Book - Buchanan's Manual of Anatomy including Embryology 2

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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!

Frazer JE. Buchanan's Manual of Anatomy, including Embryology. (1937) 6th Edition. Bailliere, Tindall And Cox, London.

Buchanan's Manual of Anatomy: I. Terminology and Relative Positions | II. General Embryology | III. Osteology | IV. Bones of Trunk | V. Bones of Head | VI. Bones of Upper Limb | VII. Bones of Lower Limb | VIII. Joints | IX. The Upper Limb | X. Lower Limb | XI. The Abdomen | XII. The Thorax | XIII. Development of Vascular Systems | XIV. The Head and Neck | XV. The Nervous System | XVI. The Eye | XVII. The Ear | Glossary
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Chapter II General Embryology

Embryology treats of the embryo and the development of its tissues and organs from the stage of the fertilized ovum to their mature condition.

Two factors are concerned in the formation of the embryo—namely, (i) the male pronucleus, formed by the head and a portion of the middle piece of a spermatozoon or male germ-cell, and (2) the female pronucleus and the cell-body of the mature ovum or female germ-cell. The two factors together lead to the fertilized ovum. The embryo is composed of cells derived from the fertilized ovum. Every cell comes from a pre-existing cell by cell-division. It will be well to consider first the general structure and mode of division of an animal cell before describing those of the specialized germ-cells.

The Animal Cell

The animal cell is a mass of a living substance called protoplasm. The essential component parts of the cell are (1) a cell-body, and (2) a nucleus. The nucleus may contain one or more nucleoli, but these are not essential elements. The protoplasm of the cell-body is called the cytoplasm, or cell-protoplasm, and it may be enclosed (as in the ovum) within an envelope, called the cellmembrane, which is simply a condensation of the peripheral cytoplasm. The protoplasm of the nucleus is called the karyoplasm, or nucleoplasm, and it is enclosed within an envelope called the nuclear membrane. The animal cell is therefore ‘ a mass of protoplasm containing a nucleus.’

Fig. 2.—The Animal Cell. distinguished from the karyoplasm,

which is the protoplasm of the nucleus. It is viscid, translucent, and more or less granular. At the periphery it may be condensed to form a cell-membrane. The basis of the cytoplasm consists of a network of slender filaments, which is known as the spongioplasm or cyto-reticulum. The meshes of this reticulum are occupied by a semifluid substance called the hyaloplasm.

The cytoplasm contains granules, which are called cyto-microsomes. The hyaloplasm, in addition to the cyto-microsomes, contains several non-protoplasmic bodies— e.g., food-particles and pigment-granules— which are known as the deutoplasm.

In most cells, usually close to the nuclear membrane, but external to it, there is a small spherical area of cytoplasm, from which lines radiate outwards into the cell-protoplasm. This area is called the centrosome or attraction-sphere, and the protoplasm around the area is known as the archoplasm. At the centre of the centrosome there are usually two small nodules of protein matter, called the central or attraction-particles , from which lines radiate outwards into the archoplasm and cytoplasm. The centrosome thus constitutes the aster, and it plays an important part in nuclear division by mitosis.

The cell-membrane, when present, is a condensation of the peripheral cytoplasm. In many cells, however, it is absent.

The Nucleus.—The nucleus is usually situated eccentrically in the cytoplasm. Its protoplasm is called karyoplasm, and the nuclear elements are as follows:

1. Nuclear membrane. 3. Karyoplasm.

2. Nuclear reticulum. 4. Nucleoli.

The nuclear membrane is a well-defined envelope which surrounds the nuclear contents and separates them from the cytoplasm. It consists of the elements of the nuclear reticulum—namely, nuclein containing chromatin, and linin.

The nuclear reticulum, .which corresponds to the spongioplasm of the cell-protoplasm, consists of nuclein, containing a stainable material called chromatin, arranged in granules. These granules are connected by threads of linin.

The karyoplasm, which corresponds to the hyaloplasm of the cellprotoplasm, occupies the meshes of the nuclear reticulum, and contains granules, known as karyosomes.

The nucleolus (sometimes absent) may be one or more in number. There are two kinds of nucleoli—true and false. The true nucleoli lie in the nuclear reticulum, or, it may be, in the karyoplasm. The false nucleoli are nodes which are connected with the filaments of the nuclear reticulum, where they intersect.


Cells increase in number by division: this is therefore a physical necessity for growth and, more indirectly, for differentiation.

There are two kinds of cell-division—namely, karyokinetic or mitotic, which is indirect division, and akinetic or amitotic, which is direct division.

Karyokinesis or Mitosis.—This kind of cell-division is of a very complicated nature, and the changes involved affect both the nucleus and the centrosome. It is convenient to consider it under four phases - namely, (i) the anaphase, (2) the metaphase, (3) the kataphase, and (4) the telophase.

Anaphase.—The anaphase constitutes the preparatory stage, and it includes three phenomena, all of which lead ‘ up ’ to the metaphase, as follows:

1. Formation of spireme. 2. Formation of chromosomes.

3. Formation of spindle.

Spireme. — The chromatin and linin of the nuclear reticulum and nuclear membrane become transformed into a coiled thread, called the spireme or skein.

Chromosomes. — The spireme is broken up transversely into an even number of segments, called chromosomes, the number of these being constant and characteristic of the species of animal. These chromosomes usually assume the form of short rods, which resemble a V.

Spindle. — Whilst the chromosome-stage is in progress, important changes take place in the stellate centrosome or aster. It divides into two segments, each division taking up a central or attraction-particle, and being furnished with radiating fibres. In this manner two centrosomes or asters are formed. Certain of the radiating fibres extend from one centrosome to the other in a fusiform manner, and these connecting fibres, called the spindle-fibres, constitute the spindle, which has a centrosome or aster at either pole. As the nucleus becomes elongated transversely, the two centrosomes take up positions one at either pole of the somewhat elliptical nucleus, the spindle-fibres becoming gradually elongated. Up to this point the spindle, with an astral centrosome at either pole, is external to the nuclear membrane, but when this membrane disappears the spindle becomes intranuclear, and the spindle-fibres extend from one pole of the nucleus to the other, where they are connected with the two astral centrosomes respectively.

The foregoing phenomena conclude the anaphase or preparatory stage.

Metaphase. — After the disappearance of the nuclear membrane, the chromosomes are brought into direct contact with the spindle, and lie at first scattered between the spindle-fibres. Very soon, however, they congregate at the equatorial plane of the spindle, which corresponds to its widest part. Here they are arranged in a stellate manner, which constitutes the aster, according to some authorities. Each chromosome now splits longitudinally into two equal parts, called daughter-chromosomes, the original number of parent-chromosomes being thereby doubled. The formation of daughter-chromosomes constitutes the metaphase or chief stage.

Kataphase. — The daughter - chromosomes at first form two rows at the equatorial plane of the spindle, lying close to each other. They soon, however, separate, those of each row travelling along [metakinesis) the corresponding spindle-fibres to either pole of the spindle, where they lie close to the centrosome. These phenomena conclude the kataphase, or leading ‘ down ’ stage.

Telophose. — The daughter-chromosomes within each aster now unite end to end, and form a spireme, round which a new nuclear membrane is formed. The spireme gradually assumes the form of a chromatic reticulum, characteristic of a normal nucleus, and karyoplasm is formed within the meshes of the reticulum. Two daughternuclei are thus constructed, one in either centrosome, each of which contains one-half of the parent-chromosomes belonging to the original cell. The cytoplasm of the parent-cell now becomes constricted at the equatorial plane, and by the deepening of this constriction the cytoplasm is divided into two halves, which separate from each other, each half surrounding the nuclear membrane of the corresponding daughter-nucleus. Two complete daughter-cells are thus formed, and the telophase or concluding stage is finished.

Fig. 3. — Stages of Karyokinesis (from E. B. Wilson’s ‘ Cell,’ by Permission of the Macmillan Company, New York).

A, resting-cell; B, early anaphase; C, later anaphase; D, later anaphase; E, latest anaphase; F, cell ready for karyokinesis; G, metaphase; H, kataphase; I, telophase; J, division complete.

The complex changes concerned in the mitotic division of the parent-cell are concluded with the formation of two complete daughtercells.

Summary of Karyokinesis, or Mitosis.—There are four phases— namely, anaphase, metaphase, kataphase, and telophase.

The anaphase consists in (i) the conversion of the linin—and chromatin—reticulum of the nucleus into a spireme, or skein ; (2) the breaking up of this spireme into chromosomes ; and (3) the formation of a spindle from the spindle-fibres which connect the two centrosomes, these centrosomes gradually separating from each other, and the nuclear membrane disappearing.

The metaphase consists in the congregation of the chromosomes at the equatorial plane of the spindle.

The kataphase consists in (1) the splitting of each chromosome into two daughter-chromosomes, and (2) the migration of these daughterchromosomes from either side of the equatorial plane of the spindle along the corresponding spindle-fibres to either pole of the spindle where they enter the aster.

The telophase consists in (1) the formation of a daughter-nucleus within each aster, and (2) the cleavage of the cytoplasm of the parentcell into two halves, each of which surrounds the corresponding daughter-nucleus, two daughter-cells being thereby formed.

Amitosis. — This is direct cell-division. The nucleus is simply cleft into two daughter-nuclei, with accompanying cleavage of the cytoplasm. It is possible that this mode of division occurs more frequently than is usually thought to be the case.


The Spermatozoon

Spermatozoa are male germ-cells. They are end products of a division series of genital cells; cells termed spermatids are the last stages of division, and these are transformed into spermatozoa.

A spermatozoon is essentially a cell, though it has undergone considerable modifications from the usual cell-type. It is an elongated body, which is endowed with remarkable power of movement, the movement being of a lashing or vibratory nature. It consists of the following parts:

1. A head, somewhat pointed, and compressed from side to side. It is provided with a head-cap, which forms the perforaculum, for penetrating the ovum. The head represents the nucleus of the parentspermatid, the archoplasm of which forms the head-cap.

2. The neck, which is a thick disc behind the head, containing the anterior centrosomal body.

3. The tail, containing the posterior centrosomal body, and con sisting, behind this, of an axial filament surrounded by certain coverings. The tail has three parts: ( a ) a middle piece, in which the axial filament is surrounded by a spiral filament and covered by a mitochondrial sheath ; S'

(b) a flagellum or main piece, fig. 4 . — Head of Spermatozoon enlarged, in which the axial filament Main Piece considerably shortened. is surrounded by a cytoplasmic sheath, which becomes thinner as it is traced backwards; (c) the terminal filament or end piece, in which there is no covering for the axial filament.

The average length of the entire spermatozoon is about 0-05 to 0-06 mm., of which the head measures about one-twelfth. It has been calculated that there are about 200 million spermatozoa in an average ejaculation, and that the production of spermatozoa during life may lie between 300 and 400 billions.


Spermatogenesis is the development of spermatozoa, which are formed in immense numbers within the tubuli seminiferi of the testes. Each spermatozoon is developed from the germinal epithelium, its original source being known as the primordial germ- or sperm-cell, which is of large size. These cells undergo Fig. 5. —Diagram showing Spermato- several mitotic divisions, and genesis (after Boveri). from the last generation sperma togonia are developed, which correspond to the oogonia of the female. These spermatogonia divide, by mitosis, and give rise to primary spermatocytes, two for each spermatogonium, and these correspond to the female primary oocytes. Each primary spermatocyte divides, by mitosis, into two cells, which are known as the secondary spermatocytes, and correspond to the female secondary oocytes. Each secondary spermatocyte, in turn, divides, by mitosis, into two cells, which are called spermatids, and each of these corresponds to the mature ovum of the female. Each spermatid now undergoes transformation into a spermatozoon, the change taking place within a cell or column of Sertoli (sustentacular cell). Prior to its full development a spermatozoon has passed through four stages—namely, (i) a spermatogonium; (2) a primary spermatocyte; (3) a secondary spermatocyte; and (4) a spermatid. When the primary spermatocyte divides to form two secondary spermatocytes, the mitotic change preceding this is of the nature of a reduction-division ,the resultant daughtercells possessing only half the specific number of chromosomes. This division is thus heterotypical. The next division, into spermatids, is homotypical, but the number of chromosomes, of course, remains the reduced or haploid number.

From one primary spermatocyte (mother-cell) there thus result four grand-daughter cells of equal size, each of which is a spermatid. These spermatids subsequently undergo transformation, each into an active spermatozoon, capable of fertilizing a mature ovum.

The Ovum

The ovum or oocyte, which is the female germ-cell, has all the characters of a typical cell, being specially remarkable for the large size of its nucleus and nucleolus. It is formed within a Graafian follicle of the ovary, and it has a diameter of inch. Its component parts are as follows:

1. Cell-wall. 3. Nucleus.

2. Cell-body. 4. One or more nucleoli.

The cell-wall is known as the vitelline membrane.

The vitelline membrane surrounds the vitellus, of which it is a peripheral condensation. External to the vitelline membrane is the zona pellucida, which is separated from the vitelline membrane by a narrow interval, called the perivitelline space. When examined under a high power of the microscope, it presents very delicate striae, which radiate across its breadth, and from this circumstance it is known as the zona radiata. These striae are regarded as minute pores or passages.

External to the zona radiata there are several layers of cells, which are disposed in a radiating manner and constitute the corona radiata. These cells, like the zona radiata, are derived from the discus proligerus within the Graafian follicle, and the innermost cells send processes through the pores of the zona radiata to the cytoplasm of the ovum.

The cell-body, as in an ordinary cell, consists of cytoplasm (ooplasm), and this presents the usual reticulum or spongioplasm, the meshes of which are occupied by hyaloplasm. The ooplasm constitutes the vitellus or yolk. Embedded in it there are several fat-globules and albuminoid granules. These granules constitute the deutoplasm or nutritive yolk. According to some authorities, the vitellus contains, in the earlier stages, an attraction-sphere and centrosome, situated close to the nuclear membrane.

Fig. 6. — Mature Ovum, Semidiagrammatic, with Figure of Spermatozoon at Same Magnification ( x 300) to show Relative Sizes. cyt, body of ovum; cs, centrosome; n, nucleus; ps, perivitelline space; zp, zona pellucida; zr, corona radiata.

The nucleus represents the germinal vesicle, and constitutes the essential part of the ovum. As will be presently described, it forms the mature ovum or female pronucleus, after extrusion of the two polar bodies. It is a large spherical body, situated at first at the centre of the ovum, but subsequently becoming eccentric. Its diameter is about 5T0 inch, and it consists of the following parts:

1. Nuclear membrane. 3. Karyoplasm.

2. Nuclear reticulum. 4. Nucleolus.

The nuclear membrane is well marked, and is formed by the chromatin and linin of the nuclear reticulum. The nuclear reticulum resembles that of a typical cell.

The karyoplasm occupies the meshes of the nuclear reticulum.

The nucleolus is often termed the germinal spot.


Oogenesis is the development of mature ova. Each ovum is developed from the germinal epithelium, the remnant of which epithelium covers the adult ovary. The original source of the ovum is known as the primordial germ-cell. These cells undergo many mitotic divisions, and, from the last generation, oogonia are developed, which correspond to the spermatogonia of the male. These oogonia divide by mitosis, and give rise to primary oocytes. Each primary oocyte represents the ovum as it leaves the Graafian follicle, and it corresponds to a male primary spermatocyte. In the process of development each primary oocyte undergoes two mitotic divisions, one after the other. In the

first division the primary oocyte (mother-cell) extrudes the first polar body, and then it becomes a secondary oocyte, which corresponds to a secondary spermatocyte of the male. In other words, the primary oocyte divides by mitosis into two cells— namely, (i) the first polar body, of small size; and (2) the secondary oocyte. In the second division the secondary oocyte (daughter-cell) extrudes the second polar body, and then it becomes a mature ovum (female pronucleus). In other words, the secondary oocyte divides, by mitosis, into two cells — namely, (1) the second polar body, of small size; and (2) the mature ovum (female pronucleus), which latter only undergoes further division if fertilized. Prior to its maturation, the ovum has passed through three stages—namely, (1) oogonium, (2) primary oocyte, and (3) secondary oocyte. The mature ovum corresponds to a male spermatid, the difference, in the case of the latter, being that the spermatid undergoes further transformation into a spermatozoon.

A reduction-division, as in the male cell, takes place at a corresponding stage. The primary oocyte divides by the modified mitosis, giving a reduced or haploid number of chromosomes to its two products. One of these, the secondary oocyte, possessing this haploid number, divides with homotypical mitosis and passes this haploid number on to the mature ovum.

From one primary oocyte (mother-cell) there thus finally result four grand-daughter cells, one large and three small—namely, the mature ovum (female pronucleus) of large size, and three small polar bodies, the first polar body, as a rule, having divided into two small cells. The mature ovum is capable of fertilization, but the polar bodies (abortive ova) are inactive and disappear.

Fig. 7. — Diagram showing Oogenesis (after Boveri).

Table of Comparison between the Male and Female Germ-cells.

Male. Female.

Spermatogonium . . = Oogonium.

Primary spermatocyte . . — Primary oocyte.

Secondary spermatocyte = Secondary oocyte.

Spermatid . . = Mature ovum.

Though there is a great resemblance between spermatogenesis and oogenesis, two differences are to be noted: (1) The final result in oogenesis is the formation of four cells—namely, (a) the mature ovum, of large size, and capable of fertilization; and ( b ) three, as a rule, polar bodies, all small, quite inactive, and subsequently disappearing. In spermatogenesis, on the other hand, though four spermatids are formed at the same stage of cell-division as in oogenesis, they are all equal in size. (2) The mature ovum undergoes no further change, unless it becomes fertilized. Each spermatid, on the other hand, becomes transformed into an active spermatozoon, capable of fertilizing a mature ovum. Spermatogenesis may therefore be said to comprise one stage more than oogenesis, but this additional stage is not one of cell-division, but is simply the transformation of a spermatid into a spermatozoon.

Fig. 8. — Extrusion of Polar Bodies (modified after Hertwig).

Reduction-Division. — This process is one by which the number of chromosomes or segments of the chromatin spireme within the nucleus is reduced to half that characteristic of the body cells in general. The reduction takes place at the first division, when the primary spermatocyte or oocyte forms the secondary spermatocyte or oocyte. In this division, when the chromosomes are differentiated from the spireme, a process of synapsis takes place, whereby pairs of chromosomes fuse together; this"leads to halving of the actual number of chromosomes. The spindle formation and subsequent stages of cell-division now proceed more or less as usual, the resultant daughter-cells consequently possessing only half the number of chromosomes presented by the original unmodified cell. The division differs, therefore, from the typical one in the occurrence of synapsis and its results, and is spoken of as atypical or heterotypical.

  • This term is used here in a simple sense, as a convenient label for the process in which the number of chromosomes is reduced. It must not be confounded with the expression ‘ reducing division ' as originally used by Flemming, which was applied to a process leading to a qualitatively unequal division of chromosomes.

The secondary spermatocyte and oocyte pass on to their subsequent divisions, by which the final products are formed. These divisions proceed in the ordinary way with, of course, the already reduced number of chromosomes. They are therefore homotypical. The final result is that, from each original spermatogone or oogone, four cells are produced of the same nuclear value, although their functional value from a reproductive point of view differs in the two sexes, as will be seen later. Each of these nuclei has a chromosome number [haploid number) half that of the original (somatic) number ( diploid number).

It may be noted also that a certain amount of nuclear chromatin is discharged from the nucleus in later stages of the mitotic changes, and lost in the surrounding cytoplasm; this is most marked in the female cell.

The synaptic fusion of chromosomes, which leads to the acquisition of the haploid number in the heterotypical division, is a matter of much interest. When the ovum is fertilized by the conjugation with it of the nuclear material of the spermatozoon, the number of chromosomes is raised to the full by the addition of those of the male nucleus. The male and female chromosomes pair, but do not fuse. Thus the individual formed from this ovum contains in his ceils an equal number of chromosomes from each parent, paired, but not fused. In the heterotypical division, however, these male and female parental elements fuse for the first time. Hence the mature sex-cells of the individual have only the haploid chromosome number, and the inter-relations of these elements are different.

It is obvious that such marked and constant changes occurring in the chromatin content and chromosome structure of the nucleus afford physical grounds for both qualitative and quantitative theories of variation and heredity, but these cannot be entered upon here. It may be said, however, that the reduction in chromosome number is a preparation for sexual conjugation, while the occurrence of conjugation is probably less concerned with actual reproduction or rejuvenescence than with the provision of material for the play of the forces of variation and heredity.


The ovum lies for some time within a Graafian follicle in the ovary. At this period it is embedded within a heap of cells, known as the discus proligerus. The innermost cells of this discus, which are in direct contact with the ovum, form the zona pellucida or zona radiata, and two or three layers of the succeeding cells give rise to the corona radiata. Within the Graafian follicle, besides the discus proligerus and ovum, there is some fluid, called the liquor folliculi.

Ovulation is the extrusion of the ovum from the Graafian follicle. As a follicle becomes mature, it approaches the surface of the ovary, being distended with fluid, and, when quite mature, it lies close beneath the surface. This part of the follicle presents a slight projection, on which there is a pale spot, called the stigma. The stigma, becoming very much attenuated, ruptures. The liquor folliculi then escapes, carrying with it the ovum, surrounded by the corona radiata and the zona pellucida or radiata, these, as stated, being derived from the discus proligerus. The expelled ovum, as a rule, gains the ostium abdominale of the Fallopian tube. Here it enters that tube, and is gradually conveyed into the cavity of the body of the uterus, where, if previously fertilized, it undergoes development into the embryo, and then into the foetus.

Abnormal Conditions. — (1) The ovum may never leave the Fallopian tube, and, if fertilized, it would give rise to tubal pregnancy. (2) When expelled from the Graafian follicle and ovary, the ovum may drop into the abdominal cavity, and, if fertilized under these conditions it would give rise to abdominal pregnancy. (3) In extremely rare cases the ovum may not leave the Graafian follicle, even though that follicle and the ovary should rupture in the usual way. If fertilized under these conditions, it would give rise to ovarian pregnancy. These three abnormal conditions are spoken of as cases of extra-uterine pregnancy.

The periods of ovulation or extrusion of the ovum from the Graafian follicle and ovary, which occur at more or less regular successive intervals, are attended by certain changes in the mucous membrane of the cavity of the body of the uterus.

These changes are of the nature of a preparation of this membrane for the reception and embedding of the fertilized ovum, and are apparently brought about by the action of a hormone formed in the ovary under pituitary influence. When fertilization fails, the prepared mucosa breaks down and is partly cast off, constituting the phenomena of menstruation.

Maturation of the Ovum

The immediate cause of the rupture of the follicle may possibly be the action of the involuntary muscle fibres within the ovary contracting in a period of sexual excitement. It is not known by Fig. 9.—Maturation of the Ovum.

what mechanism the extruded

ovum, whatever the position of the follicle may be on the surface of the ovary, is conducted into the opening of the tube. Having entered the tube, it probably takes about a week or less to traverse this canal and enter the uterus, from whence, if it has not been fertilized, it is discharged with uterine secretions or during menstruation.

In the majority of animals the maturation of the ovum, although it must, of necessity, precede the actual conjugation of the male and female pronuclei, is not completed before the entrance of the spermatozoon. In other words, the entrance of the sperm may take place either before, during, or after the maturation of the ovum. In the human ovum, however, the observations of Professor Arthur Thomson and Professor Dixon seem to make it reasonably certain that maturation, with discharge of the polar bodies, is completed before or as the ovum enters the Fallopian tube, in which, normally, it is fertilized.

When the ovum is discharged from the Graafian follicle it is probably mature and capable of being fertilized. The processes through which it has passed to attain to this state, possessing the reduced number of chromosomes, have been described already (see Oogenesis and Reduction-Division).

