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tortuous, and dilatations form in their walls a short distance from their outer closed extremities. At the same time the interglandular tissue increases in amount, and as a result of the various processes the decidua is thicker, softer, more spongy, and more vascular than the mucous membrane from which it was evolved.
Partly on account of the dilatation of the deep part of the glands and partly on account of differences in texture of the internal as contrasted with the external part of the decidua, the membrane may be looked upon as consisting of three layers. (1) An internal layer, next the cavity, the stratum compactum. (2) An intermediate layer, the stratum spongiosum, formed largely by the dilated parts of the glands. (3) An external layer, the unchanged layer, in which lie the comparatively unaltered outer ends of the glands.
When the zygote, in the morula stage, reaches the uterus, from the tenth to the fourteenth day, it acts as a parasite, it eats its way through the epithelium on the surface of the decidua, and implants itself in the stratum compactum.
The zygote may penetrate the decidua at any point of the wall of the uterine cavity, but it usually
enters at some point of the dorsal or the ventral wall. The entrance generally takes place between the mouths of adjacent glands, which are pushed aside, and the zygote becomes at once surrounded by the interglandular tissue of the stratum compactum of the decidua. The aperture through which it passes may be closed by a fibrinous plug or its margins may converge rapidly and fuse together.
The portion of the decidua in which the zygote is embedded is thicker than the other parts of the membrane, and it is separated by the zygote into an internal part, the decidua capsularis, and an external part, the decidua basalis. The junction of the decidua capsularis with the decidua
Inner mass of cells
Dilated part of gland
FIG. 74. SCHEMA OF A FRONTAL SECTION OF THE UTERUS, showing the various parts of the decidua and a zygote embedded in the decidua.
basalis is the decidua marginalis, and the remainder of the decidua, by far the larger portion, is the decidua vera.
As soon as the zygote becomes embedded in the decidua its trophoblast undergoes rapid proliferation. The superficial part of the growing trophoblast becomes converted into a mass of nucleated protoplasm, the plasmodial or syncytial layer, but the inner part remains more or less distinctly cellular.
The plasmodial portion of the trophoblast invades and destroys the surrounding maternal tissue, and at the same time spaces appear in its substance. plasmodium destroys the walls of the dilated maternal blood-vessels, channels are made through which the maternal blood flows into the spaces in the plasmodium, and thus maternal blood begins to circulate in the trophoblast of the zygote.
In the meantime the extra-embryonic coelom has appeared in the primary mesoderm of the zygote, and the outer layer of the mesoderm has associated itself with the trophoblast to form the chorion.
The spaces in the plasmodium enlarge rapidly after the maternal blood
begins to circulate within them and the plasmodium becomes divided into
FIG. 75.-SCHEMA OF A SECTION OF A PREGNANT UTERUS AFTER THE FORMATION
three series of parts. (1) The parts which lie between adjacent blood spaces, the primary chorionic villi. (2) The parts which lie in contact with the mesoderm of the chorion, and which form with the mesoderm chorion plate. (3) The parts which cover the maternal tissues and form the outer boundaries of the blood spaces, the basal layer. The blood spaces themselves are called the intervillous spaces (Figs. 76, 79). After a time each primary villus differenti
ates into a cellular core and plasmodial periphery, and thereafter the villi are invaded
by the mesoderm of
the chorion and are thus converted into secondary villi (Fig. 76).
The first-formed villi are non-vascular, but by the time the secondary villi have developed the umbilical arteries have grown through the body-stalk (allantoic stalk) into the mesoderm of the chorion, and branches from them enter the mesodermal cores of the villi, which thus become vascular.
When the secondary villi are fully developed each consists of a vascular mesodermal core continuous with the mesoderm of the chorion. The mesodermal core is covered
FIG. 76. SCHEMA OF A FRONTAL SECTION OF A PREGNANT UTERUS AT THE PERIOD OF THE FORMATION OF THE EMBRYO. Note extension of amnion as contrasted with stage shown in Fig. 75.
by a layer of cellular trophoblast, Langhan's layer, which lies neshe uterine cavity, and a layer of plasmodium external to the cellular layer. T'of the uterus is of each villus is continuous with the chorion plate of the 2nd month, and as formed by the chorion, and the distal extremity is connected w, pus at its basal layer of the trophoblast, which forms the outer boundary intervillous spaces and which is fused with the maternal decidual tissue.
After a time branches are projected from the sides of the secondary villi into the intervillous spaces. In this way two sets of secondary villi are differentiated, (1) the anchoring villi (Fig. 79), which cross from the chorion to the
FIG. 77.-SCHEMA OF A SECTION OF A PREGNANT UTERUS AFTER THE FORMATION OF THE UMBILICAL CORD, Note that the expanding amnion has almost obliterated the extra-embryonic coelom which lies between it and the chorion.
basal layer of trophoblast and are attached to the latter by cell columns, which are the remains of the primary villi which have not been penetrated by the foetal mesoderm, and (2) free or absorbing villi (Fig. 76), which extend from the sides of the original secondary villi into the blood, in the intervillous spaces.
Whilst the trophoblastic invasion of the compact layer of the decidua is proceeding, not only are the interglandular elements of the decidua destroyed, but the walls of the glands also, and, as a consequence, some of the glands in the decidua basalis open for a time into the intervillous spaces, and become filled with blood which passes from the spaces into the gland cavities. In many cases, however, before the glands are destroyed their walls are converted into solid strands of cells, and thus the cavities of their more external undestroyed portions are converted into closed spaces.
