decidua reflexa goes through changes of a very similar kind : the glands first become dilated and lengthened, and then, as the decidua reflexa becomes more and more distended through the enlargement of the chorionic vesicle, the glands gradually atrophy, and the entire layer degenerates, and ultimately completely disappears.

In the decidua serotina, from which the maternal portion ot the placenta is formed, the changes have been followed in more detail, but are still only imperfectly known. There is the same division into two layers as in the decidua vera : (i) an inner or superficial layer, from which the surface epithelium and all traces of the uterine glands disappear entirely, and in which decidual cells are developed in large numbers ; and (ii) an outer or deeper layer, in which the gland cavities remain as irregular clefts, from which the epithelium has disappeared ; except in the very outermost layer, in immediate contact with the muscular wall of the uterus, where the outer or blind ends of the glands persist, greatly compressed, but retaining their epithelium. It is from these outer ends of the glands that the epithelial lining of the uterus is regenerated, after the separation of the placenta.

The further changes that occur in the decidua serotina, and more especially the relations of the blood-vessels, are described in the next section.


5. The Placenta

The fully formed placenta, at the close of gestation, is a discoidal or cake-shaped body, of spongy consistency, measuring from 16 to 21 cm. in diameter and 3 to 4 cm. in thickness. It is attached to, or rather forms part of, the inner wall of the uterus (cf. Fig. 254), and to its inner or free surface, usually a little distance from its centre, the umbilical cord is attached, the opposite end of the cord being connected with the foetus.

The placenta consists of outer or maternal, and inner or foetal layers, derived respectively from the decidua serotina, and from the chorion. The distinction between the foetal and the maternal elements is easily made in the early stages of development ; but in the fully formed placenta, owing to the intricacy of the relations between the chorionic villi and the maternal blood-vessels, and the profound histological modifications which almost all parts undergo, it becomes a matter of the greatest difficulty to determine the real nature of the several structures met with ; and there are important points, more especially in regard to the relations of the maternal blood-vessels, on which our knowledge still remains imperfect and unsatisfactory.

The placenta consists of three chief layers or strata : (i) an inner layer formed by the chorion ; (ii) an outer layer formed by the decidua serotina, or modified mucous membrane of the uterus ; (iii) a middle or intermediate layer, which is much thicker than the other two, forming four-fifths or more of the entire thickness of the placenta, and which consists of the intricately branched foetal villi, together with the maternal sinuses with which these are in relation.

Of these three layers, the inner one is distinctly foetal, and the outer one maternal in origin ; the middle layer is foetal as regards the villi themselves, but the precise relations of the maternal vessels are still undecided. This middle, or villous zone is the characteristic, and the functionally active part of the placenta. At the margin of the placenta it thins out and disappears, and the inner and outer layers, chorionic and decidual, come into contact with each other in the region of the decidua reflexa.

The inner or chorionic layer of the placenta has already been described (pp. 601 to 605). The amnion is closely united with its inner or free surface over its whole extent. The outer surface, towards the villous zone, is characterised by the presence of patches of the substance spoken of as canalised fibrin, an^ shown by Minot to be produced by a peculiar mode of degeneration of the surface epithelial cells of the chorion. Round the margin of the placenta, the chorionic and decidual layers are so intimately fused that it is impossible to make out a boundary line between them : the decidual cells invade the chorion, and the two layers undergo degenerative changes in common. .

The outer or decidual layer of the placenta, formed by the decidua serotina (Fig. 254, ds), is about 1-5 mm. thick, and shows the same distinction into inner or compact, and outer or spongy layers already noticed in the decidua vera. Both layers contain very numerous decidual cells, formed, as elsewhere, by modification of connective-tissue cells. The decidual cells of the compact layer are smaller and more crowded than those of the spongy layer. Their actual size varies very greatly, and the largest ones may contain as many as ten nuclei.

The uterine glands have disappeared from the compact layer of the decidua, but their outer or blind ends persist in the spongy layer as irregular slit-like cavities, filled for the most part with fine granular matter, and retaining in places their lining of glandular epithelium. The outer surface of the spongy layer is closely united with the muscular wall of the uterus ; and groups of decidual cells may penetrate between the muscle fibres at places.

The middle or villous zone of the placenta is much the thickest of the three, and is also the most important, and the most complicated. It may be described, roughly, as a huge sinus filled with maternal blood, and divided up into a labyrinth of intercommunicating loculi by a framework of fibrous bands and partitions, which run across between the decidual and chorionic walls ; the loculi being occupied by forests of arborescent villi, which arise from the chorionic wall and are richly supplied with capillaries derived from the allantoic vessels of the foetus.

The villi branch extraordinarily freely : their stems arise from the chorion, and the majority of the branches end freely, but many are attached either to the uterine decidua, or to the partitions separating the loculi.