Fertilization of the Ovum

Fertilization is otherwise spoken of as impregnation , or fecundation. It consists in the conjugation or fusion of the male pronucleus, or head of a spermatozoon, with the female pronucleus, or mature ovum, and it constitutes the commencement of the development of a new individual to propagate the species. As a general rule, conjugation of spermatozoon and ovum takes place in the outer part of the Fallopian tube, or oviduct, into which spermatozoa have made their way through the vagina and uterus by the lashing movement of their tails.

Fig. 10. — Fertilization of Ovum. A, entrance of spermatozoon; B, extrusion of polar bodies; C, male pronucleus; D, compound nucleus (male and female pronucleus); E, female pronucleus (ovum); F, female pronucleus.

When the spermatozoa come into contact with the mature ovum one of them as a rule passes through the zona pellucida, or radiata, into the yolk. At the point of entrance the yolk forms a conical protuberance, called the receptive , or entrance cone. As the spermatozoon passes through this cone it parts with its tail, the surrounding vitellus becoming disposed in a radiating manner. Meanwhile a delicate membrane is formed round the yolk, called the vitelline membrane, which prevents the entrance into the yolk of other spermatozoa as a rule.

The head, or nucleus , of the spermatozoon now constitutes the male pronucleus, or sperm-nucleus, and the middle piece contains a centrosome, called the spermo-centre. The male pronucleus advances towards the centre of the ovum, near which, up till now, the mature ovum, or female pronucleus, is lying quiescent, being destitute of its original centrosome, which has disappeared. As the male pronucleus, along with its centrosome, advances, the centrosome leading the way, the female pronucleus shows receptive signs, and moves slightly to meet the approaching visitor. The two pronuclei then come into very near contact, not far from the centre of the ovum, but they do not as yet fuse. The male centrosome, or spermo-centre, now divides, and two centrosomes are formed, one of which passes to the distal side of the female pronucleus. Conjugation or fusion of the two pronuclei now takes place, and the mixed nucleus thus produced is called the segmentation, or cleavage-nucleus.

This completes the stage of fertilization. A chromatic spireme, or skein, chromosomes, and a spindle are subsequently formed within the segmentation-nucleus, and the segmentation-stage is entered upon— that is to say, cell-division with mitosis, or karyokinesis, takes place.

Certain authorities maintain that the ovum, after parting with the second polar body, retains a centrosome, known as the ovo-centre. The male pronucleus, as stated, brings with it a centrosome or spermo-centre. Each ovocentre and spermo-centre divides into two, and each division of the ovo-centre joins a division of the spermo-centre. When, therefore, fusion of the two pronuclei has been effected, the resultant segmentation-nucleus has two mixed centrosomes (male and female), one on either side, or at each pole.

Segmentation of the Ovum

Segmentation consists in the division of the fertilized ovum into a mass of cells.

After the mature ovum, or female pronucleus, has been fertilized by fusion with the male pronucleus, mitotic or karyokinetic celldivision commences, and the ovum is ultimately transformed into a great number of cells, which are called blastomeres,f or segmentation

The word ‘ ovum,’ it will be noticed, is used to denote the egg-cell after fertilization, as well as before this. Such employment of the term has some disadvantages, which can, however, be neutralized with a little care. No suitable word has been coined which does not replace these disadvantages by others, and, as the older method is used in general by writers, it is retained here.

| The prefix blast, derived from a Greek word meaning ‘ germ ' or ‘ beginning/ is used properly in those terms that describe the earliest-known morphological stage of development of some structure. Thus, a blastoderm is the earliest state of cell-layers from which an embryo will be formed, and a cells. At the first division, which affects primarily the segmentation nucleus, the ovum is cleft into two cells, which lie close together, the opposed surfaces being flattened. At the second division each of these cells is cleft into two, so that four blastomeres now occupy the interior of the ovum. Each of these, in turn, divides into two, thus giving rise to eight blastomeres. This process of cell-division goes on, sixteen blastomeres being formed, succeeded by thirty-two , and so on. Finally, the ovum, originally simple, becomes transformed into a heap of nucleated blastomeres, or segmentation-cells, the superficial cells being clear, whilst the more deeply placed cells are granular. These constitute a solid, spherical, mulberry-like mass, called the morula, and this stage is hence known as the morula-stage.

Fig. 11. — Segmentation of Ovum.

Segmentation is not to be looked on as an actual part of specific development, but as a process of changes within the ovum which are necessary to allow it to begin its development. Every organism has a definite quantitative relationship between its nuclear and cytoplasmic masses, which must be attained before development can be carried on: segmentation is the process bringing this about. The rapid subdivision of the excessive cytoplasm of the ovum, with the gradual building up of the nuclei after their first formation by division, bring the mass of cells gradually toward the cyto-nuclear ratio. The additional material for the nuclei is obtained from the cytoplasm, within which a certain amount of nuclear constituents have been situated. The last stage of segmentation thus corresponds with the production of the first potentially developmental cells, and thus with the beginning of true development.

Segmentation has never been seen in the human ovum, but it must be assumed to occur as a preliminary to development.

The stages of development of the fertilized ovum, so far as they have been followed at present, may be said, in a general way, to be common to all the members of the animal kingdom. As development proceeds, the embryonic rudiment begins to take shape and to become definitely vertebrate in character, and subsequently grafts on to this generalized vertebrate form the large and small characteristics which place it in its special subdivisions, genus, and species. The characters of a vertebrate (Fig. 12) are the possession of a longitudinal axial blastomere is the earliest form of cell in the segmenting ovum. The affix en< ^ the word, as, for example, in the term ‘ mesoblast,’ met A! later. Here the word, properly used, implies the very earliest state of e mass of undifferentiated cells which make the middle layer of the ovum an embryo, but, as soon as development has led in any evident way to an advance on the undifferentiated state, the word ‘ mesoderm ' is used instead 01 mesoblast.

skeleton, lying below the central nervous system, and the presence of a body-cavity or coelom between the body-wall and the alimentary canal.*

The axial skeleton may be, as in the simplest forms, a non-jointed cartilaginous rod, or it may be hidden in a complicated system of vertebrae. The coelom may be a common cavity within which the alimentary canal projects, and the heart is placed towards the cephalic end, or these two structures may be more or less shut off from one another by septa forming in the coelom, and, in air-breathing animals, the pleural sacs, containing the lungs, are also derived from the coelom. The alimentary canal opens in front (mouth) and behind (anus), and the central nervous system forms a brain at its front end, which may secondarily project forward over the situation of the mouth.

The human embryo, subsequent to the general development of the ovum, which has been already described, evolves from the cells of the ovum as a living vertebrate animal, possessing a central nervous system, a longitudinal axial skeleton, the notochord, and a coelomic cavity separating its body-wall from the simple alimentary or ‘ visceral ’ structures. This may be looked on as the first period of development. The subsequent evolution of the details of bodily structure are engrafted on to the general vertebrate stage; they will consist, firstly, of the building up of an air-breathing type, and then of the necessary changes and additions which bring the mammalian human being out of this generalized state.

It must not be imagined that these developmental periods are distinctly marked off from one another. They overlap in many ways, and the order of development given above is only true of the development as a whole. Nevertheless, it is convenient from a descriptive point of view to make use of such a conception of the evolution of the embryo, and the developmental changes, to the account of which we are just about to pass, may be looked on as leading up to the evolution of the general vertebrate type from that representing a more primitive animal stock.

Fig. 12. — Sectional Scheme to show Vertebrate Type of Body.

The vertebrates are divided into three great classes: Ichthyopsida, or fishes; Sauropsida, including reptiles and birds; and Mammalia. It must not be thought, because the other members of the animal kingdom are termed ‘ lower ' by man, that they are necessarily simpler or more primitive in their structure than man. Most of them are highly specialized for their own particular sphere in life, in some ways more specialized than so-called higher animals. But among these divisions the more primitive fishes and reptiles may be considered to be nearest to the line of human descent. Since the adults have undergone more or less specialization, and since the embryo is-a nearer approach to the primitive than the adult can be, the most reliable contributions to morphology can be obtained by comparison of the embryonic forms. Birds lie outside the line of descent, being only highly specialized developments from more primitive reptiles; hence morphological purposes are best served by comparison between mammalian, reptilian, and primitive fish embryos.

It may be said, finally, that man is a placental mammal; the embryo depends for its nutrition on supplies obtained from maternal sources, and, to do this effectively, develops an absorbent organ, the placenta, which comes into intimate relation and connection with the walls of the uterus. The functional mass of the placenta is formed from the wall of the ovum.

Formation of Blastula and Blastoderm

It must be understood that the early stages of development, such as are now being described, have not yet been seen in the case of the human subject. Segmentation and the formation of a morula are among these unknown stages in man, but, from their universal

appearance in other types, their existence among the human stages can be assumed almost certainly. The next stages, however, those of the formation of the blastula and of the earliest embryonic layers (blastodermic layers), being equally unknown, can only be assumed in their details on much less certain grounds, and the following account must be taken as little more than a suggestion of the lines along which these developments take place. The assumptions are based on what is known of the stages in other types resembling man in certain developmental characteristics.

The mass of cells composing the morula is soon converted into a hollow sphere consisting of a wall of cells surrounding a central cavity; the cavity is known as the segmentation cavity or blastoccele, the whole structure being termed a blastula or blastocyst.

The formation of a central cavity may be looked on as the natural biophysical result of the necessity of the cells obtaining their nutriment from the surrounding media, and hence occupying as superficial positions as possible in the sphere.

The peripheral disposition of the cells, however, is not equal throughout the sphere, for whereas the layer is thin—possibly only of the thickness of one or two cells—throughout the greater part of the wall of the blastocyst, yet a mass of cells is gathered near one pole, and projects into the segmentation cavity. This is known as the inner cell mass or formative cell mass (Fig. 13).

As a general statement it may be said that the inner cell mass is particularly concerned in forming the embryonic structures, the extra-embryonic structures of the ovum being associated with the remaining parts of the blastula. The position of the inner cell mass marks the upper, apical, or animal pole of the ovum, the opposite end being termed the abapical or vegetative pole.

Fig. 13. — The Blastodermic Vesicle, showing the Segmentation Cavity and Inner Cell Mass.

Restriction of the area of actual embryonic formation to one part of the whole ovum is a secondary development found among certain animals possessing a large amount of yolk stored in their egg-cells. Yolk (see p. 14) is a fatty material provided for the nutrition of the developing embryo. In the fertilized ova of different species it is found that the yolk may be more or less evenly distributed, thus making isolecithal eggs, or may be gathered towards the lower or abapical pole, these being known as telolecithal eggs. On the other hand, the yolk may be practically absent; this is an alecithal condition. The concentration of yolk granules leads to a comparative inertia in the cell-division of the region concerned, so that a telolecithal egg, when it segments, shows large and slowly forming cells (macromeres) towards its lower pole, and a large number of smaller cells (micromeres) near its upper pole, where the yolk granules are few or absent. The difference in rapidity and completeness of cell-formation between these two regions is more marked in those eggs that are heavily yolked and markedly telolecithal. In these cases of extreme and heavy telolecithality the actual proliferation of cells is practically confined to the upper pole, at any rate in the earlier stages, and the egg is termed meroblastic. In alecithal, isolecithal, or only moderately telolecithal eggs, division of the whole egg takes place, although the lower cells may be considerably larger than the upper ones, and the egg is then said to be holoblastic. In the meroblastic egg the area from which the embryo is forming is confined to one part of the ovum, for this is the only part admitting of the rapid cell-division necessary for the formation, but in the holoblastic egg the whole mass of cells is used in the actual formation of the embryo.

Among the placental mammals the eggs are, if not absolutely alecithal, only very moderately yolked, and are consequently holoblastic, yet they form an exception to the general statement made in the last paragraph: the embryo is developed from one polar region only of the ovum. This is explained by the assumption that their moderately yolked condition is a secondary one, that they were derived originally from heavily yolked telolecithal forms, but that, when they acquired the power of deriving nourishment directly from maternal sources, the storage of yolk became unnecessary, and was gradually lost. This change would allow holoblastic division to replace the original meroblastic type, but the limited blastodermic area still persists, being connected later with the acquisition of certain embryonic appendages, which will be dealt with later.

Changes occurring in Association with the Inner Cell Mass.

The first changes in the human blastocyst are not actually known as yet. They lead up to the earliest known stage, and may be assumed to be of a nature such as is schematically shown in Fig. 14. Spaces appear between the cells of the inner cell mass, and run together, making a single rounded cavity; this is the amniotic cavity , and is surrounded by a layer of cubical cells, the epiblast, or primitive ectoderm.

A second enclosed cavity is formed below this, the archenteric cavity, surrounded by hypoblast or primitive entoderm. Nothing can be said definitely about its mode of formation; its cellular wall may spread round the wall of the blastoccele from the inner cell mass, or may be formed by splitting occurring among the lower cells of this mass.

The two-layered plate of ectoderm and entoderm, separating these cavities, is the embryonic plate ; this is the earliest form of definition of the region from which the embryo itself will be developed later.

The most superficial cells of the original blastocyst now form a complete covering layer or trophoblast , so called because it is directly concerned, now and subsequently, with the absorption of nutriment for the contents of the ovum. The zona radiata has disappeared completely, but it is not known under what conditions this occurs.

Fig. 14.—Diagram to show Spaces between the Cells of the Inner Mass joining to form the amniotic cavity (a). E, archenteric cavity; S, segmentation cavity; T, trophoblast.

Attainment of Earliest Known Human Stage

All the stages described so far are passed through, in all probability, during the transit of the fertilized ovum through the tube, and during its short stay in the uterus preceding its fixation there by the process of embedding ’ to be described later. This preliminary stage, of transit probably takes a week or ten days to accomplish.

The next stage of development of the ovum after those described above is shown in Fig. 15. This practically corresponds with - the earliest known fertilized human cvum.

This ovum had apparently just embedded itself, but its structural stage may be assumed to have been the same, to all intents and purposes, immediately before the embedding.

The figure shows the small amniotic and archenteric cavities as before, but there are marked differences in other respects. The interior of the ovum is now occupied by mesoblast, or primitive mesoderm, this does not extend between the ectodermal and entodermal mesoderm em ^ r y on ^ c plate, whence it is termed extra-embryonic

The origin of this extra-embryonic middle layer is exceedingly doubtful, and it is possible that there may be more than one site from which it arises. Reference is made to this matter later from another standpoint.

The external covering of trophoblast has become much thicker, and it can be seen to be composed of two layers; one, deeper and next the mesoderm, is more or less distinctly seen to be cellular, but the more superficial and extensive layer shows no cell-boundaries, and is nothing but a nucleated plasmodial covering. These two layers are called cyto-trophoblast (cellular trophoblast) and plasmoditrophoblast respectively, and, taken together, they constitute the trophoblast.

Fig. 15. — Ovum filled with Primitive Mesoderm (M).

Plasmodium (P) and cellular layer (C) compose the trophoblast. A, amnion; E, archenteron.

It is possible that the plasmodium, which by its digestive powers effects the embedding of the ovum, is a solid and continuous layer before and during this process, but, as soon as the ovum is embedded, the layer grows and pushes itself out towards the surrounding mucous membrane of the uterus; in doing this it develops cavities and lacunae in its mass, and forms a plasmodial network, as is shown in the figure.

The Embedding in the Uterine Mucosa

The mucous membrane of the uterus, known as the decidua or decidual membrane , is a thick and vascular layer when the fertilized ovum comes into contact with it. The ovum proceeds at once to embed itself in this membrane, bringing this about by the necrotic and digestive action of the plasmodi-trophoblast on the decidua.

The uterine decidua contains many glands, and is applied to the muscular wall of the organ without any intervening submucosa. Deeply, nearer the muscular wall, its gland-tubes are much convoluted

Fig. 16. — Ideal Section through Ovum in situ.

A, cavity of amnion, lined by ectoderm; AP, aperture of entry; C, cytotropho blast; M, primitive mesoderm; Y, archenteron.

and enlarged, giving this part a spongy appearance, whence it is called the stratum spongiosum. The more superficial part, nearer the uterine cavity, shows smaller gland tubes running towards the cavity, and is termed the stratum compactum. The stroma is composed of cells of embryonic type, and is very vascular. The uterine cavity is lined by a single layer of cubical ciliated cells.

The trophoblast covering the ovum quickly destroys the superficial parts of the decidua with which it is in contact, and in this way the ovum is very soon lying in a small ‘ implantation cavity ' in the stratum compactum of the decidua. Here it increases rapidly in size owing to the absorption of fluid from the blood and semi-fluid detritus which surround it in the implantation cavity. The cavity is also increased in size to contain it, not only by the enlargement of the ovum, but also by the continuing action of the trophoblast covering it. This layer is now, as already seen, in the form of a plasmodial network surrounding the ovum and stretching between it and the necrotic walls of the cavity. The diagrammatic section in Fig. 16 will give an idea of the conditions. The aperture of entry (AP in the figure) is quickly closed by swelling and infiltration of the covering decidua, and the subsequent growth and development of the ovum goes on within this cavity apart from the uterine cavity: the associated changes will be dealt with in the appropriate place. The site of embedding is frequently, but not by any means always, on the upper part of the posterior wall of the uterus.

Formation of Extra-Embryonic Coelom, Chorion with its Villi and Body-Stalk

To return to the ovum. The primitive mesoderm filling the ovum, in the stage last described, is a mass of loosely packed cells. Spaces appear between these and, running together, produce a fluid-filled cavity in the midst of the extra-embryonic mesoderm (Fig. 17). This cavity is the extra-embryonic coelom. Its formation leaves the mesoderm applied, as a more condensed layer, on (a) the internal surface of the trophoblast, and (b) the surface of the ectodermal and entodermal vesicles that constitute the primitive amnion and archenteron respectively.

The new containing wall of the ovum, made in this way, is known as the chorion, and consists of chorionic mesoderm (CM, Fig. 17), covered externally by trophoblast ; this layer is sometimes referred to as chorionic ectoderm. It can be seen in the figure that the chorionic mesoderm is invading the bases of the trophoblastic projections which pass from the surface to the surrounding network; as a matter of fact, the mesodermal growth into these begins before the formation of the extra-embryonic coelom, and has made definite progress by the time this occurs. Each mesodermal outgrowth carries on it, as it grows, the two constituent layers of the trophoblast, the whole projection forming a chorionic villus; thus a chorionic villus consists of a core of mesoderm covered by cyto-trophoblast and plasmodi-trophoblast. Each villus has several side branches, but the growth of the mesoderm within it does not effect continuity with neighbouring growths through the plasmodial network. Each villus is a separate branched structure; it may be joined, by plasmodial bridges only, with a neighbouring villus here and there, but shows no other or more intimate connection with them, and even this plasmodial junction disappears after a short time. The villi are the special agents of absorption of nutriment from the maternal blood through their trophoblastic covering, and, like the chorion, soon become vascularized for this purpose. Further details about the chorion and its villi are given in the section dealing with uterine connections of the ovum.

Turning to the embryonic rudiments, it is seen (Fig. 17) that the wall of the archenteron is now composed of yolk-sac* mesoderm with an inner lining of entoderm, while the amnion has a layer of amniotic mesoderm with an inner ectodermal lining. The mesodermal coverings of these two sacs are continuous with each other all round the margin of the embryonic plate; when, later, the intra-embryonic mesoderm

Fig. 17. — A Composite Diagram showing Different Stages of the Ovum, more advanced than in the Last Figure.

The mesoderm (stippled) has split to form the extra-embryonic coelom (black). CM is the chorionic mesoderm; A and E are the cavities of the amnion and archenteron respectively; the mesoderm covering them externally has corresponding designation, and is continuous with chorionic mesoderm through the body-stalk, BS; an allantoic diverticulum, All, passes into the stalk from the archenteron; Cyt., Plas., are the cellular and plasmodial layers which make the trophoblast.

forms, between the ectoderm and entoderm of the plate, it will necessarily become continuous at this margin with these other mesodermal layers.

The embryonic mesoderm is already in process of formation at the stage represented in general by the figure, but has been purposely omitted.

The embryo and its associated vesicles (amnion and yolk-sac) are not completely separated, however, from the chorion. In Fig. 17 a thick mesodermal strand is seen at BS passing up from the caudal end of the embryonic plate, where it is continuous with the mesodermal coverings of the amnion and yolk-sac, and, between these, with the intra-embryonic mesoderm (not shown in figure). This is the body stalk ; it reaches the chorion above the amnion. It is seen in the figure to contain a prolongation from the archenteron, the allantoic diverticulum. The body-stalk is a connection between the embryonic plate and the chorion, by means of which the circulatory systems of the embryo will become continuous with the vessels developed in the chorion and its villi; the body-stalk is therefore necessary for the proper nutrition of the embryo until it is discharged from the uterus, and, in the later period of intra-uterine life, is elongated to form the main part of the umbilical cord connecting the foetus with the chorionic placenta.

The archenteric cavity and its walls form more than the definite yolksac of the human embryo, but it is customary, nevertheless, to use the term ‘ yolk-sac ’ for this part even at this stage.

The amniotic folds ( a , a) are shown, derived from the body-wall of a meroblastic ovum. They grow up over the embryo and meet. Having met, the inner wall of the fold becomes the amnion, and the outer wall (serosa) forms the superficial part of the chorion. The amniotic fold is made of ectoderm and somatic mesoderm. The coelomic cavity of the embryo is shown black; when it joins the outer coelom the amnion is left in continuity with the body-wall.

Fig. 19. — To show the Origin and Extension of the Allantois, which ultimately spreads over the deep aspect of the ‘ serosa.’

The extra-embryonic coelom is frequently traversed at first by irregular bands of mesoderm, due to incomplete splitting; it contains fluid, in which floats a fine reticulum, the magma reticulare, also probably of mesodermal origin, although not showing any definite cellular structures. The existence of the extra-embryonic coelom allows the embryonic rudiments contained within the ovum to expand and alter their form independently of the containing wall, and its appearance therefore precedes the onset of a period of rapid growth and change in the embryonic plate and its associated structures.

Morphologically, the amnion must be looked on as part of the body-wall) therefore, only able to develop as a covering of the embryo in meroblastic ova; the body-stalk and chorionic mesoderm are to be considered modifications of the allantois, a functional organ of nutrition in the Sauropsida; the extra-embryonic coelom can only exist in a meroblastic type of development, and represents the most ventral part of the body-cavity of the holoblastic body, secondarily and precociously enlarged to admit of further changes taking place within the ovum.

The amnion develops in the Sauropsida and Mammalia. It was produced originally by an upgrowth (Fig. 18) of the wall of the ovum around the relatively small surface area which gives rise to the embryo. The amniotic folds, growing up, meet above, the embryo. When they begin to grow the entoderm may be included in them, as in some reptiles, but the formation of mesoderm, with its (extra-embryonic) coelom, releases the entoderm and allows it to fall back from the fold, which is now composed of (somatic) mesoderm and ectoderm only. An amniotic fold, made in this way, has two walls or sheets, continuous with each other at the free edge of the fold; the inner wall, which will be the definitive amnion, has its ectodermal surface looking toward the embryo, while the more superficial wall, the false amnion or serosa of von Baer, is directly continuous with the outer layer of the ovum, which outer wall it, in fact, now forms altogether. When the folds meet above the embryo and join together, the septum between them, formed by the fused walls, may persist in part or altogether, or may be completely absorbed, allowing the continuity of the ccelomic space from one side to the other. The amniotic cavity is thus cut off from the exterior, with which, however, it may retain continuity for a little time by means of an elongated * amniotic duct ’ or ‘ stalk '; such connections have been found at times even in the human subject, and have been explained on morphological grounds such as these. This mode of formation of the amnion is found among reptiles, birds, and some mammals; but in higher mammals the formation has been shortened, and the hurn^n amnion is an example of abbreviation and precocity in the way in which it is developed.

Fig. 20. — Scheme showing the Sauropsidan Elements in the Mammalian Ovum.

The amnion is represented with an opening to the surface, the amniotic duct or stalk, which is only occasionally and temporarily present. Cf. with preceding figures.