In the early stages the trophoblast is differentiated in a similar manner over
constricted into the form of three flat purse-like diverticula which, by the partial obliteration of their cavities, become converted into the three semicircular canals (see Sense Organs). The more ventral part of the dorsal section of the vesicle is divided, by a constriction of its lateral wall, into a dorsal part, the utricle, which remains in connexion with the semicircular canals, and a ventral part, the saccule, which is united to the cochlea by the canalis reuniens. The apex of the constriction which separates the utricle from the saccule passes into the mouth of the ductus endolymphaticus, which is thus transformed into the Y-shaped canal which connects the utricle with the saccule. At a later period the closed extremity of the ductus endolymphaticus dilates and forms a small saccule, the saccus endolymphaticus. In the adult the saccus endolymphaticus lies in the posterior fossa of the skull,
FIG. 69.-FIGURES, MODIFIED FROM HIS, ILLUSTRATING THE FORMATION OF
ternal to the
num and the auditory tube (O.T. Eustachian)
from the first visceral pouch.
part of the pouch disappears at an early stage. The dorsal extremity expands and is converted into the cavity of the tympanum, whilst the stalk connexion
with the pharynx is gradually constricted off from its lateral towards its medial
end, and is converted into the auditory tube. The constriction commences when the embryo has attained a length of about 20 mm., that is about the beginning of the eighth week, and is completed about the end of that week when the embryo is about 25 mm. long.
After the auditory tube is defined it grows rapidly in length, and cartilage appears in its walls during the fourth month.
As the tympanic cavity increases in size the auditory ossicles-stapes, incus, and malleus, which are differentiated from the dorsal ends of the cartilages of the first and second branchial arches, are invaginated into it.
The membrana tympani, which separates the tympanum from the external acoustic meatus, is formed from the separating membrane which intervenes between the first branchial pouch and the first cleft. It consists, therefore, of an external covering of ectoderm, an internal lining of entoderm, and an intervening layer, of fibrous tissue, derived from the mesoderm.
The external ear is developed from the cavity and the boundaries of the first branchial cleft. The cavity of the cleft is transformed into the cavity of the external acoustic meatus, and on the mandibular and on the hyoid margins of the
cleft three eminences appear. From the eminences on the two arches, and the skin immediately posterior to the eminences on the hyoid arch, are formed the various parts of the auricle, but the exact part played by the individual eminences in the human subject is as yet a matter of some doubt.
THE PROTECTION AND NUTRITION OF THE EMBRYO DURING ITS INTRA-UTERINE EXISTENCE.
Whilst it is passing down the uterine tube, and for a brief period after it enters the uterus, the zygote, or impregnated ovum, depends for its nutrition the yolk granules (deutoplasm) embedded in its cytoplasm, and upon the fluid medium surrounding it which is secreted by the walls of the uterine tube and the uterus.
As the human ovum is very small, and as it contains but little deutoplasm, its nutrition is practically dependent, almost from the first, upon external sources of supply. The urgent necessity for the formation of adequate arrangements whereby the external sources may be utilised leads to the early establishment of an intimate connexion between the zygote and the mother, which is one of the characteristic features of the development of the human embryo.
During the third week after fertilisation, as the embryo is beginning to be moulded from the embryonic region, and before the paraxial mesoderm commences to separate into mesodermal somites, a primitive heart and the rudiments of some well-defined blood-vessels are distinguishable in the embryo; but the details of the development of the vascular system and the establishment of the embryonic circulation cannot be well understood until the formation and structure of a group of closely associated extra-embryonic organs or appendages, derived from the zygote, has been considered.
This group includes the chorion, the placenta, the amnion, the umbilical cord, and the yolk-sac.
THE MEMBRANES AND APPENDAGES.
The Chorion. It has already been noted that when the zygote becomes a blastula it consists of three vesicles, a large vesicle enclosing two smaller vesicles and a mass of primary mesoderm (Fig. 29).
The wall of the large vesicle is composed of trophoblast (trophoblastic ectoderm), and its inner surface is in direct contact with the primary mesoderm.
A little later a cavity, the extra-embryonic cœlom, appears in the primary mesoderm, separating it into two layers, one lining the inner surface of the trophoblast and the other covering the outer surfaces of the two inner vesicles (Figs. 70, 71).
As soon as the extra-embryonic colom is established the chorion is formed; it consists of the trophoblast and its inner covering of mesoderm.
In the meantime the trophoblast has differentiated into two layers, an inner ellular layer, and an outer plasmodial layer. In the plasmodial layer cell territories are not defined, and it consists, therefore, of nucleated protoplasm.
The differentiation of the trophoblast into two layers occurs after the zygote embedded in the mucous membrane of the uterus which is modified for its reception and which, after the modification has occurred, is called the decidua.
As development proceeds the trophoblast increases in thickness and it invades the decidua. As this invasion occurs the plasmodial layer of the trophoblast becomes permeated with spaces which are continuous with the lumina of the maternal Hood-vessels in the decidua, and are filled with maternal blood.
By means of the spaces the plasmodial trophoblast is separated into branching processes which intervene between the blood-filled spaces. The processes are
the primary chorionic villi, and they soon develop cellular interiors (Fig. 72).
After a time the primary villi are invaded by the chorionic mesoderm, and are thus converted into the secondary chorionic villi, which become vascularised by the