The foetal blood-vessels. The blood is carried from the foetus to the placenta by the two allantoic arteries, which run in the substance of the umbilical cord. On reaching the placenta the arteries branch freely, and very irregularly ; the branches spread over the surface of the placenta, between the amnion and the chorion, and dip rather suddenly into the substance of the placenta. Within this they branch freely, and approach the uterine surface of the placenta by a series of terrace-like steps, spreading out horizontally, and then dipping suddenly towards the surface, two or three times in succession ; ultimately they enter the villi, and follow their branches to their finest ramifications. ,

The capillaries of the villi, into which the arteries finally pass, are variable in diameter, and exhibit irregular dilatations and constrictions ; their average size is, however, very large, as they frequently admit from four to six red-blood corpuscles abreast.

The capillaries unite at their further ends to form veins, which follow generally the same paths as the arteries, and finally leave the placenta as the allantoic vein, which, running along the umbilical cord, returns the blood from the placenta to the foetus.

The fcetal vessels thus form a closed set of blood-vessels, which have proper walls of their own, of the usual structure, along their entire course, and which show no special peculiarities except the large size of the capillaries in the villi.

The maternal blood-vessels are derived directly from the uterine arteries and veins. The arteries, which are of comparatively small size, run with a very tortuous course, from which their name ' curling arteries ' is derived, through both the spongy and compact layers of the decidua, and open suddenly into the great sinuses or loculi of the placenta. From these sinuses the blood is returned by veins, which run very obliquely through the decidua, and eventually join the veins of the muscular wall of the uterus.

The foetal villi are thus bathed by a slowly moving stream of maternal blood ; and the necessary nutritive and respiratory interchanges must be effected in this middle or villous zone of the placenta. Nothing is known as to the exact mode in which these are Carried on ; whether by simple diffusion, or whether aided or modified by active participation of epithelial or other cellular elements. The one thing that is quite clear is, that in the placenta the foetal and maternal blood streams are kept apart, and that no actual mixing of blood from the two sources can occur.

The real nature of the sinuses, or loculi, in which the maternal blood lies has been much discussed, and is not yet determined with certainty.

It was formerly held that they are produced by enormous dilatation of the capillaries, which normally connect the uterine arteries and veins together ; and Waldeyer has shown that these sinuses have a distinct epithelial lining, continuous with that of the uterine vessels.

Kolliker and Langhans point out that, on the foetal side of the placenta, the walls of the sinuses are formed by the chorion, and show no trace of decidual structure ; they, therefore, suggest that the sinuses are not really maternal capillaries, but are .spaces between the maternal and foetal portions of the placenta, i.e. between the decidua and the chorion, into which the blood has penetrated by extravasation, or by rupture of the uterine vessels, consequent on the degeneration which the uterine mucous membrane is known to undergo.


On the other hand, a comparison of what is known concerning the human uterus, with the facts ascertained in regard to the formation of the placenta in the rabbit and in other Mammals, leads to the belief that the sinuses may prove to be spaces formed by absorption, not within the maternal tissues, but in the chorionic epithelium itself; in which case the whole thickness of the villous zone of the placenta would be of fcetal origin.

Separation of the Placenta at Birth

In parturition, the contraction of the muscular walls of the uterus, and the consequent pressure on the uterine contents, more especially upon the amnionic fluid, causes the investing membranes of the embryo, i.e. the combined decidua vera and decidua reflexa, the chorion, and the amnion, to bulge through the os uteri. On rupture of the membranes, the amnionic fluid first escapes, and subsequently the foetus is expelled.

The further contraction of the uterus detaches the placenta from the uterine wall, the plane of separation passing through the outer or spongy layer of the decidua, in which the deeper parts of the uterine glands still persist ; and the placenta, with the decidua, the chorion, and the amnion forming a membranous fringe around it, is in its turn expelled as the after-birth. The continuation of the uterine contraction, after the expulsion of the placenta, checks, and in normal cases reduces to a minimum, the haemorrhage that necessarily results from the tearing across of the maternal vessels along the plane of separation of the placenta.

The deepest part of the spongy layer of the decidua, in which are the blind ends of the uterine glands, remains in the uterus as a thin lining to the muscular walls ; and, from the epithelium of these persistent parts of the glands, the entire uterine epithelium is speedily regenerated.


Bibliography

List of the more important Publications dealing with, or bearing on the Development of Man.


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Marshall (1893): 1 Introduction | 2 Amphioxus | 3 Frog | 4 Chick | 5 The Rabbit | 6 Human Embryo | Illustrations

Marshall AM. Vertebrate Embryology: A Text-book for Students and Practitioners. (1893) Elder Smith & Co., London.

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