The allantois appears in the Sauropsida as a hollow outgrowth from the ventral wall of the gut near its caudal end, projecting into the extra-embryonic coelom (Fig. 19). It grows out through the coelom to come into contact with the ‘ serous ' layer of the amniotic fold, lining the inner aspect of the shell of the egg. It spreads over this, and, being very vascular, is the agent of oxidation for the embryo.

In the mammals which have a chorionic placenta (made from chorionic villi), and develop in the uterus, the allantois is no longer of use for direct oxidation, and its vessels serve the placenta; hence its mesodermal stalk, which carries these vessels, becomes thickened and forms the body-stalk, and its extension over the internal aspect of the wall of the ovum constitutes the main part of the chorionic mesoderm in which these vessels ramify. The allantoic diverticulum is the degenerated remnant of the cavity of the allantois. It is evident that the coverings of the embryo or foetus, its membranes, as they are styled by obstetricians, are all derived from structures which were secondarily adapted during what may, for convenience, be termed the ‘ reptilian stage ' of evolution, and for their formation and development it was necessary for the ovum to be of the meroblastic type.

A scheme is given in Fig. 20 to show these structures inherited from a sauropsidan-like stage in phylogeny; comparison with Figs. 18 and ig makes the homologies apparent.

The Embryonic Area: Embryonic Plate or Disc

If the amnion were cut away at in Fig. 17, and the embryonic plate be seen to be more or less circular in outline (Fig. 21). The ectodermal surface exposed in this way would show a longitudinal line, the primitive streak, in its posterior half. This line is produced by proliferation and thickening in the ectodermal celllayer in this situation, and it is from this proliferating area that the embryonic mesoderm is formed and grows out.

The primitive streak may present a groove in the middle line, as the mesoderm grows on each side of it.

The embryonic mesoderm extends into the plate, separating the ectoderm from the entoderm, except in the middle line; here the two earlier layers remain in contact for some little time. At the extreme front end of the plate, however, the right and left mesodermal layers meet. This is the region in which the primordium of the heart must be considered as existing (though not structure), lying partly in the yolk-s

a stage such as was represented looked at from above, it would

Fig. 21.—The Embryonic Plate or Disc, viewed from Above, and exposed by cutting away the Amnion (A).

The yolk-sac is the shaded structure on which the plates rest; PS, primitive streak; hp, its headprocess; BS, body-stalk cut; All., the allantoic diverticulum within it.

recognizable yet as a definite ic mesoderm by the embryonic rim and partly in the plate, on the edges of the right and left mesodermal layers as they come together. This region can therefore be termed for convenience the proto-cardiac area. The mesodermal sheet becomes continuous with the extra-embryonic mesoderm round the periphery of the embryonic disc. A trilaminar blastodermic area is thus formed, and replaces the original bilaminar one (Fig. 22).

The forward growth of mesoderm, from the region of the primitive streak into the plate, leads to its antero-posterior elongation in front of the streak, which thus assumes a position relatively more posterior, occupying less than the hinder half of the (now oval) disc. The area thus gained just in front of the streak shows now a longitudinal median groove in the ectodermal layer, which is slightly thickened here and raised at each side of the groove by mesodermal thickening; it is still in contact with the entoderm on its deep aspect. This groove is the neural or medullary groove, and its raised margins are the medullary the Bilaminar Plate (i) into the Trilaminar (2) by the Appearance of Intra-Embryonic Mesoderm on Each Side of the Middle Line: it becomes Continuous with the Extra-Embryonic Layer ( mes .) at the Margin of the Plate.

No. 3 shows the appearance of the neural folds and groove on transverse section, the intra-embryonic mesoderm being still shown as a solid plate on each side: actually, in a stage such as shown in the last section, this plate would be split by the embryonic coelom.

ridges; they diverge and fade away beside the primitive streak. The formation is seen in Fig. 23. This, the first indication of the formation of the central nervous system, is separated from the proto-cardiac region in front by a small area which contains no mesoderm, the buccopharyngeal area , which will become the bucco-pharyngeal membrane. The longitudinal section through the plate in Fig. 24 shows the relative position of these different parts in a schematic manner; Fig. 23 gives the surface view of such a plate. In this figure a perforation, the neurenteric canal, is seen in the plate at the anterior end of the primitive streak, and is also shown in the longitudinal section in the last figure. The neurenteric canal, as its name implies, passes to the enteron (as the gut-cavity may be termed) from between the diverging neural ridges; it is a little variable as to the time of its appearance, is not always well marked, and usually disappears very soon.

The early rudiment of the notochord is present, on the lower or entodermal aspect of the plate, in the form of a median longitudinal groove, the notochordal groove ; this extends forward from the lower opening of the neurenteric canal, lying immediately below the neural groove. It does not reach the level of the anterior end of the neural groove at first, but subsequently extends to the bucco-pharyngeal area, where the notochord is attached. The notochord is formed by the deepening of the groove and the subsequent separation of its walls from the roof of the cavity.

The neurenteric canal, mesoderm, and notochord are associated in their appearance and in their probable phylogenetic history. They are connected with the process known as gastrulation, a mode of formation of the enteric cavity which appears to be fundamental, and therefore to be taken into account in considerations of phylogeny. The simplest form of gastrulation is seen in Amphioxus. Here the blastula has a thin wall made

Fig. 23. — The Embryonic Plate, SEEN AS BEFORE, BUT AT A LATER Stage.

Fig. 24. — Schematic Longitudinal Section of Embryonic Plate at this Stage.

The neural groove is seen in the middle line, bounded by low ridges. Behind this is the opening of the neurenteric canal and the primitive streak, with ‘ caudal ’ swellings on either side of it, made by forming mesoderm. BP, bucco-pharyngeal area; BS, bodystalk, cut. The whole enbryonic area is surrounded by the cut amniotic edge.

N, neural region; PS, primitive streak; NE, neurenteric canal; BP, buccopharyngeal area; PC, proto-cardiac area. The amnion is shown entire, and the body-stalk and allantoic diverticulum are seen.

by a single cell-layer surrounding a large segmentation cavity. Invagination of this wall at one part leads (Fig. 25) to the obliteration of the segmentation cavity and the formation of a new enteric cavity, surrounded by two layers (ectoderm and entoderm) and opening on to the surface by a blastopore. The result is aided by actual backward growth of the upper or ‘ dorsal lip ’ of the blastopore, this being termed an epibolic growth. This growth of the dorsal lip in these eggs, which are holoblastic, is accompanied by a slow extension of the embryonic surface layers over the whole ovum, gradually closing in on the blastopore. In Fig. 26 are given some views of the blastoporic region in such eggs, showing how the ‘ opening,’ filled in reality by a mass of yolk, is ultimately closed as a linear ‘ scar.’ The primitive streak of higher forms may be considered to correspond with this scar, its appearance being largely of the nature of a phylogenetic memory; the neurenteric canal is the anterior or dorsal part of the old blastopore, its open condition being doubtless associated with the nature of the formations connected with this margin in all vertebrates. This front part of the opening and invaginated area is that which makes the roof of the entodermal cavity as it is turned in, and from this roof, in Amphioxus, certain outgrowths arise; they are shown in Fig. 27, which gives diagrams of sections through the body of the embryo just in front of the blastopore. The outgrowths are separated from the entoderm a little later. The outgrowth in the middle makes the notochord, and those at the sides, which are in segmental series, become the mesodermal segments, and extend ventrally round the entoderm (Fig. 28). The notochordal invagination is not segmented, but is in the form of a continuous groove, which necessarily runs into the front part of the blastopore; in other words, the lining of the groove is directly continuous with the front margin of the opening, while the lines of segmented mesoderm would, if carried back, lie on either side of the opening. There is an active growth of mesoderm also at the sides of the blastopore, which growth is then really continuous with the series in front of it; but it is necessary for certain purposes to distinguish between them, and the mesoderm formed from the roof of the gastrula cavity is termed gastral mesoderm, while that formed beside the blastopore is prostomial mesoderm. The difference between them is probably only one of times of formation.

If, now, we consider the primitive streak to represent the closed blastopore, we must look on the mesoderm arising from it as ‘prostomial' The blastopore as a whole is closed, probably because the meroblastic embryonic area is so small, and does not include the yolk-mass (as in amphibians). Its front end, on the other hand, is open, and its front edge free, because this front edge is actively growing forward into the roof of the gut-cavity, as the plate elongates, and is forming there the notochordal groove as in Amphioxus.

Fig. 25. — The Gastrula of Amphioxus (modified after Wiedersheim).

Fig. 26. — To show the Closure of an Amphibian Blastopore.

Ihe eggs are supposed to be viewed from behind. The arrows indicate the direction of spreading of embryonic ectoderm, the central one showing the growth of the dorsal lip. A yolk-plug occupies the blastopore before it is closed.

There is evidently some kind of invagination here, though it is not quite so simple as appears so far. In Fig. 21 a prolongation forwards, the headprocess ( hp ), is seen in front of the primitive streak, from which it is formed. ine head-process can be looked on as the anterior part of the thickening of the streak, and extends downwards and forwards from the ectoderm to the entoeim, thus making a prolongation from the streak, more or less visible through the ectoderm. The neurenteric canal, when it appears, opens through this head-process and the entodermal extremity of the process, as it is carried forward m the elongating roof of the enteron, forms the notochordal groove, which is thus continued into the neurenteric canal behind. So far, then, the relations between canal and notochord are much as in Amphioxus ; moreover, the lower and front part of the headprocess appears to be prolonged at the sides into mesodermal cells, which may thus be taken to represent gastral mesoderm, continuous behind with that formed at the closed blastopore. But the recognition of the head-process and its associated formations as being part of a regular invagination is complicated by the fact that a layer of entoderm lies below it, already formed before the appearance of pnmiti\ e streak or head-process, and it is only by secondarilv breaking through this that the neurenteric canal comes to open into the entodermal cavity; the archenteric cavity is formed, in fact, before the occurrence of the invagination which, in other forms, brings the cavity into existence. The fact that the head-process is at first solid does not really affect the matter, but a similar state of things in which, however, the ingrowth is hollow, exists in some reptilian ova, and can be exemplified by the accompanying figure (Fig. 29). This shows a true invagination, which opens into what is essentially a segmentation cavity, the roof of the invagination temporarily replacing the roof of the second cavity, which has been broken through. A notochord and mesodermal outgrowths are formed from this temporary roof, the former taking first the aspect of a widely open groove; later, the original entodermal roof closes over again below these formations.

The process is essentially similar, though not so evident, in the human embryo, and the wall of the neurenteric canal (or anterior edge of the primitive streak) can be looked on as being ‘ paid out ’ in a forward direction as the embryonic area increases in length, laying down, as it does this, the notochordal rudiment; this rudiment is closely associated with neighbouring mesoderm, though not continuous with it. The temporary substitution of another layer for the earlier entodermal roof, and the development of the notochord from this, has led to the description of the notochord as a structure owning primitively a mesodermal origin. The argument does not seem to be well founded, for the supplanting layer is entodermal, being of the nature of an invagination, whence the description of the rod as being split off from the entoderm is fundamentally true. But to class it with the mesoderm because it arises, like the gastral mesoderm in more primitive forms, from the entoderm is another matter, and is a question of opinion depending on the definition that may be given to the word ‘ mesoderm.’

Fig. 28. — Transverse Section of Body of an Amphioxus (modified after atschek).

Fig. 27. — Sections, compounded of Various Stages, through Amphioxus Embryos, just in Front of Blastopore.

The outgrowth of entoderm to form notochord (not.) and mesoderm (mes.) have closed in and separated off from the entoderm in the second section. The mesoderm will extend ventrally at a later stage (Fig. 28). N, neural tube.

Finally, phylogenetic considerations may offer a theoretical explanation of what is found. In most fishes and amphibians, the alimentary cavity is made

Eggs of Certain Lizards

Upper row, longitudinal sections; lower row, transverse sections of the same stages, made as along the long arrow in the first figure. An invagination occurs, growing forward from the region of the primitive streak (ps) in the direction of the short arrow. The floor of this invagination is separated from the yolk-sac cavity by the ‘ primary entoderm ’ (interrupted line). In the second stage this primary layer and the floor of the invagination have given way; the roof of the invagination is now the roof of the yolkcavity, and is beginning to form notochordal and mesodermal outgrowths. In the last stage the entoderm is reconstituted, and the notochord and (gastral) mesoderm are not connected with it.

by the process of gastrulation, delayed and otherwise modified, it may be, by the amount of yolk present, but ultimately forming a true invagination entoderm which obliterates the ‘ segmentation cavity.’ In some amphibians, however, and possibly in some fishes, the invagination cavity breaks into the segmentation cavity, and the definitive enteron is lined in its lower part by cells which were not carried there by gastrulation. In reptiles the proportion of cells derived from the invagination decreases, the other cells correspondingly increasing their area in the wall of the cavity. In birds the process seems to go further still. Thus we reach a stage where only the lining cells of the roof are derived from the gastrular invagination, all the rest being of the nature of lining cells of what is probably a segmentation cavity. Though so far lacking demonstration, it seems at any rate a theoretical possibility that in the higher mammals the process has gone so far that the only derivatives of invagination cells now left are those forming the notochord; hence, when this has separated off, the lining cells have no gastrular representatives among them. Whether or not this view ultimately proves to be correct, it at any rate gives a present explanation of the complicated conditions which appear in the early stages of the human embryo. If it turns out to be correct, the archenteric cavity of the earliest known form will then be classed as a segmentation cavity.

Developmentally, the head-process or chordal process, extending forward from the neurenteric canal, is the first indication of formation of the notochord. It is a cellular process, containing a minute central lumen, which lies on the entodermal roof in the middle line. Its cells fuse with those of the entoderm, then breaking down, so that a longitudinal groove in the archenteric roof remains, and elongates forward below the growing neural region: from the mode of formation it is evident that this groove is continued behind into the neurenteric canal. As the embryonic growth progresses, the length of the groove is increased by addition from behind, and at the same time it deepens, closes, and separates from the entoderm in its front part, and continues to do this from before backwards as it increases in length. All these changes belong to stages later than that reached so far, but the subject may be conveniently considered here. By continuation of these processes the notochord forms (when the proper proportionate length of the embryo is completed) a longitudinal rod of cellular composition lying between the entoderm and the neural structures, and extending from the bucco-pharyngeal region in front to the end of the caudal region behind. It is only a phylogenetic remnant, having no function in the body and no further development, and is surrounded by the vertebrae as they form.

Further Growth of Embryonic Area

The rapid increase of the mesodermal layer is accompanied by some increase in size of the embryonic disc, which is now of an elongated oval shape. The margin, relatively fixed by the surrounding extra-embryonic mesoderm, is slower in its expansion, wherefore the embryonic area—which may now be definitely termed the embryo begins to stand up in a curved fashion above the level of the margin, and projects into the amniotic cavity. This result is apparent in the transverse section shown in Fig. 22, and would be more marked in a longitudinal section. The rate of increase in length begins to exceed markedly that in width; this is due to the rapid growth of the central nervous axis, while the mesodermal layer alone is responsible for the general increase in width of the area. The neural groove is deepened by the increasing height of the medullary folds, its ectodermal lining begins to grow and thicken, and it receives constant addition to its hinder end by the pushing forward of the developing area which lies round the neurenteric opening and front end of the primitive streak; the medullary folds are produced here by the rapid mesodermal proliferation from the streak, so that the addition to the neural groove from behind goes on pari passu with the formation and pushing forward of mesoderm from this region. The neural region and its mesodermal boundaries may thus be said to increase in length largely by addition from behind, but there is also a process of growth going on in the neural wall itself, and this is particularly marked in the front portion of this wall, the part which was first formed; it will become apparent subsequently that it is this front part which, by its increasing length, leads to projection forward of the anterior end of the embryo.

Formation of Somites and Closure of Neural Groove

As the embryo lengthens, the mesodermal layer undergoes a striking change, which is very apparent on the surface. A longitudinal groove appears on each side, some little distance from the medullary margins. The mesoderm medial to this groove, lying under the medullary folds and beside the neural groove, can be termed paraxial, while that lateral to the longitudinal groove is the lateral sheet.

The paraxial mesoderm now begins to segment into blocks (Fig. 30), which appear quadrilateral when seen from the surface. These are termed somites or primitive segments, and lie in series on each side of the neural groove. They do not extend to the front part of the groove, nor are they visible beside its hinder end; here they are in process of formation, like the groove itself, and are added as they are formed to those already in position. These primitive segments are therefore produced from before backwards.

Although this is true of the majority of the somites, there is reason to believe that three or four additional segments are added in front of those laid down in series as in the figure; these additions are produced by aggregation in the mesoderm beside the hind-brain, and their delayed production is no doubt associated with the rapid growth of this part.

When the full number of somites is complete it amounts to thirtyfive pairs or more. At the stage being considered at present, however, they are few in number, though increasing rapidly. They form thick mesodermal blocks lying beside the neural groove, and the edges of the neural folds in this neighbourhood come together (Fig. 30) and join, thus closing in the groove and converting it into a neural canal. The neural canal, however, remains open at first in front and behind (as seen in the figure), the openings being the anterior and posterior neuropores. The formation of somites is the earliest indication of segmentation * in the body.

The word is used now in a new sense. The morphological conception of the body is that it is composed of a series of successive segments fundamentally resembling one another. A complete bilateral segment would theoretically possess its own segmental skeleton, muscles, vessels, nerves, body-cavity, alimentary tube, and excretory organs. A body composed of such segments, unmodified, would be akin to a tapeworm, and the bodies of all animals higher than this class have their basic segmental structure largely obscured by secondary modifications. Nevertheless, the segmental basis shows in many places, as will be seen, and under many conditions. Body segments, then, imply by their name something more than when the word is used as meaning a division or part as m segmenting ovum ’ or ' segments of a limb,’ and confusion in meaning can be avoided by attention to the context.

Their successive repetition is a simple example in development of ‘ serial homology,’ an expression which implies a succession in series of structures in the body which possess essentially a genetic similarity.

Fig. 30. — Showing the Formation of Somites and the Closure of the Neural Groove to form a Canal.

The open ends of the canal are the neuropores. The somites (s) are segmentations in paraxial mesoderm, and are separated by a groove from the lateral sheet (Is). The rapid elongation of the front end of the neural walls leads to their projection beyond the heart region (h).

The lateral sheet of mesoderm never exhibits any trace of segmentation like that in the paraxial portion. The intra-embryonic coelom (described in the next section) splits the lateral sheet into somatic and splanchnic layers when it appears, about the time of the formation of the early somites; the somatic layer is applied to the ectoderm, and constitutes with it the somatopleure, while the splanchnic layer makes with the entoderm, to which it is applied, the splanchnopleure. A transverse section through the body, therefore, as in Fig. 31, would show at this stage:

(a) The closed neural tube in the centre.

(b) The notochord below this.

(c) A somite on each side of (a) and (b) . This is roughly triangular on section, and composed of somewhat elongated mesodermal cells arranged round a small enclosed cavity; a longitudinal section would show a certain very small amount of loose mesodermal cells between the successive somites.

(d) A solid cellular mass, the intermediate cell mass , connecting the walls of the somite with—

(e) Parietal and splanchnic layers of the lateral sheet, separated by the intra-embryonic body-cavity, which is continuous with the extraembryonic cavity through the split margin of the embryonic area. The parietal layer, covered by ectoderm, is directly continuous with the amnion; the splanchnic layer, lined by entoderm, forms the wall of the enteron, as that part of the visceral cavity may be termed which is destined to form intra-embryonic structures.

Fig. 31 — Diagram of a Section through the Embryonic Body.

The neural folds are represented as just closing, to show how the closure takes place (X).

The intermediate cell mass, which is the connecting link between the segmented paraxial and the non-segmented lateral mesoderm, is constricted, and thus produces the longitudinal groove on the surface already mentioned. It is a continuous, non-segmented cell condensation, running longitudinally along the roof of the body-cavity, and i^ the region from which the excretory organs of the embryo will develop.

Formation of Intra-Embryonic Coelom (Fig. 32).

This appears about the time of formation of the first few somites, and is a splitting of the mesoderm into two layers, as has been seen already. The cavity begins at first in relation with the rudiments of the heart, and quickly extends round the front part of the plate: it is separated , however, from the external coelom by the thick mesodermal margin of the plate in this region. The cavity extends backwards by two lateral prolongations towards the lateral sheet of mesoderm; here they join with cavities in these sheets. The body-cavity in each lateral sheet is separated from its fellow by the somites and neural region, and, at first, from the extra-embryonic coelom by the lateral mesodermal margin, as yet unsplit. The split, however, very soon extends into this margin, and the inner and outer coelomic cavities become continuous with one another. The front part of the margin, lying round the pericardium—as it can be termed now—and its lateral prolongation, remains unsplit. Hence the pericardium communicates directly only with the rest of the body-cavity, by means of its lateral channels, the lateral recesses , and only indirectly with the extra-embryonic cavity through these recesses and the body-cavity (peritoneal cavity).

Fig. 32. — Schemes to show the Relations between the Intra-Embryonic Coelom and the Fundamental Constituents of the Embryonic Area.

Coelomic spaces are shown as shaded areas projected on to (very schematic) embryonic plates. In the first figure the pericardial cavity is forming in front of the bucco-pharyngeal area (BP), and cavities are beginning to form in the lateral sheet. In the next scheme the cavities in the lateral sheets are formed, and the pericardium has two * lateral recesses ' extending back toward them. In the third figure the cavities of the lateral sheets are not only continuous with the pericardium, but have also opened into the extra-embryonic coelom by breaking through along the margin (A); the front portion of the margin (B) remains unsplit. The last figure gives the disposition of these cavities as they might be shown from the side, the embryo standing up somewhat into the amnion.

The schemes in the figure show the disposition of the general cavity at this time.

It can be understood that, by the establishment of continuity between the outer and inner ccelomic spaces, the visceral or gut-wall loses its attachment to the body-wall, which (Fig. 31) now remains only continuous with the amnion. This loss of attachment, however, is only where the mesodermal margin has been split, and the two walls are still connected with one another along the front part of the margin. Here, then, as will be seen subsequently, the vessels of body-wall and gut-wall can meet and reach the heart. Persistence of connection occurs also (see Fig. 32) at the hinder part of the margin, but this is not so important, and is modified to a certain extent later.

Formation of Fore-Gut and Hind-Gut

These are intra-embryonic recesses of the enteron. Rapid increase in axial length of the neural region is the dominating feature of the embryonic development at this stage. This is accompanied by addition to the number of somites and increasing length of the closed part of the neural tube; on the entodermal surface of the area there is corresponding addition to the notochordal formation, which is continually freeing itself from the entoderm and closing in from before backwards. The region of formed somites and closed neural tube, however, may be looked on (for present purposes) as relatively fixed, but the free anterior part of the neural groove (see Fig. 30) and the open posterior end, where actual addition is being made, are regions in which active growth and increase in length take place and affect the relations of neighbouring areas. Increasing length in both these regions leads to projection of the growing parts over the anterior and posterior limits of the original embryonic disc. This implies that the areas originally lying between the growing regions and the terminal margins become reversed and turned under the projecting ends of the neural structure. 1 he progress and results of these changes are illustrated in a schematic manner in Fig. 33. At the anterior end, the extremity of the neural region projects forward and passes over the bucco-pharyngeal and pericardial areas, reversing these below itself as it grows forward, and a diverticulum of the general gut-cavity is necessarily produced and included between it and the reversed parts; this diverticulum is the £ore-gut or primitive pharynx, which therefore lies above the pericardium and is closed, at what is now its front extremity, by the buccopharyngeal membrane.

The projection of the hinder end (tail-bud) has a similar effect. The region between it and the posterior margin of the embryonic area is that of the primitive streak, and this is reversed and turned under the growing caudal projection. The included cavity, comparable with the cavity (fore-gut) included in the anterior projection, can now be termed the hind-gut ; but there is a complication that is not present m the case of the fore-gut. The allantoic diverticulum passes into the body-stalk just below the level of the original embryonic margin, but the connection between body-wall and gut-wall remains here when the coelomic split occurs, as has already been seen, so that, as the tailbud projects and carries body-wall with it, this part of the enteron, with its diverticulum and attached stalk, is drawn into the body of the embryo. Thus, as shown in the schemes, the allantoic cavity comes to open into the ventral wall of the ' hind-gut.’ A large cavity is formed in this way which receives the allantoic diverticulum ventrally, and, dorsally, a prolongation from the gut-cavity itself; it is

Fig. 33 To illustrate the results of rapid elongation of the Neural Axis, shown in Solid Black.

ps, primitive streak; BP, bucco-pharyngeal area or membrane; P, pericardium, ine fore-gut is made by the forward projection of the neural structures reversing the pericardium and bucco-pharyngeal membrane below itself.

he hind-gut is made, in a similar fashion, by the growth of the caudal end reversing the area of the primitive streak below itself. X is the front margin of the original embryonic plate, which, when reversed, comes to lie behind the pericardium, and separates it (septum transversum) from the abdominal region.

convenient and customary to distinguish these parts by speaking of the common cavity as the cloaca, and restricting the term hind-gut to the tube (from the gut-cavity) which opens into the cloaca dorsally. As is apparent from the figures, the ventral wall of the cloaca is made m the middle line by the primitive streak, and this forms the cloacal membrane, closing off the cloaca from the surface, like the buccopharyngeal membrane in the fore-gut.

That part of the intra-embryonic enteron which lies between the anterior and posterior prolongations is termed the mid-gut ; it very soon undergoes alterations in its disposition and appearance, but at this stage it is widely open and continuous below with the extraembryonic yolk-sac*

In Fig. 33 two other points may be noticed. The pericardium enlarges rapidly (keeping pace with the rapid growth of the heart), and is not only reversed by the neural elongation, but, owing to its own growth, bulges forward and ventrally over the original embryonic margin (shown at X in the figures). The neural tube not only elongates behind the bucco-pharyngeal area and pushes this forward in the reversing movement, but also extends forward over and in front of this as a free projection, which is the fore-brain. The prominent fore-brain and pericardium are separated by a depression, transversely disposed, which is the stomodseum; the bucco-pharyngeal membrane lies at the bottom of the stomodaeum and shuts it off from the fore-gut.

Condition of Intra-Embryonic Coelom

Consequent on the growth-processes just considered, the bodycavity shown schematically in Fig. 32 undergoes a certain amount of change affecting its anterior and posterior parts; this can be easily understood by comparing the last scheme with those in Fig. 34, which represents the results of reversal. The reversal of the pericardium has carried each of its lateral openings dorsally and forward, but as the great relative size of the pericardium, when the movement is completed, is due to rapid growth and bulging forward of that structure, the ultimate openings of each lateral recess are on the dorsal side of the cavity, some distance in front of its caudal wall; each recess passes back from this to join the lateral cavity. The bulging of the pericardium, with its reversal, has led to the unsplit marginal mesoderm, which (p. 43) lay round it and its lateral recesses, being now concentrated behind it, below the lateral recesses. This concentration of mesoderm forms a septum between the pericardium in front and the peritoneal cavity behind, and is termed the septum transversum. The lateral recesses, therefore, pass back to the peritoneum above the septum transversum, which is forming the posterior or caudal wall of the pericardium; in doing this, the recesses must lie on either side of the fore-gut in this region, for this is above the pericardium and its caudal wall (see figures).

The septum transversum, being the original unsplit anterior margin of the embryonic area, is (as was previously pointed out) an area where body-wall and gut-wall still retain their connection. Hence the veins (vitelline) from the gut-wall meet here with those from the body-wall (umbilical and others), and at the junction form a large venous sinus. This sinus venosus is therefore embedded in the septum transversum, from which it opens into the venous end of the heart. At the caudal end the external coelom, which is between body-stalk and yolk-sac at their junction, has been drawn into the body with these structures, and forms a peritoneal recess passing from side to side between the hind-gut and the allantoic stalk. Sections through the embryo, in the directions indicated by arrows in the previous figure, are shown in Fig. 35 to facilitate comprehension of these relations of the body-cavity.

See note on p. 30.

Lateral mesodermal sheet removed, exposing cavity. Rim of plate unsplit in first figure, but has given way (broken line) in others. Numbered arrows in third figure correspond with sections in Fig. 35.

The embryo may now be fairly said to have reached the vertebrate level. It has an alimentary tube separated by a body-cavity from the body-wall, with a heart and pericardium ventral to the tube, and an axial skeleton (notochord) and central nervous system dorsal to it. It is very minute still; the embryonic plate, when the neural groove is first formed, is only about 1 mm. in length or less, and the greatest length of the embryo when it has completed the processes of reversion is not much more than 2 mm. The stage is reached during the third week, but exact data cannot be given about this matter. The broad details of subsequent development which are about to be considered are comprised under the term organogeny , including as they do the modes of formation of the various structures and organs that make up the regions of the body. In attaining to their mammalian form, these several structures pass in a general way through stages that can be ermed fish-like and reptilian, but the recognition of such stages is arge y a matter of opinion; and in any case the phylogenetic influence o the past is much overlaid by the present and insistent influence of ontogeny. T ^e action of the rapid neural growth does not cease with If' W / h n h been reached so far; it is only when the full number of somites (about thirty-five) has been produced that the rate of growth of the nervous axis included between the somites falls to that of the

Fig - 35 - Sections i, 2, 3, and 4, passing through the Directions shown by the Arrows in the Last Figure.

ST, stomodaeum; YS, yolk-sac; CL, cloaca; BS, body-stalk.

iTstiF reh the anterior Part from which the brain is formed,

change goes on here for a considerable period. The result the dorsal^su^ rate ° f f 0wth bein S in the middle line, on

curved form, the elongating dorsal tube encircling the slower-srrowinu

5 S KrKKJV? is " d C ,ed m «* ifS iKTS

figures, but is in reality much more marked than appears in them.

Fig. 35A. — Schemes showing Attainment of General Form of Embryo.

A, enveloping layer, whose fate is uncertain, represented by a line. Entoderm has grown round and enclosed the archenteron, and the inner cell mass (M) shows amniotic spaces. In B, primitive mesoblast, of uncertain origin, fills the ovum, surrounding the ectodermal amniotic sac and the entoderm. The embryonic plate lies between the two, made by ectoderm and entoderm only. C shows this mesoblast split, forming the extraembryonic coelom; this leaves the cells deposited either on the wall of the ovum (chorion), or on the included embryonic structures. A connection, however, persists (B) between these, the body-stalk, by which vessels can run between embryo and chorion. Situations of anterior and posterior poles of the disc are shown at a and p respectively. For rest, see text.

Fig. 36 gives a representation of an embryo considerably older (though still under 5 mm. in length) in which the extreme curvature is well seen, and it appears very soon after the movement of reversal has taken place. This curved state of the embryo remains a marked feature for some weeks, but, as the visceral growth within the body begins to make its influence felt, the tightness of the curve is relaxed; towards the last half of the second month it is much less in evidence, and in the third month it is not at all striking. The foetus, however, always retains more or less of the curve of these months, owing to the necessities of the space it occupies, and the thoracic curve of the adult vertebral column is really a remnant of the old embryonic dorsal convexity.

Fig. 36. — Human Embryo 4*9 Mm. in Length. The body-stalk is cut short; limb buds are visible.

In the third schematic section in Fig. 33 a dorsal concavity is shown. This is not a necessity of the diagram, but is actually found in this and somewhat later stages. It is customary now to refer its existence to the effects of preparation of the embryo, but, since nearly every known embryo of these stages presents this angled bend, it is not improbable that the older view was correct, and that it is really a temporary normal stage, due to irregularities in growth-rates, which is corrected later, so that the full convexity is produced.

Before proceeding to describe the formation of organs and regions, however, it is necessary to consider two things to which reference has been occasionally made in the preceding pages—the yolk-sac and the early formation and circulation of the blood.

The Yolk-Sac. This is a hollow sac, with the embryonic plate as part of its roof. Its general form and appearance can be gathered from the figures. It is empty, and, as seen in hardened and sectioned embryos, is wrinkled and collapsed.

The emptiness of the sac refers to the absence of any volk-like material t actually contains fluid, and presents a rounded appearance when fresh* its collapsed state is a result of the processes of preservation. The absence of yolk is a secondary result of the establishment of placental nutrition The sac, especially m the later stages (m which the name is applied with most Satt" 8 representative of the lower part ofThe visceral

The wall of the yolk-sac is composed of mesoderm, lined internally by entoderm, from which short glandular outgrowths project into the mesoderm; it is possible that these secrete the fluid contained within the sac. The mesoderm between the short glands is the seat of the earliest formation of blood-cells. The aggregation of bloodforming areas, or blood islands, causes the irregularly bossed appearance of the yolk-sac wall which is apparent in the figures. The subsequent fate of the structure will be dealt with later.

Blood Formation and Early Circulation

The blood-corpuscles of the embryo are nucleated. They are first formed in the wall of the yolk-sac, probably from the mesoderm. Separate collections of special cells, erythroblasts, appear in the mesoderm, and these develop quickly into blood-cells and endothelial containing cells. The vascular islands formed in this way extend and run together, thus producing a vascular network on the yolk-sac. About this time spindle-shaped cells appear in the chorion and body-stalk, and, becoming connected, form another network in these parts of the ovum. Actually, vessels appear to be formed in the chorion before they are definitely recognizable in the yolk-sac. Blood-cells also seem to be formed in the chorion. The vitelline network soon joins up with the chorionic system by connection with the vessels in the body-stalk. At the same time, although the details are not known, the intra-embryonic circulation appears, connected with the chorionic and vitelline systems of vessels. This takes place at (or just before) the time when the somites begin to appear, and the circulation at this very early period may be represented as in the upper scheme in Fig. 37.

Vitelline veins run to the venous end of the heart, which at this stage is at the front end, and from the heart two aortcc run back, lying beside the bucco-pharyngeal area, and passing in the embryonic plate along the roof of the enteron, to reach the body-stalk; here they become placental or umbilical arteries, and reach the chorion and its villi. Blood returning from the chorion runs in the umbilical veins, which pass back in the body-wall close to the amniotic attachment; thus they reach the anterior part of the margin, where they join the vitelline veins in the venous end of the heart. Branches from the aortae run down on the wall of the yolk-sac, and thus complete the vitelline circulation.

The process of reversal puts the venous junction at the caudal end of the heart, in the septum transversum, and the arterial end of the heart, now in front, gives off aortae, which must turn back to run their course; this is at first on the roof of the fore-gut. The lower diagram in the figure shows these results of reversal. The necessary changes in position of the umbilical and vitelline veins are also seen. In addition, when the excretory system begins to form along the dorsal wall of the body-cavity, a longitudinally running cardinal vein is developed in association with it; this passes forward, and joins with a primitive jugular, draining the region of the growing head. The junction of the two forms a large vein, the duct of Cuvier, which reaches the septum transversum by turning down in the body-wall on the outer side of the lateral recess of the pericardium. On each side of the body, therefore, there are three main veins which meet in the septum transversum: the duct of Cuvier, and the umbilical and vitelline veins; when they join they form a large venous lake known as the sinus

Fig. 37. Two Schemes to illustrate the Circulation in the Embryo.

The fl a wd g K re iS \ representation of the ' plate ' stage, opened out on the ni ’ e ^ cc °-pharyngeal area Is seen between the aorta; at their begin mi thXSErf KfcSSrfg’ Th « "W

Broad Outlines of Development of Systems and Organs

Nervous System

The spinal cord is formed from the neural tube, lying between the rows of paired somites. The ectodermal walls of this tube, proliferating rapidly and thus thickening the walls laterally, soon convert it into a structure (Fig. 38) in which a cavity, elongated dorso-ventrally, lies between two thick lateral walls connected by a thin roof-plate and a thicker floor-plate. The cavity becomes ultimately the central canal of the cord, its shape being much modified by the developments in the walls. The side walls are syncytial in structure, the nuclei being embedded in a protoplasmic network, the myeloplasm. The nuclei proliferate in the layer immediately adjoining the cavity, the additional nuclei formed in this way being pressed out into the myeloplastic network. In this way certain layers can be distinguished before long in the walls:

(a) An inner ependymal layer surrounding the canal; there are large clear ' germinal cells ' in this layer, which are thought to be particularly concerned in the proliferation.

(b) A mantle layer, covering the ependymal layer, and showing many nuclei in the syncytium.

(c) A marginal layer, consisting of protoplasmic processes, without nuclei, sent out from the syncytium.

Fig. 38. — Three Sections from Different Levels of Cord in Embryo of 4-9 Mm.

Left lower figure, under higher power, shows nerve-fibres leaving ventro-lateral wall.

The mantle layer cells, when they differentiate from the early syncytium, are neuroblasts and spongioblasts’, the former develop into nerve-cells, the latter into neuroglial (supporting) cells. The mar

Fig. 39. Compound Schematic Figure to show the Various Points


It is supposed to represent part of a transverse section through the body.

fihA Til f0rn T A net r rk or scaftoldi ng within which the white p of the cord, when they develop, can pass up and down.

theventrn w ma i 10n i°i neu ro blasti c nuclei takes place especially in Darts bnloW V dor ? odateral P arts of the side walls, so that these dorsal T 1? th ® d v A ty ' A nd P roduce longitudinal columns, the sulcus (Fig 3g” d V6ntral baSd lamin8e ’ se P arated b y an interlaminar

theTentTaTT^nt “ ‘I® ba ? al 1 f m “ a send out Processes which form

the dorsawir nnT 1 r °° t J S ° f th ® s P inal nerves - The ganglia of

longitudinal rid vp r> Til ro ° ts are derived from the neural crest, a

neural folds pT ^ ro ° f T} at ? made b y fusion of the edges of the

of cells aDDlied to tfi°d ° f C w fr ? m thlS makes a continuous sheet

within thTsheet atT d0rS °;T eral aS P ect of the cord on each side:

definite masses which TT E lntervals - the cells are gathered into asses which are the precursors of the ganglia. These ganglionic rudiments, therefore, are connected with each other at first by the remnants of the sheet lying between the masses, and only become ‘ free ’ when such remnants disappear. The ganglion cells send out processes to meet the ventral roots and complete the nerve.

Each spinal nerve extends by degrees into its proper position in the developing body. There are at least three main theories concerning the nature of their extension:

1. Outgrowth theory of His: This, the most generally accepted, describes the nerve-fibres as direct growths from the neuroblasts, hence ectodermal in origin, but acquiring mesodermal sheaths.

2. Cell-chain theory of Balfour: This regards nerves as chains of cells, but opinion is divided as to the ectodermal or mesodermal origins of these cells.

Fig. 40

3. Primitive continuity theory of Hensen: This view postulates the (syncytial) continuity of ectodermal and mesodermal layers from the beginning, so that the potential path of the nerve impulse, and so of the fibre, is present ab origine. An increasing number of recent observers adopt this view.

Brain. — The most anterior part of the neural groove, that first laid down, forms the brain. The widely open portion shown in Fig. 30 is the hind-brain ; its anterior end, where the two folds meet in a low ridge, contains the primordia of what will later develop into mid-brain and fore-brain. The hind-brain is the part which, by its rapid elongation, leads to the initiation and carrying out of the movement of reversal already considered. When the reversal is accomplished, the hind-brain extends practically along the whole dorsal length of the fore-gut; the notochord lies between it and the roof of this recess. As it is prolonged forward it closes in from behind forwards, so that the position of the anterior neuropore is moved forward. Before the reversal is completed its anterior end begins to grow rapidly, and projects (Fig. 33) beyond the bucco-pharyngeal membrane; this makes the fore-brain, which is therefore a free projection forward from the front end of the neural region. As the fore-brain projects forward, the hind-brain closes in completely, and the anterior neuropore is now only a small opening which has been carried forward on the projecting fore-brain, and closes rapidly. The fore-brain is now a small and rather elongated part, connected with the large hind-brain by a short constricted neck which will soon develop into the mid-brain, and may be so called now.

Mid-Brain Flexure

Fig. 41. To show how the Brain Tube, between the Fixed Point (X) Grow™ ' ReGION ° F THE Somites ( s )> is bent to admit of its Rapid

N, notochord; Bp, bucco-pharyngeal membrane.

The brain at this stage is seen in Fig. 40; the relation of the forewarn to the remnants of the bucco-pharyngeal membrane is seen, and can be observed that the notochord is attached to the upper part of this membrane The anterior neuropore has closed. The lower aspect of the fore-bram is in direct contact with the covering ectoderm

middfehtTn mt “ g: this ^d attachment persists Tn the

ddle hne for a short time, and for a considerable time just in front of

i?^“^^ phaiyngeal membrane < after the membrane

incomplete S coverina° Ve it n ^i T for ®' b / ain “ at first very scanty, and forms an layer, but no segmlntat^o^cTrlt “

atta^hed P t°o n tL X o J‘ g ' 41 ,T rks T e pIace where the fore-brain is

With the nosterior erm f , £ T the bu cco-pharyngeal membrane,

p stenor layer of which the notochord is continuous. This part, then, can be looked on as a relatively fixed point, round which further processes of growth may effect rotation, or may produce flexures against its resistance. Both these results are seen. They are schematically represented in Fig. 41, in which the black line represents the brain as in the last figure, and the interrupted and dotted lines two imaginary later stages. The ‘ fixed ’ point is at X, where ectoderm, bucco-pharyngeal membrane (BP), and notochord (N) meet. Between this and the region of somites (S) the disproportionately increasing length of the brain leads to flexures; the hind-brain, opening out with a wide roof-plate, makes a sharp pontine flexure, convex ventrally, and the mid-brain stands up in a mid-brain flexure, concave ventrally.

Fig. 42. — Outlines illustrating Stages in Development of Form of Brain.

Though diagrammatic, the four stages may be taken to correspond, in a general way, with the conditions in embryos of the fourth week, fifth week, seventh week, and third month respectively. F, M, H, fore-, mid-, and hindbrains; Tel, telencephalon; CV, cerebral vesicles; CBLM, cerebellum; PIT., pituitary; OPT., optic outgrowth, cut away in later stages, leaving only the stalk. The topmost figure shows a section through the hindbrain in its wide part; D., B, dorsal and basal laminae.

In front of X the ' push ’ of the mid-brain tends to rotate the lower part of the fore-brain backwards and downwards round X, so that it comes to be firmly pressed against the projecting pericardium; but its anterior and upper part is free, and from this region outgrowths take place to form the optic and cerebral vesicles.

Further stages in the general development of the form of the brain are shown in Fig. 42; the hind-brain is seen to become more sharply bent at the pontine flexure , so that, at this bend, it is widely opened out; thus the dorsal and basal laminae come to lie in what is now the floor of a wide lozenge-shaped cavity, the roof being formed by the broad and undeveloped roof-plate. The breadth of the cavity and roof-plate decreases as one passes forwards or backwards from the pontine flexure. This ' open ’ part of the hind-brain is called, from its shape, the rhombencephalon, and the name of isthmus is given to its narrower upper end, where it is about to join the mid-brain. A nuchal flexure is formed secondarily below it, being a compensating curve due to the presence of the pontine bend. The cerebellum develops, at a fairly late period, from the dorsal lamince of the front limb of the bent hind-brain ; it begins as a thickening in each lamina, spreading into the roof-plate and standing out prominently behind. The cavity of the rhombencephalon becomes that of the fourth ventricle.

The cavity of the mid-brain becomes the Sylvian aqueduct. The cerebral crura are formed below its floor by the fibres that grow down, at a later period, from the cerebrum and, lying in the marginal zone, obliterate the ventral concavity in this part. The roof develops symmetrical elongated corpora bigemina, which are subsequently divided by a transverse groove, thus producing corpora quadrigemina. During a great part of the embryonic period the mid-brain makes a prominent convexity (Fig. 40), which constitutes the highest part of the head, or most anterior point; towards the end of this period the relative rate of growth of the part falls, and the cerebral vesicles begin to cover it.

The fore-brain, as it appears in the early stages, is known as the thalamencephalon or diencephalon. Two optic outgrowths project from its lower part, one on each side, in contact with the ectoderm of the surface; they make the optic nerves and inner layers of the eyeballs, forming the lens in each case. The dorsal laminae of the fore-brain thicken to form the optic thalami, and the basal laminae orm the corpora mammillaria, tuber cinereum, and subthalamic nuclei. i he infundibulum of the pituitary body is an evagination from its floor, and the pineal body from the hinder end of its roof.

But, before developments take place in the thalamencephalon, here occurs a dilatation or projection forward of its anterior part, m connection with the dorsal laminae. This newly added part of the tore-brain is termed the telencephalon ; it appears when the embryo is a ou 5 mm. long. The side walls of the telencephalon very soon egm to project laterally to form the cerebral vesicles. As the cerebral vesic es enlarge they project to some extent forwards, but their main enlargement is m a backward direction, as well as upwards and outwards, they extend back, separated from each other in the middle

Z t y a me ^°d ern J a J septum (falx cerebri), and cover successively

e lore-, mid- and hind-brains. Each has a large cavity at first,

vitThpr Wlth th at of the telencephalon at its site of origin; the

cenhTn k T S r he 1 fa l ™ ntncle ’ and its opening into the telencenhn on ww - The thalamencephalon and telen cotiola f tute - the . th ”* ?™ tride - The cerebral vesicles have the

at fdst thrkTTT S m F lr fl x 0rs and outer walls : their roofs, thin hemispheres U sec f uen ^T to ^ orm main masses of the cerebral

The elongated brain of the embryo is altered, by the changes shortly sketched out above, into the compact organ of descriptive anatomy. The hind-brain, originally extending along the dorsal length of the fore-gut, is much shortened by flexure; this, with the rapid growth and elongation of the fore-gut, soon leads to the definite cranial situation of this part of the brain. The fore-brain, at first forming the roof of the stomodaeum, is soon separated from this by the growth of mesoderm below it, as will be described when dealing with the formation of the face. It is then hidden from view by the growth of the cerebral vesicles, and these, growing upwards and backwards, become the highest parts of the whole structure when the head is brought into the vertical position.

Vertebrae and Body-Wall

In the region of the somites the inner wall of each of these mesodermal blocks begins to break up, with rapid proliferation of its cells. A large extension inwards of cells takes place toward the notochord; this extension is known as the sclerotome. Each sclerotomic ingrowth (Fig. 43) meets its opposite fellow round the notochord, which thus comes to be embedded in a mesodermal mass lying between the neural tube and the structures connected with the roof of the alimentary tract. The vertebral centra and intervertebral discs are developed in this mesodermal mass, and extensions from it pass dorsally round the tube and form the neural arches and the ligaments between them.

The vertebrae, however, are not segmental in position, like the somites from which they are indirectly made, but inter segmental. This result is attained (Fig. 43) by the splitting of each sclerotomic ingrowth into two parts, anterior and posterior; the anterior part of one sclerotome joins with the posterior portion of the sclerotome in front of it, and thus an intersegmental mesodermal mass is formed, in which the vertebral centrum is developed. The dorsal extensions, and lateral ones, are also between the somites, and therefore intersegmental. The lateral extensions form transverse processes, and also the vertebral ends of the ribs ; the latter extend gradually into the somatopleure or body-wall.

Body-Wall. —The lateral outgrowths from the sclerotomes are still intersegmental in the body-wall, for, between them, extensions from the somites pass ventrally into the body-wall to form the muscles of this wall; such a downgrowth from a somite is, of course, segmental in position. In the abdominal wall there are no costal extensions from the sclerotomic derivatives, and the muscle growths from the somites are separated from each other only by mesodermal cells of the lateral sheet into which they are growing. No muscles are formed from the mesoderm of the lateral sheet where this has been split by the development of the body-cavity, but they are all derived from these ventral downgrowths from the somites. There is a part of the lateral sheet, however, which is not involved in the coelomic splitting; this lies on each side behind the bucco-pharyngeal area (see Figs. 32 and 34), and makes the side-wall of the front portion of the fore-gut when this comes into existence, and here certain muscles are developed directly, as will be seen later.

43- Sections of Human Embryo.

notochord t T)wJr e ^PrV ,S ^°T S s< ^ er °t°mic ingrowths from somites towards anterior siihrlivicin 1( i n ' h° nzon tal, shows the (dark) posterior and (lighter 1 tube. ^ ns sc ^ er °tomes. a, intersegmental arteries; N, neural

growing- Tun-vSl ( ^ vn § r °wths from the somites are accompanied by supplies of the wall VCSSe S ’ WhlCh become the vascular and nervous

Changes in the Mid-Gut.

It has been seen that the mid-gut is at first a cavity within the embryonic body, into which the fore-gut and hind-gut open, and is in wide continuity with the yolk-sac outside the embryo (see Fig. 33). Its roof shows at first the notochordal groove, but this soon closes in and separates from it. The width of its roof (Fig. 31) is small compared with that of the embryo, owing to the presence of the bodycavity. During the third week the roof of the mid-gut begins to come away from its original position immediately below the notochord, and the dorsal parts of the body-cavity on each side are approximated above it; in this way a two-layered mesentery is made, in the middle line, between the receding roof of the gut and the dorsal wall of the body, carrying between its layers the vessels that were at first supplied more directly to the gut-wall. At the same time the region of continuity between the intra- and extra-embryonic gut-cavities begins to show a constriction, which definitely marks off the permanent gut from the outer yolk-sac. The constricted region is drawn out to form the vitello-intestinal duct or canal. Thus, by the fourth week, there is an angled loop of embryonic mid-gut (Fig. 44) suspended by a median dorsal mesentery and, at its most ventral point, attached by a narrow and elongated vitello-intestinal duct to the yolk-sac. The hind-limb of the mid-gut loop is continuous with the hind-gut, and the front-limb with the fore-gut, but in this case the place of continuity is not so evident at first sight. Examination of the diagram will show that the gut-tube, when followed forward, passes up behind and in contact with the septum transversum, and so forward over the pericardium; all that part in relation with the septum is to be regarded as fore-gut\ whence the mid-gut must be described as beginning at the lower edge of the septum, or, in other words, as soon as the ventral wall of the tube is free from this attachment (X in Fig. 44). The hinder end of the fore-gut is thus seen to have a median dorsal mesentery directly continuous with that of the mid-gut, and coming into existence with it.

Fig. 44. — Diagrams to show Changes in Mid-Gut.

In the first figure the dotted lines indicate the successive limits of the mesentery, and, at the junction of intra- and extra-embryonic parts of the enteron, the increasing constriction producing the vitello-intestinal canal. The second figure shows, by transverse section, the resulting production of a mesentery and a vitello-intestinal canal.

It will possibly have become apparent already that the division into foremid-, and hmd-gut is more descriptive than morphological in value and that the fore-gut and hind-gut are only comparable in a very general wav The beginning of the mid-gut at the lower end of the septum is necessary for the conception of the human fore-gut, but does not apply to some other forms The beginning of the hmd-gut can be more clearly appreciated later.


Fig. 45 — Compound Scheme to show the General Relations of the Chief

Derivatives of the Alimentary Tube.

Th %h n an il tha 1 tm°ustrlted t in W th en T* firSt x f ? r “ ed - is much Sorter and simpler sent ^ ~pre . .T'b mld ;g ut now grows in length somewhat rapidly between its two extremities, which may be considered to be more or less fixed

t fo^a lonu by T- n P tion of the mesentery and

t lorms a long U-shaped loop which projects ventrallv out of the hodv dUCt 15 T aCheS t0 P e ventral convexity 6 of°the

The loop external to f* the beginning of the second month.

IRu. i?’ 1 t0 the body, lies against the front of the bodv-stalk

S “ F “= it IS, of course, in a 3 l part

here in an ‘ umbilical sac.’ ° m ' Zt ls sometimes described as lying

whicVi 1 berins li t 0 n i i lteStinal - d V Ct n ° W se P ar ates from the loop of gut

the posterior limTTthe u's]! 1 * rapldl y b y growth of its front limb', r nmp ot the U-shaped loop does not elongate to any extent, and shows an early swelling a little distance from its lower end, which develops into the caecum and appendix. The coils formed by the anterior limb lie on the right side of the non-coiled posterior limb. This goes on through the second and into the third month, by which time the caecum is a long cone-shaped projection from the straight posterior limb, while the closely packed coils lie to its right; the lower portion of the posterior limb, beyond the caecum, also shows some tendency to elongate the coil. In the tenth week (40 mm. embryo) the umbilical loop enters the abdomen, and its constituent coils are brought into their normal relative positions after this has taken place. The contents of the sac do not enter the abdomen en masse , but in continuous sequence, the anterior limb of the loop passing in first, from before backwards, and then the posterior limb. Their subsequent disposal in the belly, and the factors concerned in the movement, will be dealt with in the appropriate sections. The common median mesentery still connects them with the dorsal wall, and the definitive peritoneal arrangements and attachments are only acquired secondarily and subsequently as the bowels attain their positions, some weeks after their withdrawal into the abdomen.

The vitelline artery runs in the common mesentery of the loop, and gives branches, in front and behind, to the two limbs of the loop as it passes between them. It is carried on to the vitello-intestinal duct and yolk-sac at first, but, when these separate, it is confined to the mesentery, and can now be called the superior mesenteric artery. When the coils enter the abdomen and are disposed within that cavity, the artery and its branches are laid out with the mesenteric folds, and supply all that part of the intestine which is derived from the mid-gut. This extends from just below the entrance of the bile and pancreatic ducts (derivatives of fore-gut) to the junction with the hind-gut; the junction is at first about half-way along the ‘ transverse ' colon, but subsequent modifications shift the point more towards the left to about two-thirds of the length of the transverse colon.

The vitello-intestinal duct very rarely remains as a cord extending from the gut to the umbilicus, or even as a patent canal. Less rarely, however, its intestinal end may remain as Meckel’s diverticulum, a hollow gut-like protrusion from the ileum, of variable length, about 40 inches from the caecum; this 40 inches of the ileum is derived from the lower end of the posterior limb of the U-loop, below the caecal projection. The vitelline artery may persist as a fibrous cord passing from a corresponding place in the mesentery to the umbilicus. The distance from the caecum is very variable.

The vitelline vein does not accompany the artery, but passes, free in the peritoneal cavity, in front of the mid-gut loop, and runs to the pancreatic region ; its persistence in later life is excessively rare.

The Umbilicus.

The margin of the embryonic disc, after the movement of reversion has taken place, extends, as has (Fig. 33) been seen, from the posterior wall of the pericardium [septum transversum) in front to the anterior end of the [reversed) region of the primitive streak behind; round the margin, at and between these points, the amnion is attached. If the amnion and the visceral wall below the embryo were removed by section along the margin, the embryonic area, viewed from below, would be as shown in Fig. 46. The first diagram represents the condition before the embryonic body-cavity has broken through the margin; the second shows the result of this split after the proximal part of the body-stalk has been drawn into the embryo. The bodystalk now forms a projection into the opening enclosed by the margin; two umbilical arteries, two umbilical veins, and the allantois are cut in its section. It can be seen, in the second figure, that the umbilical veins run forward from the body-stalk to the septum transversum in the body-wall , close to its continuity with the amnion.

Fig. 46. — To show the Formation of the Umbilicus. (Description in text.)

T the U-loop of gut is cut as it passes out of the body, and the vitelline vein is ‘ free ' in front of it, no longer attached to the septum at this level.*

lonn'hai a ^4^ a ?'+u 1 sta t e of the opening when the intestinal

It Tan L ^ the vitelline vein has disappeared,

corresnonrk itvf +l° that, at the beginning, the embryonic margin the embrvo m-n e . Slze °t the embryonic area or embryo. But, as Thus th° rnarfrin WS ’ 1 over ^ a P s the margin to an increasing extent, increases slowfv inT mar ^ s ou ( an area which, even though it actually of the erowin/pmU* 26 ’ n ® vert ^ e ^ ess relatively, compared with the size U growing embryo, becomes rapidly smaller and smaller. It is

This is a sec °ndary change that need not be considered now.

already considerably shorter than the embryo at the stage represented by the second diagram, and by the time the intestinal loop is about to enter the abdomen, it is represented by a region, only a few millimetres long, on the ventral wall of the belly. When the passage of the gut has taken place, the amnion and the muscular wall of the abdomen contract down on the body-stalk attached to the middle of the bellywall, where it forms the foetal end of the umbilical cord. When the cord separates from the wall after birth, the resulting scar constitutes the umbilicus ; therefore, from this scar the remnants of the allantois (urachus) and umbilical vessels pass to the parts with which they were connected in intra-uterine life. The umbilicus of the adult is not, then, quite the same thing as the umbilical opening of the embryo; the scar really corresponds with the posterior margin of the opening, which would be represented by a small and almost linear area in front of this. If this area is taken into account, then it would not be very wrong, though not exactly true, to speak of the umbilicus as representing the area from which the body originally grew.

Fore-Gut and its Derivatives.

It has been seen that the early fore-gut has its floor formed by the roof of the pericardium and the septum transversum. It has also been seen that, in the development of the mid-gut, that part of the fore-gut which passes down behind the septum transversum acquires a median dorsal mesentery directly continuous with the similar structure suspending the mid-gut. It will be convenient to consider separately this last portion, the part with a dorsal mesentery, and to restrict the account for the present to the part lying above the pericardium and above the septum. The roof of the fore-gut is made by the mesoderm beside the notochord. A strip of mesoderm (Fig. 32) on each side, between the roof above and the pericardial wall below, makes the side-wall; this mesoderm is continuous with the lateral sheet farther back, and is, of course, covered by ectoderm and entoderm. There is no ccelomic split in the mesoderm of the side-wall where it lies above the pericardium, but a split is present where it lies above the septum transversum ; this split is continuous, below and in front, with the pericardium, and opens into the lateral body-cavity behind, and is, in fact, the ‘ lateral recess ’ of the pericardium (or pericardio-peritoneal canal) already described.

We find, accordingly, that the fore-gut can be divided here into two parts. In front, dorsal to the pericardium, is a cavity with unsplit side-walls, which extend from the covering of the hind-brain above to that of the pericardium below; therefore the general width of this cavity will correspond with that of these structures—particularly the hind-brain—and its growth is associated with changes on the surface. The pharynx and related structures will be developed from this part, and it can be termed the primitive pharynx.

This front part is continued backwards into the second portion, which has the lateral ccelomic passage on each side of it; this is the part above the septum transversum. Here, then, is a definite splanchnic wall separated by the body-cavity from the somatic wall, and the splanchnic wall of this portion of the fore-gut can be narrowed and elongated without reference to the dimensions of the somatic wall and without affecting surface form. This is what occurs here, the part being gradually elongated to form the oesophagus , and the name of primitive oesophagus or oesophageal part of the fore-gut can be given to it.

Primitive Pharynx.

The thin unsplit mesoderm of the side-wall of the front part begins to increase in thickness. The growth is not uniform, but takes place in a series of dorso-ventral ridges or bars of condensation placed one behind the other. These regions of growth and condensation are known as visceral arches, or pharyngeal arches, and the intervening lines, where no growth takes place, are the corresponding grooves. As the condensations involve the whole thickness of the side-wall, this wall presents, on both ectodermal and entodermal aspects, a series of visceral arches, visible as projections and separated from one another by grooves; 'on the ectodermal aspect they are external pharyngeal arches and grooves respectively, and on the entodermal surface they are internal arches and grooves . The arches do not appear simultaneously, but rapidly in sequence from before backwards, and are six in number on each side.

,, sur i?f 1 e A iew ? hows onl y five arches, but the last one is really the sixth, the true fifth being buried, as will appear later.

They are numbered from before backwards, the first arch being usually spoken of as the mandibular arch. When their number is complete they extend back to the lateral recess of the ccelom the condensation of the last arch being in relation with this.

The grooves between the arches are numbered according to the arches m front of them; thus, the first groove is between the first two arches, the second groove between the second and third arches, and

s ° ° n ' - hctoderm and entoderm meet at the bottom of the grooves, where these are to be seen on the surface of the side-wall. The meeting

°; these tw ° la y ers here constitutes what is sometimes termed the closing membrane.

established^ & Tn a ™P hibia thi f J membrane gives way, and thus the gill-slits are to be Derforatld S t f u , r 0 P slda th e closing membrane mav be frequently seen ever a nart nf th, b m mammals it is doubtful whether such perforation is in the^human embryo ° f develo P meab It certainly does not occur

of th^onnTui 6 ^ 3 ° f t d® a .r hes extend ra P idl y towards their fellows

P dial root. In this way a new mesodermal floor is made for the primitive pharynx, separating it from the pericardium. This frees the pharyngeal walls, which can now grow forward with the rapidly elongating cranial end of the embryo, leaving the pericardium in its original position.

The change in relation between pharynx and pericardium, in which the latter is found farther and farther back as the development proceeds, is usually described as a retrogression of the pericardium, but, as this retains its relation to the fixed parts of the ventral portion of the embryo, the explanation of the change given above is a truer description.

The rapid growth forward, which succeeds this freeing of their ventral ends, leads to obliquity of the arches, which are now found

to be directed downwards and forwards, and inwards at their lower ends.

The short fore-gut of the early embryo, after reversal, has reached a considerable length by the time that all the arches are visible (embryos of 4 to 5 mm.), as the result of rapid anterior elongation and pericardial growth. Ihe shape of the primitive pharynx corresponds with that of the hind-brain, which covers it. It is broadest a little behind its front end, where it lies under the widest part of the hind-brain, narrows slightly just behind this, and continues to lose width as it is traced back, finally reaching the small size of the primitive oesophagus, into which it is continued. Thus its arches become shorter and smaller in the posterior part of the primitive pharynx.

Changes in (Ectodermal) Surface Form.

In big. 47 the head of a young embryo is seen from the left. The external pharyngeal arches are shown plainly marked in Roman figures.. Ihe second arch is the most prominent, tending to overhang the third; this tendency becomes marked a little later. The third and fourth arches are thus somewhat sunk in a triangular area bounded in front by the second arch, above and behind by a thick, rounded ridge (made by downgrowths from occipital myotomes)' from the lower end of which a lower limiting ridge (epipericardial ridge) turns forward above the pericardium. This sunken area is the precervical sinus, which becomes deeper as the second arch enlarges. In some animals this arch seems to cover in the third and fourth arches more completely by fusing from above downwards with the posterior border, so that an ectodermally lined recess is formed, opening to the surface ventrally and having the two next arches in its floor. In the human embryo, however, this does not take place. The second arch soon begins to atrophy, except at its upper part, where it is forming part of the pinna ; its lower portion, diminishing rapidly, becomes an appendage of the dominant first or mandibular arch.. Thus the third arch, remaining on the surface, becomes flatter; the line of the second groove becomes the f flexure line ’ of the neck,' so that the area of the third outer arch corresponds practically with the ‘ carotid triangle ' of human dissection.

Muscle-cells extend from the second arch superficially over the first arch in front and the depressed area behind; these form the platysma and subcutaneous facial musculature, supplied by the facial nerve—the nerve of the second arch.

The upper end of the first groove remains, however, as the external meatus. The pinna is formed round this by the coalescence of certain auricular tubercles, which develop behind, above, and in front of it. It results from this that the outer ear is situated at first at a much lower level, compared with the face, than that which it occupies later.

Fig. 47 Head of Embryo nearly 5 Mm. in Length, showing External Pharyngeal Arches I., II., III., and IV.

Dotted areas mark placodes for yth, gth, and 10th nerves. Ci, first cervical somrte; OM, occipital myotonies; OT, otocyst; G, trigeminal ganglion; OP eye; Mx maxillary process; P, pericardium; EpR, epipericardial ridge continuous with caudal boundary of the arch field. 6

Deeper Changes.

The mesodermal masses, which make the visceral arches, not onlv extend ventrally toward their fellows of the opposite side, but reach the level of the roof of the primitive pharynx, and turn inwards over this roof for a very short distance; the greater part of the roof, between these roof-processes of the visceral mesoderm, is covered by par these TrW 16 ™ r ° Und th ? , n ? tochord ( Fi g- 48, A). Associated with Indin 11 roof -P rocesses > and ying on their inner edges, are two longitudinally running arteries, the right and left dorsal aortse, into which

arc^^Thesl 01168 T^u v P * hf ? Ugh the mesoderm of the visceral tv +L+ Th T vessels , Wl11 , be dealt with later; it is enough here to

from a ventral TtT f ch h - as a , corres Ponding aortic arch arising

the aortlc m ,~ “T d ° rSally to the dorsal vessel ; that

more or ess whh thl a d T l0pe a successivel - v fr °m before backwards,

”nd that ttfe first TtT t “ ar ® &11 f ° Und at the same sta g e i

a n^ tne first tw 0 b rea k up very soon after their annpamnrp 8

arches EaJh arch a h ilaSi r US) ** develo P ed in tb e first three or four Lach arch has also a nerve which is distributed to it, and muscles are developed from the mesodermal cells. The nerves (Fig. 48, B) pass down from the hind-brain, thus lying lateral to the dorsal aortae, and enter their arches in front of the corresponding arterial arch.

This relation between nerve and artery within the arch will be better understood if it is pointed out that the nerves are really distributed to the clefts (Ichthyopsida) in front of the arches; thus, for instance, the nerve of the fourth arch would be more truly described in some ways as the nerve of the third cleft. But its larger division (known as post-trematic ) lies in the fourth arch near the posterior edge of the cleft, and its smaller branch ( pretrematic ) loops over the top of the cleft to run down the hinder part of the third arch. In the human embryo the absence of clefts co-exists with the disappearance of distinct pretrematic branches, the only one that seems to come under this heading being the chorda tympani from the facial, but even this is more than doubtful.

A, diagram showing on one side the mesodermal mass of an arch, projecting within and without; on the other side the section is supposed to pass along a groove, between two arches, showing the dorsal and ventral angles of the ‘ lateral pouch,' where ectoderm and entoderm are in contact. The line A indicates the course of an aortic arch from the ventral aorta (VA) to the dorsal aorta, and the line N, showing the course of the nerve of the arch, crosses in front of it to reach its internal distribution. B, a plan to illustrate the fundamental relations of vessels and nerves in the arches. Remnants of the first two aortic arches are shown, and the forward prolongation of the dorsal aorta on the side of the fore-brain.

The nerves of the different arches are:

I. Mandibular division of fifth. IV. Superior laryngeal of tenth.

II. Facial. V. (? possibly joined with next.)

III. Glosso-pharyngeal. VI. Recurrent laryngeal.

Entodermal Aspect.

The arches begin to form at the stage of about 2*5 mm., and after this they present definite prominences, with intervening grooves, in the floor of the primitive pharynx. The buccopharyngeal membrane breaks up and disappears about the time that the arches begin to appear.

As the arches form, and project internally, they extinguish the original appearance of a side-wall to the cavity; this has actually only a floor and roof when the arches are fully developed.

If a horizontal section were made along the pharynx, separating the roof from the floor, and the latter were exposed from above in this way, the aspect of the floor at the 6 mm. stage, when the arches are well formed, would be as in Fig. 49. The oblique direction is seen in the hinder arches, and becomes more pronounced as growth goes on; they are, in fact, 'telescoped’ within one another, so that each of these arches is not only behind, but also internal to, the one in front of it. A small tuherculum impar lies in the middle line between the. first and second arches. An elongated median hypohranchial eminence lies behind this, and receives the ventral ends of the third and fourth arches; the second has a later temporary connection with it.

The pulmonary outgrowth , which arises at an earlier period, when the arches are first beginning to appear, takes place from the floor of the posterior end of the primitive pharynx; at the stage shown m the figure, the opening of the pulmonary outgrowth is seen in the shape of a median sagittal slit, immediately behind the hypobranchial eminence and between the two last (sixth) arches, which are approximated by their obliquity.

The grooves are clearly seen between the arches. Towards their outer ends there is contact between entoderm and ectoderm, these being the parts of the grooves corresponding with those seen externally; but farther in, where the growing arches have replaced the pericardial roof m the floor, the grooves (see Fig. 48) have mesodermal floors. Hence it comes that the outer parts of the grooves are much eepei than the rest, and constitute the lateral pouches of the pharynx. A lateral pouch is shown on one side of the section in Fig. 48, and it can be seen here that each pouch, made in this way, presents dorsal

certain growths take place from each of these angles, as will be described later.

m J 1 he 'tV th l . ateml P ouch ' is re My a complex, because the rudiIZl!V fifth a PP ear / m its fl°°r for a short time, and it presents outgrowthF 6 6 SetS ^° rSa * and ver *tral angles giving origin to

spoild withtwmh? are he ° my - of the s y stem of grooves which correnot to be , se/ n' bearlng . regl0n L n fishes ' But the term ' branchial' ought arch offi® h erL“ 3 “ cfaoa Tn h ^ visceral arches. The first branchial would be avoided if tb d » 7* the th l rd vlsceral arch, and much confusion mammalia, with which £ has^thing todF alto S ether when dealin g with

It * f k !t plaC ? V6ry 6arly in the middle line outgrowth from the nl 10n 'j P u ^ monar y bud, the only median

impa° It comes in?£ T X ’ a V d occur ? i ust behind the tuherculum t • mes into close relation with the dividing aortic stem

coming out of the pericardium, and is ' carried back ’ with this as the pharynx is growing forward. It soon loses its connection with the entoderm. It forms the lateral lobes and isthmus of the thyroid gland.

Fig- 5° shows the floor of the pharynx at later stages, schematically

Figs. 49A and 49B.—Reconstruction Models, showing the Pharyngeal Floors in Embryos of 5 Mm. and 12 Mm.: the Arches are Numbered.

P, pericardium; T, tuberculum impar; S, stomach. In the first embryo (A), which is somewhat twisted, the two lateral recesses have been opened where they are becoming continuous with the abdominal cavity, and the roots of the lung-buds, which have been cut away, are seen in them. In the 12 mm. (B) embryo the tubo-tympanic recess is apparent.

combined, and indicates the modifications taking place in it. The tongue is seen to be formed, as to its front part, by a swelling on the mandibular arch, which is formed from the tuberculum impar. Later, a forward growth from the front part of the hypobranchial eminence comes against the back of the earlier formation; it is a paired growth

Fig. 50. — A Compound Scheme to show the Development of Various Parts from the Simpler Pharyngeal Floor.

On the left the tympanum has been separated from the pharynx by the third arch, which has grown forward in the direction of the arrow and obliterated the inner part of the original tubo-tympanic recess, only leaving the front part as the tube. The meatal plate (X) has extended below the tympanum, with the handle (m) of the malleus interposed. The tympano-hyal lies behind this. The palate-fold has formed along the inner side of the (maxillary process of the) first arch, below the tubal orifice, and has a backward extension derived from the third arch. This arch has also sent a process (a) forward to the back of the tongue. On the right side the cartilaginous bars are shown. M is Meckel’s cartilage, and m its manubrial process. R, Reichert’s bar. The remnant of the third bar is seen behind this. The relations of the formations from the lateral pouches are shown. The thymus passes back between the thyroid lobe, and the epithelial growth (E) from the fourth pouch.

with an included angle, and is really derived from the third arch, growing over and burying the second arch which originally lay in 1 on ° it. This compound development of the tongue accounts for me difference m nerve supply of its front and back portions, i he foramen ccecum becomes evident as the hinder part of the tongue orme , and is the depression corresponding with the angle between the two halves of this hinder part; owing to its development in this way, its position is necessarily over the point from which the thyroid outgrowth had originally grown, and it marks this point in this way, although it is not directly connected with it at all.

The epiglottis is formed from the posterior part of the hypobranchial eminence, just behind the part that is built into the tongue. The epiglottic portion is connected with the third and fourth arches; the third arch becomes the pharyngo-epiglottic fold and the fourth forms the aryepiglottic fold.

The larynx is formed, behind the epiglottis, by swelling of the sixth arches, which stand up behind the hypobranchial eminence, and thus produce a transverse slit-like cavity, at the bottom of which is the original sagittal slit of the pulmonary opening. The sixth arches have the fourth arches lying outside them, and, when they swell up, a somewhat similar upgrowth of the fourth arches keeps pace with them; as the fourth arches pass on to the hypobranchial eminence, it follows that the ‘ transverse slit ' is bounded laterally by these connections—-that is, by the aryepiglottic fold. The larynx, therefore, has two parts developmentally: an upper part is developed by modification of the pharyngeal floor, and is enclosed by the epiglottis in front, the sixth arches ( arytenoids ) behind, and the aryepiglottic folds laterally; the lower part is a modification of the opening of the pulmonary outgrowth, and is bounded by the deeper parts of the sixth arches, in which the cricoid develops. The rima glottidis and incisura interarytenoidea mark the line of the original sagittal slit.

The cavities of the tympanum and Eustachian tube are developed from the lateral part of the pharyngeal cavity. The widest part of the cavity (Fig. 49) is opposite the second arches. As development progresses, the upper end of the first arch thickens in front of this on each side, and the third arch behind it, so that a tubo-tympanic recess is formed; this has its front and back walls made by the first and third arches , and in its floor are small parts of these arches , the first and second grooves, and the whole breadth of the second arch. The second groove here is the lateral pouch; the dorsal and outer angle is at the outer end, the lower and inner angle is not within the limits of the recess. There is no apparent lower angle in the first groove. Contact with ectoderm is lost along the floor of the second lateral pouch; this is a result of the growth of the arch already seen on the surface. It is on the roof of the tubo-tympanic recess that the otic capsule lies, and quickly becomes surrounded by cartilage (Fig. 51).

The outer part of the tubo-tympanic recess is cut off from the general cavity of the pharynx (see Fig. 5 °) by a forward growth of the third arch affecting its more medial part ; this obliterates the intermediate part of the second lateral pouch, spreads over the inner portion of the second arch in front of this, and comes up against the first arch. It presses against and fuses with the lower part of the inner portion of this arch, exposed in the front wall of the recess.

In this way the first groove is obliterated in this region, but the process stops there, and a narrow passage, the tube, is left between the (upper part of the) first arch and the growth from the third arch; the tubal passage leads outwards and backwards from the pharyngeal cavity to the tympanum. It is more or less parallel with the first groove, but is not formed from it, being at a higher level.

The tympanic cavity is the outer part of the recess, the part which has not been obliterated by the growth thrown forward from the third arch. Its floor, which slopes upwards and outwards (Fig. 51), is thus seen to be made by the outer end of the second arch, with a small part of the first ; the extremity (dorsal angle) of the second

Schemes to show on the right, how the otocyst begins as an invagination (A) of ectoderm beside the hind-brain, and, on the left, how this invagination becoming a closed ectodermal sac and separating from the surface becomes

P -fwr° n ! he T ? 0i ° f the Wlde . st P art of the pharynx (tu bo-tympanic recess) with the dorsal aorta below it. The displacement is probably due partly

out the n hind g b 0 re n e ^ t0 “ si T widt^and

mt ol the hind-brain. The cartilaginous capsule of the otocvst is con?

uous with the cartilage of the basis cranii of this region.

?h°e 0 s V e e cond b ar h ch d ’ ^ ^ ° Uter 6nd ° f the first 8 roove in Rout, of

tb e T fi he + UPPe [ Part ° f the first skeletal bar ( Meckel’s cartilage) lies in he first arch area mesoderm, but a secondary downgrowth from it (with surrounding mesoderm) passes over the upper end of the

SS ST The 0 the t area 0f y® f sec ° nd -Ch, and Occupies a° large part ol it. The posterior part of the second arch area however

Skdetal bar (Reichert^ cartilage)

I he upper end of Meckel’s cartilage forms the malleus the sernnrl

up a h rcbbehind bein s its manubril I) and the' IP pa t of Reicherts bar becomes the tympano-hyal ; this bar,

at a higher point still, and at an earlier prechondral stage, turns inwards above the tympanic roof and, coming into relation with the otic capsule, is continuous with the rudiment of the stapes, which separates from it subsequently.

The roof of the tympanum is made by third arch mesoderm passing forward below the otic capsule; it becomes the inner wall (Fig. 50). On the lateral side the growth of the mesodermal masses has led to great thickening, but contact between the ectoderm and the first dorsal angle has practically persisted, so that there is at this point a depression leading inwards from the surface; this is the upper end of the first external groove. A secondary solid ingrowth of ectodermal cells (meatal plate) takes place from the lower part of this depression, and passes (Fig. 50) below the floor of the tympanum, but separated from it by the handle of the malleus and its mesodermal bed. Thus, when this solid ingrowth hollows out centrally to form the meatal canal, the tympanic membrane is left in position, the manubrium, etc., being between its entodermal and ectodermal layers.

To sum up, the tympanum is the persisting outer part of the tubotympanic recess, separated from the pharynx by the forward growth from the third arch; it has in its floor (outer wall) remains of first and second arch structures, and of the outer ends of their grooves, and its roof (inner wall) is made by structures derived from the third arch. The Eustachian tube is not derived from the first groove (which is obliterated here), although it lies more or less parallel with the line of this, but at a higher level. It is the remaining front part of the tubo-tympanic recess, that part which has escaped obliteration owing to the fact that the third arch has only been applied to the lower part of this portion of the first arch, and has fused only with this part; the lateral wall of the tube is therefore derived from the first arch, and its medial wall from the third.

The forward growth of the third arch, which separates the tympanum from the pharynx, is a lateral manifestation of the active growth of this arch, which forms the back of the tongue. Between these two regions a similar process, as will be seen later, makes the pharyngeal pillars of the palate and the tonsil. The second arch drops out altogether from the floor of the pharynx, except where, at its outer extremity, it forms the tympano-hyal.

A distinct tympanic cavity first makes its appearance among the amphibians.

The Skeletal Structures of the Arches.

1. Meckel's cartilage, well developed in its whole length, meets its fellow in the middle line. The lower jaw is formed in membrane near it, and encloses it secondarily. The lower end of the cartilage appears actually to undergo ossification in the process. The upper end forms the malleus (see Fig. 50), and the intervening portion disappears.

2. Reichert’s bar. The dorsal end, in the precartilaginous stage, forms the stapes, which separates from it; below this, the bar makes the tympano-hyal and stylo-hyal (styloid process). Its ventral end forms the small cornu (cerato-hyal) of the hyoid. The stylo-hyoid ligament (epi-hyal) is a part of the bar which never actually chondrifies.

3. Only the ventral portion of the bar of the third arch remains. It makes the great cornu and part of the body of the hyoid.

4. Still less of this bar remains. It is represented by the thyroid cartilage.

5 and 6. No recognizable bars are formed, but they may possibly be represented in a highly modified form by the other laryngeal cartilages.

Pharyngeal Grooves and Lateral Pouches.

A typical lateral pouch is supposed to give origin to two entodermal outgrowths, one from each of its angles (dorsal and ventral). In some cases these remain as small undeveloped epithelial bodies, in others they develop into glandular masses. When the angles are modified no growth takes place from them. The formations in the human embryo may be shortly summed up as follows:

First Pouch. Dorsal angle retains contact with ectoderm, and has no outgrowth. There is no ventral angle.

Second Pouch.—Dorsal angle, in the tympanum, has no outgrowth. Ventral angle, in side-wall of pharynx, has no proper outgrowth but tonsil is formed (by third arch tissue) in it.

Third Pouch. Dorsal angle gives off lower parathyroid. Ventral angle gives off a bud to form the thymus.

Fourth Pouch. This is a complex of true fourth and rudimentary th pouches. From the true fourth, the upper parathyroid arises rom the dorsal angle, while the ventral angle affords origin to an epithelial body with which the lateral lobe of the thyroid comes into contact, but which does not appear to undergo further development.

I he rudimentary fifth pouch gives origin (? dorsally) to a small epithelial body [ultimo-branchial body) which soon disappears.

The thymus, like the median thyroid growth, already mentioned, comes in o re ation with the ventral arterial channels, and pericardium, below the pharyngeal arch system, and assumes with these a more

ntbp a P 0Sltl0 ^ m re jation to the overlying pharynx. See relations to other outgrowths in Fig. 50.

lohTnf Z d T t 1 hyr0i , <1 F° wth ’ dividin S into two, forms the lateral 01 th -® g lu nd ’ and b, 656 ’ extendln g laterally, come into secondary on wi 1 the ventral buds of the fourth pouches (see Fig. 50).

Position of Former Pouches in Adult Pharynx.

nonchl f hpT ati0n K n° m A® P ouches having separated, the lateral

cSin theS.phaSS' ^ > W

iS d P o p S p £L only,: To " siUir

fourth pouch: Lower end of pharynx, beside cricoid cartilage.

The lateral fossa (Rosenmuller) of the pharynx is not a pouch-remnant, but a later enlargement.

All the changes so far described as taking place in the primitive pharynx are effected by the third month. At the end of the first month the early condition of the simple arches has been modified by the beginning of the tongue-swelling, the early growth of the sixth arches in the larynx, and a definite but shallow indication of the tubo-tympanic recess. During the second month changes progress rapidly as described, and at the end of this month the hinder part of the tongue is formed. Thus, early in the third month, the human condition of things is present, but not, however, in proper proportionate size and disposition; this is brought about during later development by different growth-rates, etc.

Formation of Lungs, Trachea, and Pleural Sacs.

The pulmonary outgrowth takes place at an early stage, in the shape of an elongated unpaired groove or pouch, from the middle line of the extreme end of the primitive pharynx; owing to the obliquity of the last pair of arches, the opening of the outgrowth is placed between them, thus leading to the impression that it is situated farther within the pharynx than is really the case.

The pulmonary bud lies dorsal to the pericardium and, to some extent, the septum transversum, and thus comes to lie between and above the front parts of the lateral ccelomic recesses. It elongates and divides quickly at its caudal end into two lung-buds. Each of these thus comes into direct relation with one of the pericardio-peritoneal recesses, and, growing outwards, backwards, and somewhat dorsally, invaginates the inner wall of the recess into the cavity of the canal, which it thus comes to occupy—covered, of course, by the reflection over it of the inner wall. Increasing in size within the lateral recess, each lung projects caudally into the peritoneal cavity, and in front is in relation with the dorsal surface of the venous end of the heart through the pericardial opening of the canal.

In the meantime the original unpaired common root of the lungbuds, connecting them with the pharyngeal floor, has lengthened with the general growth of the parts. The front end, situated between the two sixth arches, has been seen already to be converted into the infraglotiic part of the larynx ; the remainder becomes the trachea, in which cartilaginous rings are developed from mesodermal condensation during the latter part of the second month.

The outgrowth, taking place between the sixth arches, apparently derives its mesodermal basis from these arches. This view is supported by the nervesupply to the trachea, and by the derivation of the right and left pulmonary arteries as side branches from the sixth aortic arches.

The trachea is sometimes described as formed from the original longitudinal groove, the margins closing over to separate it from the oesophagus. This view appears to lack confirmation by absolute observation in the human embryo.

Pleural Cavities.

The lungs continue to grow in the lateral cavities , which enlarge with them. Their caudal ends are withdrawn from the abdominal cavity with the increasing length of the parts, and the opening into this

cavity is closed during the sixth week. The pericardial opening is closed earlier. The pleural sacs enlarge with the lungs, splitting the septum transversum to a very small extent ventrally and internally, but growing mainly at the expense of the body-wall. They extend into this and split it into deep and superficial layers , gradually extending ventrally and cranially round the pericardium, and to a smaller extent caudally beside the abdominal cavity. The superficial layer of the body-wall thus split off affords i . , i . . an area for the downgrowths,

which make nbs and intercostal muscles, the deep layer becoming the ven ro-lateral wall of the pericardium and, in the abdominal region, the muscular part of the diaphragm. The unsplit portion of the septum

T, n t VeiSl T c e r W” ds wl * h that P art of the diaphragmatic tendon which is attached to the pericardium.

full use in breathing v.-iniZ ,i U, T S ^ , ^ rrl collapsing, and for allowing their

secondary importance T alue U T llscle attachment has become of

is an a 1" 11 dia P hra § m - then,

in formation, attainment by the embryo of a high or mammalian level

Fig. 52.—Diagrams to show the Enlarging Pleural Sacs splitting the Body-Wall.

The inner layer of the split wall forms part of the pericardium. The arrow passes through the recess, which is dilating to make the pleural sac.

Ihe Post-Pharyngeal Portion of the Fore-Gut.

behSd S it )ar it° f is tl in rTlttf Ut ^ ab ,? ve tbe se ptum transversum, and recesses above the sew tl0n atera ! 1 T Wlth tile two pleural (lateral) (see Fig. 49) b Um ’ anb Wltb abdominal coelom behind it

The^enlamernent of^th ? a})ove A tbe se P tum becomes the oesophagus.

of the region, and development oFa^hora’ ^ and th * ^ ^

concomitant elongation of the oesophagus d ^ “ a necessar r

Part behind the Septum Transversum.

Liver. —The oesophageal part leads directly into that part lying behind the septum. This part is at first (see Fig. 46) broadly applied to the septum, but when the intra-embryonic coelom breaks through the margin of the * plate/ its attachment to the septum becomes relatively and actually narrower. At this time an ingrowth into the abdominal aspect of the septum takes place from the lower end of this part of the fore-gut. This is the liver bud. It spreads rapidly in the mesoderm of the septum, breaking up the veins ( vitelline , see Fig. 37) which are running through this region to reach the sinus venosus farther forward; thus the foundation of the liver is laid in the septum transversum, consisting of columns of hepatic cells surrounded by vascular channels connected with the alimentary system.

The attachment of the fore-gut to the septum transversum has now narrowed considerably, and it soon comes away slightly from the septum, drawing this attachment after it in the shape of a ventral mesentery.

This is a convenient term in description; but it must not be forgotten that the ventral mesentery is nothing more than an attenuated part of the mesoderm of the abdominal surface of the septum transversum.

In the meantime the dorsal mesentery has been formed, and extends forward (see Figs. 44 and 45) on to the fore-gut, as already explained. It extends for a little distance above the septum, and forms here a partial meso-oesophagus, but the continuity of this with the common dorsal mesentery is broken later by the growth of the diaphragm.

Stomach, Bursa Omentalis, and Mesoduodenum.

The posterior part of the fore-gut is now held by dorsal and ventral mesenteries. At its lower end, just before it passes into the midgut, the elongated stalk (bile-duct) of the liver outgrowth extends from it to the septum between the layers of the ventral mesentery. The upper part of the gut-tube here shows a dilatation, the early stomach, but this is still in the middle line. When the embryo is about 3 mm. long an outpouching of the dorsal mesentery begins, pushing this out towards the left in a bag-like manner; this occurs just dorsal to the stomach. Thus there is quickly produced a thin-walled, hollow, and rounded projection (Fig. 53) into the left half of the abdominal cavity. The hollow bursa omentalis, as it is termed, has its relatively small opening or ‘ mouth ' on the right side of the dorsal mesentery, or mesogaster, as this part can now be called.

The bursa omentalis carries the stomach on its anterior or ventral wall, so that this viscus now has its right side looking into the cavity of the bursa, and its left side looking ventrally and toward the left. In the meantime the liver has grown so rapidly that it projects into the abdominal cavity from the abdominal aspect of the septum. Thus the ventral mesentery is now attached to the projecting liver, and becomes the gastro-hepatic omentum.

The dorsal mesentery, immediately below the opening and projection of the bursa omentalis, supports that part of the tube from which the hepatic outgrowth took place. It can therefore be termed the mesoduodenum, although the portion of the tube which it supports, and which will become the curved duodenum, is very short at first.

Fig - 53. Section through Abdomen of 15 Mm. Embryo.

The V lrs ? < ’, m ! Ilti !! is is seen free in the left peritoneal cavitv. It is only

attached to the median dorsal mesentery. The section just misses the

opening from the right into the bursa. J

The mesoduodenum is thicker than the neighbouring parts of the dorsal mesentery and its upper end forms the lower edge of the opening into the

Pancreas and Duodenum.

f after the hepatic outgrowth has taken place two secondary

formations appear, growing from it close to its origin from the duo ’ these are the n gil and dft pancreatic buds. The left growth disappears ve ry soon. The ventral attachment to the gut of the

+ P l n T eat l C out g rowths now becomes shffted to the dorsal aspect of the tube by the operation of certain agencies which are not fully understood. Thus the common opening of these two growths is on the dorsal wall of the duodenum, and the pancreatic bud grows into the mesoduodenum ; developing here, it forms the head of the pancreas.

The neck and body of the gland are formed by a second dorsal outgrowth, springing a little higher up from the dorsal wall of the tube. Thus this second bud enters the upper end of the mesoduodenum, and passes from this directly into the lower part of the dorsal wall of the bursa omentalis, where it extends towards the left.

The lumina of the buds, hepatic and pancreatic, become the lumina of their ducts; the dorsal pancreatic bud thus forms the duct of Santorini, and the right (originally ventral) bud forms the duct of Wirsung.

The duodenum is elongated and gradually moulded into its curve by the growth of the head of the pancreas within the mesoduodenum, the process going on during the second and third, and even the fourth, months.

The spleen begins to develop during the first half of the second month. Mesodermal thickenings occur simultaneously at several neighbouring points in the lateral and dorsal part of the bursa omentalis ; they are produced by proliferation of the outer lining cells (mesothelium) of the bursa. The thickenings join, but the presence of variable notches in the adult spleen indicates its original development in distinct parts.

The bursa omentalis grows to a considerable size, lying ‘ free ’ on the left side of the median mesentery. The liver, enlarging rapidly, fills all the available space in the abdomen, lying on the right beside the median mesentery, but on the left separated from this to a great extent by the bursa omentalis. When the intestines enter the abdomen, in the tenth week, they pass below the liver and push the median mesocolon to the left ; they also, in doing this, raise up the lower and dorsal part of the bursa, invaginating it into the sac. Thus the lower and ventral part of the bursa lies on the ventral aspect of the intestinal coils, forming the great omentum. The bursa, however, is as yet quite free, except where it is continuous with the median mesentery, and it is only at a period considerably later that it becomes adherent to the mesocolon and to the dorsal wall, and thus forms nearly the whole of the small sac of the peritoneum.

Development of Regions of Face, Mouth, and Base of Skull.

The transversely disposed depression which exists at first between the pericardium and projecting fore-brain, and is termed the stomodceum, is quickly modified by the appearance of partial side-walls to it. These side-walls are the maxillary processes, which are continuous behind with the upper parts of the mandibular arches. They come into evidence with the mandibular arches, from which they are often described as arising. They extend forward below the optic outgrowth (Fig. 47), like a bracket on each side under the overhanging fore-brain.

Each maxillary process is not applied to the ectodermal surface of the projecting structure, but its mesoderm extends forward in immediate contact with the proper (paraxial) mesodermal covering of the eye and brain, deep to the common ectodermal layer.

The maxillary processes form two mesodermal masses which rapidly

Iug. 54.

The two upper figures show the lower aspects of the projecting heads of embryos of 4 mm and 7 mm. (From reconstruction models.) The change in position of the olfactory region is due to the presence of the telencephalon m the 7 mm. specimen This not only advances the front of the head

hrJ a °n-u t r G Slte n° f the olfactor y fields ' but also, as a result of increase in breadth, turns them more on to the lower aspect. A and B in the lower

figures are sections through the olfactory fields of the two embryos, showing formation of olfactory pits and fronto-nasal process. C and D are diagram?

Se^ndkater?h t0 - 1 f UStra f% t r 6 further chan S es - The maxillary processes tion n by n 1 M 1 ? rr T U S^ d llnes to show where the y win come into posiprocess D ‘ ° NF ’ INF ' ° Uter and mner naSal folds; FNP > fr °nto-nasal

mciease m size and thickness. A view of the lower aspect of the piojectmg part of the head, as in Fig. 54, shows them as lateral prominences lying below the eyes, and having between them a hollow in which me rounded posterior part of the fore-brain is readily recognizable

thin oV?t?l ble becau , se the P a taxial mesoderm of the fore-brain is very hm on lts iower surface m the middle line, and is completely absent here in the hinder part, so that the wall of the back of the fore-brain is in contact with, and adherent to, the ectoderm here, thus the shape of this part of the brain is apparent on the lower surface, between the maxillary processes, which hide the optic outgrowths from view, though this part of the brain is continued into them on each side. The depression between the maxillary processes is the early form of Rathke’s pouch, which is thus a recess in contact from the beginning with the fore-brain.

The paraxial mesoderm on the lower aspect of the fore-brain begins to increase in depth, and the maxillary mesoderm extends over its surface, as will become evident later. The cartilaginous base of the skull is formed in the paraxial mesoderm. The membrane bones of the face, and of part of the skull-base, are developed in the maxillary mesoderm.

Formation of Nasal Pits.

An olfactory field of thickened ectoderm lies on each side on the ventro-lateral aspect of the projecting head (Fig. 54) just in front of the region of the eye; this ectodermal area is probably in connection with the lower part of the cerebral vesicle from an early stage. As the mesoderm thickens, each field, maintaining its original level to all practical intent, assumes a sunken position, with medial and lateral nasal folds projecting on either side of it (Fig. 54). The two folds are continuous in front of the nasal pit formed in this way. The two inner nasal folds, with the depressed median area between them, constitute, when taken all together, the fronto-nasal process.

Formation of Primitive Nasal Fossae.

The maxillary process, growing forward below the eye, comes into relation with the hinder ends of the two nasal folds of its own side, and applies itself to them. That part of the process applied to the lateral fold increases rapidly in thickness and depth (Fig. 54), and thus forms a definite lateral boundary to the mouth-cavity here. Its front portion then turns somewhat inwards, as shown in Fig. 55, lying below and behind the lateral nasal fold, and, extending across the posterior part of the opening of the olfactory pit, comes against the medial fold, with which it fuses.

In doing this it converts the nasal pit into a primitive nasal fossa by making a new floor to its hinder part. The unclosed anterior portion of the mouth of the pit is now a primitive anterior naris. The primitive posterior naris is at this stage merely a potential opening, lying behind and above the new ‘ floor ’ of the fossa; it is little more than a point, where the hinder end of the cavity of the pit has been covered in by the approximation and fusion of the ectoderm-covered maxillary mesoderm applied to the bases of the inner and outer nasal folds respectively. The adherent ectoderm makes an epithelial plug which fills the very small potential opening, and is known as the bucconasal membrane.

Completion of Limitation of Roof of Cavity of Mouth. —The fusion of the maxillary processes with the fronto-nasal process is quickly followed by the extension of maxillary mesoderm into and over the fronto-nasal process, and this makes a definite anterior boundary to the region of the mouth; the main maxillary mass has been seen already to form its lateral boundary. Thus the (potential) posterior nares open into the mouth far forward, just above its anterior border.

This stage is reached in the fifth week.

Fig. 55 -— Embryos of io Mm., 12 Mm., and 13-5 Mm.

The mandible has been left in position in the first, but cut away in the other two specimens. To show the extension of the maxillary process, and the early mrm of the palate fold, compare with Fig. 54. Pit., pituitary (opening of Rathke s pouch); pal., palate fold. This is not so well shown in the o der embryo. The last shows the invasion of the fronto-nasal process bv the maxillary mesoderm.

Formation of Palate Folds.

As the lateral maxillary process increases in depth, its inner surface or edge begins to show a longitudinal ridge (Fig. 55), which extends as far as the process itself—that is, it reaches the fronto-nasal process in front, passing therefore below the posterior nares, and ends behind just internal to, and below , the front part of the opening of the u 0- ympanic recess. This ridge is the main, or maxillary, palate fold.

les or e most part against the side of the growing tongue, and ncreases m size as much as the restricted space will allow. At a later stage a pharyngeal extension of the fold is formed; this is derived from “f° d r erta , of third arch (Fig. 49 ), and is associated with the

frrh nnrl FT 1 ex .t ei } slon ( ,0ln this arch, which covers in the second c ch and excludes it from the pharyngeal floor.

Formation of Definitive Nasal Fossae and Mouth.

Nasal Fossae.—The mesodermal bed, in which the small fossae are placed beneath the cerebral vesicles, increases in depth fairly rapidly. The depth of the fossae increases also pari passu with that of the mesodermal bed. Their increase, like that of the mesoderm, is in an upward direction, for the level of the ‘ floor ’ is fixed by the mass of the maxillary process fused with the fronto-nasal region, and by the fact that these are supported by the pericardium and the mandibular arch. The fossae, in extending upwards, necessarily leave a wall of undisturbed mesoderm between them, which comes into increasing evidence with their growth. This is the nasal septum, and the structures

Fig. 56._View, from below and behind, of the Roof of the Mouth of an Embryo of 16 Mm. (Sixth Week).

Showing the palate folds reaching the fronto-nasal process, the evident shape of this process, though covered by a layer of maxillary mesoderm, and the growth of the labial extension of the mesoderm only just meeting its fellow in the middle line. This will become much thicker and vertically deeper, hiding the fronto-nasal form altogether. The interrupted lines indicate the extension upward of the upper level of the posterior nasal openings.

of the septum form in this wall as it is ‘ discovered ' by the extension of the foss96. The nasal fossae, however, extend backwards as well as upwards. This must be associated with the upward movement of the upper edges of the posterior nares, which are the extreme ends of the roof of the pits or primitive fossae. The primitive posterior nares therefore enlarge in an upward and backward direction with the growth of the fossae, thus increasing the height of their openings into the cavity of the mouth. This is represented by the interrupted lines in Fig. 56. The broad strip of mouth-roof which lies between the openings is evidently the free ‘ edge ’ of the septum between the fossae; this is continuous, like it, with the fronto-nasal process below, and growing in height with the increasing height of the openings. The upper limits of the openings finally reach the level of the highest part of the pharyngeal or mouth roof (Fig. 56); at the same time the nasal fossae have attained their final situation, and the capsule enclosing them is in position in the front part of the base of the skull.

Nasal Capsule. —A cartilaginous capsule is formed round the growing fossae. It extends into the septum, but is deficient in the floor. As the fossae extend upwards and backwards, the lower and front portion of the cartilaginous capsule is made first, the rest being added as the fossae grow. When they reach their final position, the cartilaginous capsule is completed by junction between its outer walls and the septal formation, and by the fusion of the upper part of the

whole structure with the cartilaginous ala orbitalis, which lies behind it (see Fig. 59).

Roof of Cavity of Mouth. —The roof of the mouth, at first somewhat flattened, becomes arched. The concavity from behind forward is due to the mesodermal growth—in which the nasal fossae are situated —above and in front of it ; the transverse concavity is due to the growth of the maxillary processes.

The posterior nares open on this roof, and the ‘ free edge ' of the septum is a part of the roof.

At the beginning of the third month the posterior nasal openings reach their full height, extending on the roof of the mouth as far up as the level of the highest part of its curve. The cavity of the mouth is Fig. 57.—Section through Embryo occupied by the tongue, which is of 28 Mm., showing the Palate in contact with the roof, the back of Folds beside the Tongue, the septum, and the margins of the

or THE Mouth, and in Contact P^gor nares. The palate folds lie with the Septum and Nasal beside the tongue, directed downMargins. wards (Fig. 57).

Formation of Palate and Definitive Posterior Nares.

The base of the tongue is attached to the mandible, which is comparatively short up to the ninth week, so that the tip of the tongue has no reached the fronto-nasal process. At this time, however, the elongalon o the jaw carries the front part of the tongue below the process, and 16 ront parts of the palate folds, being attached to the process, are necessarily brought above the tongue. The remaining parts of the

d T ere c k es ide the tongue, soon follow the change of position initiated by the front portions. * b F

The palate folds thus find themselves above the tongue, which is now in less close contact with the septum and mouth-roof, and can broaden out to some extent in the wider space below the folds (Fig. 57). The folds, therefore, have better opportunities for growth. They get thicker, and in their growth insinuate themselves between the tongue and the posterior border of the septum. They become attached to this aspect of the septum, and to one another. The connection with the septum only occurs along its lower three-fourths or so, and to this extent the corresponding portions of the posterior openings are closed by the folds passing across them. The openings are not closed in this way for their upper fourths, more or less, and these unclosed parts constitute the definitive posterior choanse. The posterior parts of the palate folds, which lie below and behind the upper fourth of the septum, but do not get attached to it, form the soft palate.

Rathke’s Pouch and Formation of Pituitary.

The angle between the bucco-pharyngeal membrane and the early overhanging fore-brain forms a transversely disposed cleft (Fig. 33). This is quickly changed by the formation of the maxillary processes on either side; the central part now remains as a depression, Rathke’s pouch, the front wall of which is made by ectoderm covering the rounded surface (Fig. 54) of the fore-brain. After the bucco-pharyngeal membrane disappears, the back wall of the pouch is formed by the roof of the pharynx, depressed by mesodermal thickening. Increasing thickness of the mesoderm round it causes the pouch to become deeper, and at the same time its buccal portion is constricted to form a neck or stalk. This opens (Fig. 55) on the roof of the mouth. The upper part is a dilated sac, adherent by its front wall to the fore-brain prominence, as before; the pituitary body is developed from this part of the pouch.

Slow ingrowth of mesoderm separates the pouch from the brain at the beginning of the second month. The antero-lateral lobes of the pituitary body are formed by ectodermal outgrowths from the front wall of the pouch into this mesoderm. The posterior lobe is formed from the blunt end of a median club-shaped growth from the fore-brain, projecting just above the pouch. The stalk of this growth forms the infundibulum. The antero-lateral parts grow up over this stalk, and meet above it, and the upper parts of this, in contact with the stalk, form the pars tuberalis.

The original neck or stem of Rathke’s pouch is broken and destroyed by the formation of the skull-base, thus leaving the body only attached by the infundibulum. The buccal part of the stalk remains, however, below the base, and is drawn out into a long cell-strand as the mouth forms. The ultimate position of the attachment of this strand to the lining layer is near the lower part of that portion of the nasal septum which is not attached to the palate folds.

Formation of Region of Face.

The diagrams in Fig. 58 show how the face is built up, mainly by the great growth of the brain and of the maxillary process. The maxillary growth leads to the nasal region, originally more lateral, being brought into the centre of the face, to the great depth of the

Fig. 58. — Three Figures to show the Formation of the Face.

Though diagrammatized, the figures practically illustrate the conditions in embryos of about 10, 12, and 16 mm. respectively. The maxillary mesoderm is invading the fronto-nasal process in the last figure, but this does not give an idea of the depth of the formation in later stages. The eyes

the third r month lateral m positlon; the y do not come right in Font before

upper jaw, and, by its invasion and covering of the fronto-nasal process, to the formation of an upper lip; the lip is being formed in diagram d Jab ii W ° maxillary growths have not yet met to make the vertical

however ic b w n Ti! 16 £ reatest thickness of the maxillary process, ehmd the eye, where its height and breadth exceed its measurements elsewhere. This leads to broadening of this part of the head, and one of its effects is seen in the eyes, for these, originally placed laterally , are swung forwards and inwards, and thus come to look forwards from the face. This movement is not completed before the third month.

Cartilaginous Base of Skull.

This is developed in paraxial mesoderm. It can be divided, according to its place of origin, into {a) a posterior part, behind the pituitary and below the hind-brain; and (b) a front part, underlying the projection of the fore-brain.

Fig. 59. — Two Schematic Figures to show the Parts of the Cartilaginous Base.

The nasal capsule has come into position in the base of the second figure. SPH, area where the membranous great wing will come up into the base; CAR, carotid artery through paraxial mesoderm.

(a) The most anterior somites (four) lie behind the hind-brain, and coalescing here, form a basal plate of cartilage; this is situated between the cartilaginous ear-capsules (Fig. 59, A). This part of the base may be termed ‘ parachordal/ although the notochord lies below it, in relation with the roof of the pharynx. The plate extends as far as the pituitary formation, where it becomes continuous with the more anterior chondrifications; it blends secondarily with the otic capsules on each side, and later it gives extensions backwards round the neural tube, to make the cartilaginous part of the occipital (Fig. 59, B).

Only four somites are apparent in the embryo, but it is possible that a number have been concerned in forming the mesodermal condensation that lies just behind the pituitary. The number of somites taken into the skull in other types is very variable, but more than four in number.

(b) Two small ‘ bars ’ of cartilage form beside the pituitary stalk in early stages, but these soon fuse with each other and with neighbouring areas of chondrification, to form a short but wide plate (Fig. 59, A) from which extend, on each side, a long orbito-sphenoid, pierced at its base by the optic nerve, and a short alisphenoidal process, which becomes attached to the otic capsule behind it.

The front part of the cartilaginous base is completed by the nasal capsule, when it comes into position, and by (Fig. 59, B) the sphenoethmoidal plate, which connects this with the orbito-sphenoidal process.

The area of paraxial mesoderm seen in the diagram between the orbito-sphenoid and the otic capsule does not chondrify later, but forms dura mater, including the tentorium cerebelli, and the hiatus in the base of the skull is filled by the movement upward from below of a ‘ visceral ' bone, the membranous alisphenoid, which is formed in the thickest part of the maxillary process.


I he details of the formation and division of the heart are given in full in the section dealing with the description of the organ; it is enough here to point out that it has, at a fairly early stage, the form of a single and relatively simple tube, formed by the rather irregular coalescence of two vascular channels lying in the approximating edges of mesoderm near the anterior margin of the embryonic plate, and its final form is gained by modifications of this single tube. The tube is supported at first by a dorsal mesocardium from the pericardial roof. It elongates and becomes twisted, and the dorsal mesocardium gives way in the middle, thus leaving a venous and an arterial mesocardium at the corresponding ends. The perforation of the mesocardium, between arterial and venous reflections, becomes the great transverse sinus. The sinus venosus, in the septum transversum, opens into the venous end of the caidiac tube, and the arterial end passes out of the pericardium as the common arterial stem ; this divides into two ventral divisions below the floor of the pharynx. The heart-tube itself shows three latations. primitive (single) auricle, ventricle, and bulbus cordis. le common ventricle is the largest division, and hangs down below

an +n e ^ een other ^ W0, which thus become relatively approximated. As the heart grows, its cavities become divided into right and left by e ormation of a complicated system of septa, which are described a 1 s enlargement also draws the sinus venosus into the pericardial confines rom the transverse septum, and this venous cavity becomes part ot the heart when completely formed.

Arterial system.

to twl^ rChe f~ The a( T ic arches ’ passing from the ventral side \t it fi a / e nul ? bered like the visceral arches in which they

rs wo > °rmed early, break up and disappear very soon.

The others, formed rapidly in order from before backwards, are modified to make the permanent vessels. Distinct ventral aortae do not occur in human embryos, but a good idea of the changes in the aortic arches may be obtained, nevertheless, by studying the general vertebrate plan of these arches, as given by Rathke. This is shown in Fig. 60, and gives the nature of the alterations. The third aortic arch becomes, with the dorsal aorta in front of it, the internal carotid, and the fourth arch forms the aortic arch on the left, and the first part of the subclavian on the right. The pulmonary arteries are formed as side-branches from the sixth vessel, which remains itself as the ductus arteriosus on

Fig. 60. — Scheme of the Aortic Arches, showing what becomes of the Different Vessels.

The fifth aortic arches disappear very soon after formation.

the left, disappearing on the other side. The dorsal aortae join opposite the end of the pharynx, forming a single aorta, which divides again into the umbilical arteries, and the vessels of the lower limb arise as branches from these. Intersegmental arteries come off the dorsal vessels, running to the body-wall, and visceral branches go to the visceral structures. The anterior ends of the dorsal aortae are prolonged to the fore-brain as internal carotids, piercing the paraxial mesoderm beside the pituitary rudiment.

The veins are for the most part as in the earlier stages (p. 51 and Fig. 37). The vitelline veins are broken up into a network in the septum transversum by the growing liver. The left umbilical vein forms a connection with the left vitelline vein on the visceral aspect of the liver, so the blood from the placenta is diverted into the liver vessels, the rest of the left umbilical vein, like the right one, disappearing. The posterior cardinal vein, joining with the primitive jugular, forms the duct of Cuvier. This passes in the body-wall round the outer side of the lateral coelomic recess to reach the sinus venosus in the septum transversum. When the pleural dilatation of the recess spreads into the body-wall, it goes outside the duct, which thus runs in front of the root of the lung to reach the heart. It becomes the intrapericardial part of the superior vena cava on the right, and degenerates on the left side.

The inferior vena cava is formed above the renal veins by a secondarily developed subcardinal vein, and in its upper or anterior portion by the junction of this with the veins leaving the liver to reach the sinus venosus. Below the renal veins it is formed from a ‘ supracardinal ’ or ' periganglionic ’ system: details are given later.

Genito-Urinary System.

The excretory organs are developed in association with the region of the intermediate cell mass (p. 42). The genital glands are formed in relation with part of the excretory system. The products of the two systems are carried to the cloacal region by ducts; there are, on each side of the body, two ducts.

As the result of interdevelopment it comes about that the duct of the permanent kidney is a new formation altogether, and the older ducts become connected only with the genital system.

The genito-urinary system as a whole, then, can be considered as composed of glands, ducts, and modifications in the cloacal region connected with the differentiation of the two systems.

The urinary or excretory glands and ducts are the first to form.

Excretory System.

The development of this system is characterized by the appearance ot three successive glandular formations, of which the first is merely vestigiai the second is functional during a part of intra-uterine life, and the third becomes the definitive kidney. The three formations are known respectively as the pronephros, mesonephros, and metanephros. But the structure of these is essentially the same; each is

+w> P °i? d ?\F° UpS °! f xcretor y/ units/ of a very simple type, found throughout the animal kingdom in general.

in rnnnprtfn ic u ? Uch a ne P hric ' unit/ which is developed

whtw 1 T th the bod y- c avity, can be understood from Fig. 61, of thp Pfrv ^ section through a (hypothetical) simple animal

nected with/h/ / P6 'n fhe n Ca U y in the somite ( m y°coile) is conthe splanchnocoele by a narrow channel in its ‘ stalk.’

This channel is the part from which the nephric ‘ unit ’ is made. The inner or splanchnic wall is pushed in as a covering for a vascular glomerulus. The lateral or somatic wall develops an outgrowth which becomes a convoluted nephric tubule. The tubule ends in a longitudinally running excretory duct , which carries the excretion caudally to the cloaca. The coelomic channel or cavity into which the glomerulus projects is termed the nephrocoele, and the nephrostome is the opening into the tubule from the nephrocoele. Fluid exuded from the glomerulus would ‘ flush ’ the nephrocoele into the tubule, and thus carry any waste products in the coelom into the excretory duct.

In the actual animal or embryo itself, the connection between myoccele and nephrocoele is always obliterated. That between nephrocoele and splanchnocoele may be (a) so widely open that the glomerulus and nephrostome may be really in the dorsal part of the splanchnocoele, the glomerulus being then said to be external ; ( b ) a persistent minute aperture; or ( c ), most usually, completely closed secondarily, the glomerulus being then enclosed and described as internal.

Fig. 61. — Transverse Diagrammatic Section to show Situation and Component Parts of a Nephric ‘ Unit ’ in a Vertebrate Embryo.

The myotome and its stalk are in segmental series, but the splanchnocoele shows no segmentation. Thus the nephric units are in segmental series , developing from the myotome stalk. In the human embryo the stalks have no cavities and are more or less fused together, making a solid intermediate cell mass (Fig. 31) in which segmentation is not very apparent; but the nephric units develop secondarily in this mass, and show its primitive segmental value by forming in segmental series—at any rate at first.

If the opening (peritoneal opening) into the splanchnocoele is narrowed or closed, it is apparent that a large development of the nephric structures from the intermediate cell mass would lead to a projection or bulging of the roof of the splanchnocoele into that cavity.

The primitive connection of the nephric system with the coelom suggests that that cavity may have come into being originally as a drainage cavity in the thickness of the mesoderm. This would imply that the splanchnocoele was originally segmented, but such segmentation would quickly disappear to allow of the further unhampered development of the visceral structures.

In the human embryo the three successive sets of excretory organs develop in connection with the solid intermediate cell mass, and are thus related to the dorsal wall of the body-cavity.

Pronephros. — Segmental tubules are formed very early from the intermediate cell mass below the cervical myotomes. The nephrostomes reach the roof of the lateral recess of the pericardium on each side. The glomeruli, badly formed, are in part external. The outer ends of the more posterior tubules join together to form an excretory duct, known usually as the Wolffian duct. Lumina are not present, or are badly formed, in the system of tubules. The whole structure is evidently merely vestigial; it begins to disappear at its anterior end before it has fully appeared farther back, and it has disappeared by the time the embryo reaches the length of 5 mm.

The Wolffian duct extends backwards, keeping pace with the differentiation of the mesoderm from before backwards, and ultimately reaches the cloaca at about the time when the pronephric tubules disappear.

Mesonephros. — This begins to develop on each side as the pronephros is disappearing. It forms in the intermediate cell mass caudal to the pronephric area, although its cranial end somewhat overlaps this. Its glomeruli are internal from the beginning. Its tubules are much coiled, and they open at their outer ends into the Wolffian duct, which is, of course, already in situ as the result of its rapid backward growth from the pronephric region, the tubules and glomeruli are at first segmental, but subsequently secondary tubules and tufts form between them, so that a large mass of dilated tubules and glomeruli is built up and projects into the abdominal cavity at each side of the mesentery. The mass is frequently termed the Wolffian body. It reaches its greatest development about the sixth week.

Metanephros.- The caudal end of the nephrogenic area, immediately caudal to the mesonephros, is dorsal to the cloaca. Mesonep lnc formation does not extend to this part, and it is only represented

7 ' f C ?xr i^ se< ^ mes °dermal mass. When the ‘ ureteric outgrowth ' ot the VVolftian duct (see later) comes into relation with it, it forms a metanephnc cap on the dilated end of this. It is carried forwards on tnis outgrowth, passing dorsal to the caudal part of the mesonephros. Glomeruli and tubules subsequently develop in the metanephric blastema, and join with collecting tubules which grow out from the ureteric bud. 1 he permanent kidney is formed in this way.

the proper excretory duct, lies beside the growing mesonephros as it runs back to the cloaca. The tubules of the mesonephros come to open into it secondarily. Its anterior or pronephric end degenerates with the degeneration of the pronephric tubules from which it is formed, so that it comes to correspond with the extent of the mesonephros at its anterior end. It opens caudally into the cloaca.

The ureteric bud arises from it close to the wall of the cloaca (see Fig. 66), and grows dorsally and cranially, coming into relation with the metanephric mesoderm, and carrying this with it on the dorsal side of the mesonephros. The hollow bud has a club-shaped end, which soon shows a tendency to a bilobed condition; these two ‘ lobes ’ ultimately make the major calyces or infundibula of the pelvis of the kidney. The bud, with its metanephric mesodermal cap, pushes gradually forward on the dorsal wall of the abdomen, and comes into relation with the large suprarenal gland, which is formed just below the diaphragm. Outgrowths take place from the infundibula into the mesodermal cap, forming minor calyces and collecting tubules, and these form secondary junctions with tubules which are formed, in the cap, with glomeruli.

Fig. 62. — Schematic View of Dorsal Wall of Body and its Cavity, seen FROM BELOW, TO SHOW THE RELATIVE SITUATIONS OF THE THREE SUCCESSIVE Formations. Interrupted lines indicate the shutting-off of the pleural cavity.

Sometimes there is failure of junction, with the result that the secreting tubules become distended, and a congenital cystic kidney is formed.

The general relations of all these structures to one another, and to the body-cavity and regions, are shown schematically in Fig. 62.

The pronephros, on each side, is associated with the dorsal wall of the lateral recess of the pericardium, and extends to the upper abdominal region; the mesonephros, overlapping this on its outer side, lies in the abdomen, and extends about as far as, or just beyond, the middle of the lumbar region; the metanephros, the temporarily undeveloped hinder portion of the nephric ridge (intermediate cell mass region), lies immediately caudal to the mesonephros, these structures, extends caudally to oft a secondary ureteric hud.

Fig. 63. Back Wall of Abdomen of Embryo of 15 Mm.

Sh0 'Me™n 1 pnhr^r i , dS K'i, Wi ; h tu “ ridge ven tro-laterally, and gonad internally S mrfi f ls behind and between these, but shows at one or two placesuprarenal swellings appear between the ridges and the mesentery.

The Wolffian duct, lying beside reach the cloaca, and is giving

that*?t cin C not th he lonLa T nephr ? overla P s the pronephros is an indication

nephric formation It is iU™ 1 ’in aS a P rolo .ngation backwards of the pro than the pronephros as is inrii^e^h a re P lai ? ln § structure, more specialized pronepnros, as is indicated by the enclosed glomeruli, etc.

On the other side of the figure is shown a subsequent state. The pronephros disappears, the lateral recess becoming the pleural cavity, as indicated by the interrupted lines; the mesonephros is now separated from the pleura by the diaphragm, and extends caudally from this; the upper end of the Wolffian duct has disappeared with the pronephros, and its ureteric outgrowth has reached the metanephric condensation and has dislocated it cranially on the dorsal aspect of the mesonephros.

The two mesonephric masses, or Wolffian bodies, thus form elongated prominences lying beside the attachment of the median mesentery to the dorsal wall of the abdomen. The actual condition in the sixth week, the other abdominal viscera being removed, is shown in Fig. 63, and Fig. 64 is a drawing of a section through the upper abdominal region of the same embryo. The two Wolffian bodies are separated from the mesenteric line above by the masses of the suprarenal formations, but they approach the mesentery below; here the permanent kidneys can be seen, just apparent on the dorsal side of the more salient ridges.

Fig. 64. — Section through Upper Abdomen of 15 Mm. Embryo.

The structure of the Wolffian body, as seen on transverse section, is illustrated in Fig. 65, where tracings of such sections in embryos of the second month are shown.

In the embryo of 8 mm. the body forms a well-marked ridge, with the Wolffian duct (WD) in its lateral part, and the future gonad already indicated by thickening of the coelomic lining on its ventro-medial aspect. The posterior cardinal vein (V) lies dorsally. At 12 mm. the tubular structures in the body have increased considerably, the gonad is very apparent, and the Wolffian duct is now projecting somewhat ventro-laterally. The next specimen shows these changes accentuated, and a second duct (D) is seen ventral to the Wolffian duct in what is now a definite duct ridge or fold: the second duct is the Mullerian duct. At 26 mm. the gonad is well formed, and attached to the main mass by a narrow pedicle, and the plane of section cuts the ducts somewhat along their length. A key to the different parts of this compound urogenital fold ' is given in the last figure, where M is the main (meso

Fig. 65.— Tracings of Sections through the Region of the Wolffian Body at Different Times in the Second Month.

The lowest figure is a scheme of a mesonephric tubule, showing its convoluted

course, with a secretory part near the glomerulus, and a collecting part leading to the duct. ^

boc ty j WM the Wolffian or urogenital mesentery, G the gonad,

Ju £ e . ni tal mesentery, and TM the tubal mesentery supporting the tubal ridge and ducts. J ^ &

Division of Cloaca.

The cloaca has been seen to be a cavity into which open the hind-gut and the allantois The first figure in Fig. 66 gives the position of the cloaca in the tail end of an embryo of less than 5 mm. A prolongation towards the tail process is known as the post-anal gut (PG). The cloacal membrane (m) looks cramo-dorsally. The Wolffian duct (W) reaches Lie cloaca, but has not yet given its ureteric outgrowth. BS is the

The cloaca is divided by the cloacal septum into ventral and dorsal parts. The septum, deepening from above downwards, is seen to be bringing about this division in the other figures; it is completed in the last. 1 he dorsal division of the cloaca is now the rectum, and its ventral moiety is known as the urogenital sinus, because the genital ducts and the ureter open into it. The ureter is shown, arising from the Wolffian duct, in the second figure, while, in the others, its opening has been transferred to the sinus, and is assuming a higher position here than that taken by the Wolffian duct. The other duct (Mullerian) is not shown.

Fig. 66. — Outline Reconstructions of Cloacal Regions in Embryos of 4-5 Mm., 8 Mm., 13 Mm., and 18 Mm., to show the Division of this Cavity. Description in text. Also shows the atrophy of the ‘ caudal filament,' which

drops off after the 13 mm. stage.

It should also be noticed that the direction in which the cloacal membrane looks is almost completely reversed, this result being attained by rapid mesodermal growth between the membrane and the body-stalk, with lessening of growth in the caudal region. Associated with this is the formation of a prominent genital tubercle at the ventral end of the membrane. When the membrane gives way, the urogenital sinus opens on the surface, but the rectum remains plugged by an epithelial formation for some time afterwards.

Genital Glands or Gonads.

A genital gland develops on each side of the body, on the medial aspect of the Wolffian body. The lining cells of the coelom which cover this region begin to proliferate about the end of the first month. A growing mass of mesoderm cells is formed in this way on each side, making a projection (see Fig. 64) which extends nearly along the whole length of the mesonephros. The mesonephric structures lie behind and between the gonad and the tubal ridge.

Towards the middle of the second month the gland in the male embryo begins to show an arrangement of its contained cells into columns (:medullary cords , Fig. 67). At this stage there is no indication of similar arrangement in the female gonad.

Male Gonad or Testis.—In addition to the medullary cords , which are visible in the body of the testis, rete cords are formed near the attached border or hilum of the gland. These are derived from the upper part of the gland. They develop lumina. Later there are found secondary connections of the rete cords with the medullary cords , on ^ hand, and on the other hand with a number of tubules in the neighbouring part of the mesonephros. In this way the potential tubules

Fig. 67.

medullary cords


Diagrams to illustrate Development of Male and Female


vv ita ^ es ^ s ( mec lullary cords) are brought into continuity with the Wolffian duct, which therefore becomes the vas deferens.

. j I n the female gland or ovary, at a much later stage, there is some in efimte indication of attempted formation of medullary cords; definite rete cords are developed, somewhat as in the male. But a late ingrowth of epithelioid cells from the surface displaces these earlier formations which subsequently disappear. The new cells from the surface ultimately form the sex-cells and nutritive cells. There is no .iLtemp at junction with mesonephric tubules, as in the male; consevestigial ^ tubules and the Wolffian duct degenerate and become

Mullerian Duct.

This is the oviduct, which only functions in the female, although it forms in both sexes. It begins to develop about or just after the time that the gonads make their appearance. An ingrowth of lining cells takes place into the diaphragmatic end of the tubal ridge, close to the Wolffian duct. This ingrowth extends in a caudal direction by growth of its solid free extremity; it lies beside the Wolffian duct, and thus reaches the cloaca. The cloaca has divided by this time (see Fig. 66), and the Mullerian duct, with the other duct, joins the dorsal wall of its ventral subdivision. The duct fuses with its fellow of the other side before reaching its termination, so that an unpaired and median structure is implanted into the ventral division of the cloaca. Each Mullerian duct has thus, from its earliest stage, an opening into the coelom at its proximal or diaphragmatic end, and it has no connection with the nephric tubules which open into the Wolffian duct.

The extension backwards of the blind end of the duct, which lies close to the Wolffian duct, is only a secondary modification of its original mode of formation, which seems to have been by a process of splitting off from the Wolffian duct from before backwards.

Transverse Pelvic Ridge and Genital Cord.

The Mullerian duct lies, in the tubal ridge, on the ventral side of the Wolffian duct; the mesonephric tubules open into the dorsal side of the Wolffian duct. As the duct approaches the pelvis, however, beyond the mesonephric area, it changes its relation to the Wolffian duct, crossing it ventrally to lie on its inner side. Thus (Fig. 68) entering the pelvis, the two Mullerian tubes lie between the other two ducts. The four ducts lie in the pelvis in a mesodermal ridge which extends across the pelvis, and is continuous, at the pelvic margin, with the Wolffian ridge. The two Mullerian ducts fuse together in this ‘ transverse ridge ’; it is immediately dorsal to the allantoic bladder; it is separated from this by a peritoneal depression in female embryos, and becomes the broad ligament, but in male embryos it is smaller and attached to the back of the bladder without an intervening depression.

The central part of this mesodermal ridge is thickened round the contained ducts, and this condensation surrounds them as they pass to the urogenital sinus, constituting the genital cord.

Terminal Portions of Ducts.

The Mullerian ducts are well formed in female embryos, and have a large lumen where they are fused together; this fused part makes the uterus and (a large part of) vagina, the muscular and fibrous walls of these being developed from the mesoderm of the genital cord. In the male they are smaller, and the fused part has an irregular lumen; this forms the small uterus masculinus, or sinus pocularis.

The terminal parts of the Wolffian ducts lie beside the fused Mullerian ducts in the genital cord. In the female they form the vestigial ducts of Gaertner, lying beside the uterus in the broad ligament. In the male the Wolffian ducts remain functional as the vasa deferentia, and the terminal parts form the common ejaculatory ducts, which lie beside the uterus masculinus.

The various conditions and changes that have been described are shown in schematic fashion in Fig. 68. The scheme on the left illustrates the original arrangement. The change in position of the ducts entering the pelvis is seen. The upper figure on the right

Fig. 68. — Diagrams of Male and Female Developmental Changes.

Mnp W m n SOneph J rOS; M ' W ' Mmi erian and Wolffian ducts; G, gonad* UM fused Mullerian ducts; UG, urogenital sinus; U, uterus; V, vagina; VES. SEM.,

™B VGS1Cle ‘ The dlrectlon of the fibres of the gubernaculum is shown

tnWffiQ mo ^hications, the connection between mesonephric

WnffiffiTi and ° f the teStlS ’ and the conse q uen t persistence of the Wolffian, and degeneration of the Mullerian duct. The vesicuEe

ThTlowe r a fiL f ° rmed as sec ° ndar y outgrowths from the vasa deferentia. the Wolffion^r| re V re ^d? S ^ ntln ^ f ema ^ e > shows the degeneration of hnT hnn , n C t ' th !l r remnan , ts ’ with those of the mesonephric

remak ’asTidSr 6 " (G) and the MMerian ducts ’ which

Changes in Position of Urogenital Structures.

or jginal position of the structures on the dorsal wall of the abdomen as qmckly modified. The conditions present in the female at the end of the second month are shown in Fig. 69. The great extension of the liver attachment and the growth of the suprarenals are associated with marked downward displacement of the structures of the Wolffian ridge. At the same time there has been degeneration of the upper part of each mesonephros. Thus the gonad and tube, with the Wolffian remnants, are approaching their ultimate pelvic position, which they will attain at a much later period as a result of difference in growth-rates.

Fig. 69. — Dorsal Wall of Abdomen in 28 Mm. Embryo. For comparison with Fig. 63.

In the male there is the same kind of general descent that is seen in Fig. 69, but an additional factor becomes operative in the shape of the gubernaculum testis. This is seen in its early stage in Fig. 63. It is a band passing between the inguinal abdominal wall and the testis and the two ducts, and develops muscular fibres in its thickness.

As the band does not grow in length pari passu with the wall, the structures to which it is attached necessarily come to a lower level near this wall and out of the pelvis. No doubt a certain amount of muscular contraction occurs also, and aids in this movement of the testis along the pelvic brim. The other end of the gubernaculum passes through the wall and into the genital swelling, which forms that half of the scrotum. Thus the gland is brought down towards the scrotum, but it is not known by what mechanism or factors the actual passage outside the body is brought about. The testis is just outside the outer ring at birth.

A similar but weaker gubernacular fold is present in the female, but the persistence and thickening of the Mullerian tubes, to which it is attached on each side, limits its power of downward traction and the ovary moves into the pelvis with the tube to which it is attached.

Failure of the factors causing descent may lead to the condition of retained testis, when the gland is usually found on the pelvic brim, or just within the abdomen; if partially retained it is in the inguinal canal. On the other hand

afUachment 13 ' 7 ^ d ° Wn ^ situation b F failure of gubernacular

Further Intra-Uterine Conditions : the Decidua.

The embedding of the ovum has been described shortly on p 28* the conditions were brought up to the stage in which the ovum, covered by chorionic villi, lies in the intervillous space in the stratum compacturn of the decidua. The intervillous space is filled with blood

decidnTAh detrltu , s ’ results of th e action of the trophoblast on the decidua. The cavity containing the ovum is separated from the

capsulark V,ty b ' V & thm lay6r ° f mUCOUS memb rane, the decidua

erowW dlagra P in Fig ' 70 re P re sents these conditions. The growing embryo and ovum cause a projection of the capsular decidua

in o the uterine cavity; the capsular decidua is stretched and thinned. surround1ng d th a en? part ,° f the Stratum “mpactum immediately

TheTpert^e^d^jult° Ut A \

its position is possibly indieafprl K, r a b ^ a hhrmous plug, and

prominent part of the^apsular decidua. m SCar (Reichert ’ s scar ) on the most

wdWd f ? luc ° sa betwe en the intervillous space and the muscular

tt° le ptti^atdThtdTci

uterus,' is The of the cavity of th*

mucous ^ * dosed ^ a P lu S consisting of epithelial and

which does ^ nh^ mT^he^ovum b thus 1S a surr ° unded U the amnion,

, thus an extensive extra-embryonic

coelom is present. The chorionic villi are smaller (< chorion leave) over the capsular aspect of the chorion, but are well developed and rapidly growing (chorion frondosum) over its basal aspect. The body-stalk is attached to this basal region of the chorion.

It is evident that the ovum can undergo rapid growth in this position out of proportion to the more slowly enlarging uterus, for it can expand in the direction of the uterine cavity, which will ultimately be obliterated by its expansion.

The second diagram in Fig. 70 gives schematically the condition in the fourth month. The ovum has enlarged considerably, and by its enlargement has brought the decidua capsularis into contact with the parietal layer over the rest of the uterine wall, thus obliterating the uterine cavity ; the cavity is shown in the figure as on the point

Fig. 70.—Two Diagrammatic Sections of the Gravid Uterus, to illustrate the Conditions at the End of the First and in the Fourth Months.

of obliteration. A certain amount of fusion then takes place between the parietal and capsular layers. The villi on the greatly stretched capsular part of the chorion have practically disappeared, and the chorion and decidua capsularis have come into close contact, with the disappearance of the intervillous space which originally intervened. Within the ovum the amnion has enlarged with the growing foetus, and has come into contact with the whole extent of the inner surface of the chorion, obliterating the extra-embryonic coelom. The bodystalk has elongated, and is now a definite umbilical cord covered by amnion and connecting the foetus with the basal part of the chorion. This basal chorion is thickened, and has growing from it enormously enlarged and arborescent villi; these are the villi of the chorion frondosum. These large villi he in the intervillous space which is filled with maternal blood, and the ' floor ’ of the space is made by the superficial part of the decidua basalis. The basal chorion with its villi, and the superficial layer of the basal decidua, form together the placenta, which is the organ on which the foetus depends for its nutrition and respiratory exchanges.

The extra-embryonic coelom is obliterated towards the end of the second month.

The uterine cavity is obliterated in the fourth month.

The placenta is considered to be functional from the third month onward, but it can be recognized definitely before the third month.

The amnion and chorion , with their immediate decidual covering, constitute what are known, in the lying-in room, as the membranes.

The amnion is a thin transparent membrane, composed of a very fine outer layer of areolar mesoderm, lined on its inner side by a layer of cells, the amniotic ectoderm. It encloses the amniotic cavity , filled with amniotic fluid, a thin clear liquid containing a small amount of albumin.

Fig. 71. — Placenta: Fcetal and Maternal Surfaces.

The chorion, although shown in the diagram as a fairly thick wall, is really only thick and opaque where it forms the chorionic basis of the placenta. In the rest of its extent it undergoes much stretching and thinning, so that it becomes almost transparent.

Ihe capsular decidua is also stretched to such an extent that it almost becomes transparent. It is considered to become more or less attached to the chorion, but remnants of the villi of the chorion laeve can always be found, here and there, pressed between the two layers.

The placenta can be seen to be composed essentially of a ‘ floorplate of decidua (maternal part) and ‘ roof-plate * of chorion from which the villi spring (foetal part). Between these two plates is the area of villi, bathed in the blood of the intervillous space. The peripheral part of this space, surrounding the area of large villi, makes an open sinus or channel round the placental margin, hence termed the marginal sinus.

The placenta, from its mode of construction, must necessarily be continuous at its edge with the ' membranes’ (chorion and decidua) enclosing the ovum; it is lined on its foetal aspect by the amnion. Further details of its structure will be given later.

It is evident that, as a result of the fusion of the capsular and parietal decidual layers, the cavity of the ovum has replaced the cavity of the uterus. The foetus, lying in the amniotic fluid, is separated from the mouth of the uterus (which is still closed by a mucous plug) by the thin * membranes,’ consisting from within outwards of amnion , chorion , and decidua capsularis.

From the fourth month to the time of birth the conditions remain essentially as figured. The enlargement of the foetus and ovum can now only take place pari passu with that of the uterus. At full term the foetus, attached by an umbilical cord of considerable length to the placenta, is separated from the vaginal cavity by the ‘ membranes ’ already described, which bulge through the dilated cervix uteri. When these membranes are ruptured, the amniotic fluid escapes, and the uterus contracts down on the foetus, which is expelled, partly by the contraction of the uterus, and partly by that of the abdominal walls.

After the foetus has left the uterus, this contracts down on the placenta and membranes. The placenta separates from the deeper layers of the decidua, and is expelled, with its associated membranes, by further uterine contraction.

The line of separation of the placenta lies in the stratum spongiosum of the basal decidua. The growth of the uterus leads to stretching of this decidua, and the large gland-cavities of the spongy stratum are drawn out into long cleft-like spaces; it is in the plane of these clefts that the separation occurs between the c maternal part ’ of the placenta and the rest of the basal decidua. The line of clefts is shown in Fig. 70.

Histological Changes in the Decidua

In addition to the macroscopic changes which have just been considered, certain histological effects of pregnancy are found in the decidua at an early stage. The lining epithelium is destroyed before implantation in the neighbourhood of the site of this process, and subsequently elsewhere.

The decidua becomes very vascular. Its glands enlarge, and their lining cells become enormously swollen and show a tendency to break down. The stroma is much swollen, and is everywhere infiltrated with decidual cells ; these are large spherical or angular cells (Fig. 73) with well-marked nuclei of considerable size, and when present are diagnostic of pregnancy. Their origin is not definitely known; probably it is from the stroma cells. The decidua also shows a considerable infiltration with leucocytes, a fact which is of use in distinguishing decidual structures from neighbouring foetal formations.


The placenta is mainly composed of masses of villi, but its foetal surface is made by the placenta chorion from which the villi spring, and its maternal surface is covered by a layer, more or less complete, of decidua basalis, torn away from the uterine lining when the placenta and its ‘ membranes ’ are expelled.

The whole structure usually resembles a thick disc, nearly circular in outline, some 6 to 8 inches across, and i or 2 inches thick. It is covered by the amnion on its foetal surface, and has the umbilical cord implanted on this surface, as a rule near its centre.

Fig. 72. Placental Section (Schematic). Explanation in text.

in Fi UC tV ar organ is shown in the diagrammatic section

infprvflinno 11 P 10 J ect from the placental chorion into the large

enorT nfrl f 6 ’ Wh '. ch b ? lds the . arborizations produced by their

blood CMR'i They be ’ m this space, bathed in maternal

leaves bl LT , °° d . e ?T rs the S P ace by maternal arteries, and

destruction nf these vessels have been opened up by the

Some form of rT iT Jy - t trophoblast of the advancing villi.

intervilSus 1 ” 13 P US P resent in a11 probability in the

and that in the vill Pr °TK ln j 0, i the interchange between this blood

of plasmodi tronhnhl + i 6 decl j Ua l wa h closing the space has masses

be P deg”ne?ded P fo r “ * a ? d , there in contact with ^ ( T )i this may

g ated, forming one of the varieties of ‘ fibrin ’ or ‘ fibrinoid/

of which other sorts are derived from necrosed decidua. Some masses of fibrinoid make what are termed white infarcts (W) in the intervillous space, and clotted blood, usually round some necrotic tissue as a centre, may make (R) red infarcts. Some villous processes show attachment to the decidual wall through their trophoblastic covering, as at FV; these are known as fixation villi. Fixation villi occasionally degenerate, leaving their contained vessels, however, enlarged into dilated sinuses, which thus seem to run from the chorion (CDV) to the decidua, and are called chorio-decidual vessels ; it must not be forgotten, however, that there is never any direct connection between foetal and maternal

Fig. 73.— Part of a Sfxtion through the Intervillous Space in the

Fourth Week.

Villi are seen cut, covered by trophoblast. L, cyto-trophoblast or Langhans’ layer; P, plasmodi-trophoblast. One fixation-villus is in position. Masses of plasmodium and degenerated and necrosed tissues lie in the space. F is a deposit of ‘ fibrinoid ’ lying against the decidua. The decidua shows the large nucleated cells of the ‘ decidual change ’ in the stroma, and is infiltrated with leucocytes.

vessels, and the chorio-decidual vessels end in or on the decidua, and have no connection of any sort with the vessels of this layer.

The destruction of the decidua which results from the activity of the villi is naturally most marked opposite each main villous stem, so that these areas are partially separated from each other by incomplete septa projecting (S) into the intervillous space. The maternal arteries tend to open on or near these septa, the venous apertures lying rather between them. The placenta, when viewed from the maternal side, exhibits indefinite prominences corresponding with these main villous areas, separated by badly marked depressions along the lines of the septa; the prominences make the cotyledons, of which there are generally from twenty to twenty-five.

Where the villi are sparse or absent, in the peripheral part of the intervillous space, this space, containing only blood, is known as the marginal sinus ; it is shown in Fig. 70.

General Growth of Foetus

Fig. 74 shows embryos in various stages of development during the first three months, drawn of natural size. The human appearance is definitely attained in this month. Limbs begin as ‘ limb-buds/ which grow larger, form ‘ hand-plates * with marginal irregularities that indicate the early form of digits, and later develop a bend nearer

the proximal end, which, as the nearer segments grow, become the elbow and knee respectively. Limb-buds are at first condensations of mesoderm or mesenchyme, in which the central parts form the skeleton, and joints appear in these central parts secondarily. Muscles and tendons develop in situ, and not as ingrowths from somites.

After the third month the growth of the foetus goes on with fairly regular progress at about the rate of 10 mm. a week, more After this time the foetus increases in * sitting or less. During the third eight at about the rate of 1 finches a month. month the nails appear on

the hands and feet, the eyelids close, and the umbilical gut enters the abdomen. During the fourth month minute hairs can be found on the surface, and the hair begins to show on the head during the fifth month ; movements of the toetus are apparent in this month. In the sixth month the nails begin o project on the digits, and the foetus is covered with fine hairs, and w *. a fatty, sebaceous-like material, the vernix caseosa. The eyes, wnich have been closed since the third month, open during the seventh month, and subcutaneous fat begins to be deposited. In the eighth month the fine hairs {lanugo) begin to disappear from the skin, subcutaneous fat is well developed, and the testes appear at the internal i-'ng. in the ninth month the testes reach the scrotum, and the foetus approximates to the appearance of the new-born child.

ni 1 ical Cord. This is the name given to the later representative

Fig. 74.— Drawings, Natural Size, of Embryos of the End of the First Month, Fifth Week, Sixth Week, End of Second Month, and Middle of Third Month Respectively.

of the body-stalk, of which it is practically only a modification. It connects the foetus with the placenta, carrying the placental or umbilical vessels. Its length (usually less than 2 feet) is considerable, though variable, and it exhibits a twisted appearance, due to the movements of the embryo—at any rate in part. Its structure shows a semitransparent substance, Wharton s jelly , formed by changes in the mesoderm of the body-stalk, and carrying the vessels and allantoic remnants ; this is surrounded by an amniotic covering, which is said to include a portion of the original body-wall proximally. Remains of the vitello-intestinal duct may exist between amnion and the rest of the cord, the umbilical vesicle (yolk-sac) itself being usually between the amnion and the placenta.

Summary of Structures derived from the Germinal Layers

Fig. 74A. — The Germinal Layers.


  1. The nervous system—that is to say, the spinal cord and encephalon, the peripheral nerves, and the sympathetic system.
  2. The epithelial elements of the organs of sense, except the tongue — e.g., the epithelial elements of the olfactory region, internal ear, optic nerve, and retina.
  3. The epithelial elements of the posterior lobe of the pituitary body, and those of the pineal body.
  4. The crystalline lens.
  5. The epidermis and its appendages— e.g., the hairs and nails.
  6. The epithelial elements of the sebaceous glands, sweat-glands, and mammary glands.
  7. The plain muscular tissue connected with the hair-follicles, and arranged as the musculi arrectores pilorum, as well as the plain muscular tissue of the sweat-glands; muscles of iris.
  8. The epithelium of the roof and sides of the mouth, but not that which covers the tongue and back part of the floor of the mouth; the epithelium of the parotid glands; the enamel of the teeth; and the anterior lobe of the pituitary body.
  9. The epithelium of the nasal fossae and of the air-sinuses which communicate with them.
  10. The epithelium of the external auditory meatus and outer layer of the membrana tympani.
  11. The epithelium of the conjunctiva and front part of the cornea.
  12. The epithelium (modified epidermis) of the anal canal below the anal valves.
  13. The epithelium of the spongy part of the male urethra.


  1. The epithelium of the alimentary canal, except the following parts: (a) The roof and sides of the mouth; and (b) the anal canal below the anal valves.
  2. The epithelium of the tongue (including that of the taste-buds) and of the floor of the mouth.
  3. The epithelium of the glands which open into the alimentary canal (except the parotid glands)— e.g., the liver and pancreas. The epithelium of the gall-bladder is included.
  4. The epithelium of the Eustachian tube and tympanum.
  5. The epithelium of the thyroid and thymus bodies.
  6. The epithelium of the respiratory tract— e.g., the larynx, trachea, bronchial tubes, and air-cells of the lungs.
  7. The epithelium of the urinary bladder, of the prostatic and membranous parts of the male urethra, and of the whole of the female urethra.


  1. The various connective tissues— e.g., bone, cartilage, dentine, cement, areolar tissue, fibrous tissue, and blood.
  2. Muscular tissue, striated and plain, except the muscular tissue of the sweat-glands, that which constitutes the musculi arrectores pilorum in connection with the hair-follicles, and that of the iris.
  3. The vascular and lymphatic systems, together with their endothelial linings.
  4. The serous and synovial membranes, together with their endothelial linings, including all bursal sacs.
  5. The kidneys and ureters.
  6. The testes, and their complicated excretory equipments.
  7. The ovaries, Fallopian tubes, uterus, and vagina.
  8. The spleen.

The mesoderm exists under two forms, called mesothelium and mesenchyme.

Mesotheiium is that form of mesoderm in which the cells are flattened and form a definite epithelial membrane or layer, known as endotnelium, there being only a very small amount of intercellular substance It lines serous membranes, as well as the chambers of the eart, the bloodvessels, and the lymphatic vessels.

Mesenchyme is that form of mesoderm in which the cells are more or less scattered, m a homogeneous ground-substance or matrix, as occurs, say, m the various connective tissues. The cells are stellate and non-epithelial.

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