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proceeds the plate is gradually invaginated into the substance of the head, and is
FIG. 67.-TRANSVERSE SECTION OF A RAT EMBRYO.
Showing the relation of the paraxial mesoderm of the head to the lateral plates, the commencement of the formation of the otic vesicles and hyomandibular clefts, and the relation of the primitive heart to the pericardium and fore-gut.
SoM. Somatic mesoderm.
SpM. Splanchnic mesoderm.
transformed into a pear-shaped vesicle, the otic vesicle, which remains for a time in communication with the exterior by means of a short tubular stalk, the recessus labyrinthi, which is subsequently converted into the ductus endolymphaticus.1
After it is separated from the surface the otic vesicle alters its position, until its ventral end lies in close relation to the dorsal wall of the pharynx, and, at the same time, it undergoes alteration of shape. The ventral part of the vesicle grows towards the median plane, along the ventral wall of the hind-brain. It forms the cavity and the lining epithelium of the cochlea; but it remains in connexion with the dorsal part by means of a narrow tube, the canalis reuniens, and as it grows in length it becomes converted into a spiral tube. The portion of the dorsal section of the primitive vesicle, which lies to the lateral side of the recessus labyrinthi, first
OF AN EMBRYO.
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
in relation with the posterior surface of the petrous part of the temporal bone and external to the dura mater.
num and the auditory tube (O.T. Eustachian) are developed from the first visceral pouch.
The ventral 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 of 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. upon 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 cellular 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 is 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 blood-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
growth of foetal vessels into the foetal mesodermal cores. The secondary villi, therefore, consist of a mesodermal core covered by a layer of cellular trophoblast and a layer of plasmodium, the latter lying outside the former. Still later the
Anterior end of neural fold
Mesoderm lining of tropho
Mesoderm of amnion
Ectoderm of amnion
secondary villi send out numerous branches into the blood [blast spaces, and thus increase greatly in complexity (Figs. 75, 76, 77). As development progresses still further a part of the chorion is converted into the fœtal portion of an organ called the placenta, and thus the chorion is divided into placental and non-placental regions. Upon the placental part the villi continue to increase, but they disappear entirely from the nonplacental part, which is then called the chorion læve (Fig.
Cavity of entodermal vesicle
FIG. 70.-SCHEMA OF SAGITTAL SECTION OF ZYGOTE ALONG LINE A. 77).
The Amnion, the BodyStalk (Allantoic Stalk), and the Umbilical Cord.-The amnion is formed from that portion of the wall of the larger of the two inner vesicles of the zygote, the ecto-mesodermal vesicle (p. 22), which does not take part in the formation of the embryo. It consists of ectoderm cells covered. externally by a layer of extra-embryonic mesoderm, and it is continuous with the margin of the embryonic area (Figs. 70, 71).
The cavity of the ectomesodermal vesicle, enclosed the cavity of the amnion; it
between the amnion and the embryonic area, is is filled with fluid, which raises the amnion in the form of a cupola over the embryonic region (Fig. 70).
The Body-Stalk (Allantoic Stalk).-It has been noted already that the mesoderm of the median part of the posterior or caudal portion of the amnion becomes
FIG. 72. SCHEMA OF THREE STAGES IN THE FORMATION OF A CHORIONIC VILLUS.
thickened. In the thickened strand lies the allantoic diverticulum of the entodermal vesicle (Fig. 70), whilst through it, on either side of the allantoic diverticulum, pass the umbilical arteries and veins, by means of which blood is conveyed between the embryo and the chorion.
This segment of the wall of the amnion vesicle was termed by His the body-stalk. It takes no direct part in the formation of the embryo, and as it
contains the rudimentary allantoic diverticulum and represents the much more highly developed allantois of other forms, it would, perhaps, be better to term it the allantoic stalk. For the present purpose it is important to note that the bloodvessels which pass through the body-stalk enter or leave the body through the umbilical orifice, which is, at first, a relatively large aperture (Fig. 50).
As the embryonic area is folded into the form of the embryo the amnion increases in extent, filling more and more of the extra-embryonic cœlom, and the embryo rises into the interior of its cavity. In other words, the walls of the amnion bulge ventrally round the cranial and caudal extremities and the lateral borders of the embryo (Figs. 75, 76, 77). As the distension of the amnion still continues, the ventral bulging, round the margin of the umbilical orifice, becomes more pronounced, the yolk-sac is forced farther and farther away from the embryo, the vitello-intestinal duct is elongated, and it is surrounded by a hollow tube. The cavity of the tube is an elongated part of the extra-embryonic coelom, and its walls are formed by the amnion (Figs. 57,62,63).
The caudal wall of the tube necessarily consists of the elongated body-stalk (allantoic stalk).
As the distension of the amnion still continues, the walls of the tube are forced
Afferent vessel of villus
Mesoderm of villus
Efferent vessel of villus
against the vitello-intestinal duct, and FIG. 73.-SCHEMA OF A TRANSVERSE SECTION OF A SECONDARY CHORIONIC VILLUS. A loop of the afferent vessel has been cut at two points.
the amniotic mesoderm fuses with the mesoderm of the vitello-intestinal duct.
When the fusion is completed, a solid cord, the umbilical cord, is formed (Figs. 77, 78, 80). It consists of an external covering of amniotic ectoderm, and a core of mesoderm in which lie the two umbilical arteries of the body-stalk, a single umbilical vein formed by the fusion of the two primitive veins, and the remains of the vitello-intestinal duct and the vitelline vessels. The proximal end of the umbilical cord is connected with the embryo; the distal end is attached to the chorion, and in its neighbourhood lies the now relatively small vesicular yolk-sac (Fig. 62).
As the amnion grows still larger, all that part of its outer surface which does not take part in the formation of the umbilical cord is ultimately pressed into contact with the inner surface of the chorion, with which it fuses, and the cavity of the extra-embryonic part of the cœlom is obliterated (Fig. 78).
The outer wall of the zygote now consists of the fused chorion and amnion, and it contains in its interior the amniotic cavity and the embryo, which is attached to the chorion by the umbilical cord.
When it is first formed the umbilical cord is comparatively short, but, as the amniotic cavity increases, the cord elongates, until it attains a length of from 18 to 20 inches, a condition which allows the embryo to float freely in the fluid in the amniotic cavity, whilst its nutrition is provided for by the flow and return of blood, through the umbilical cord, to and from the placenta, where interchanges take place between the maternal and the foetal blood."
The Yolk-Sac or Umbilical Vesicle.-When the embryonic area is folded into the form of the embryo, the entodermal vesicle is differentiated into three parts: (1) a part enclosed in the embryo, where it forms the primitive entodermal alimentary canal; (2) a part which lies external to the embryo in the extraembryonic cœlom-this is the yolk-sac or umbilical vesicle; (3) the third portion is the vitello-intestinal duct, which connects the primitive alimentary canal and the yolk-sac together (Figs. 40, 62).
The walls and the cavity of the yolk sac are, therefore, continuous with the walls of the primitive alimentary canal, and the structural features of the two are identical, each consisting of an internal layer of entodermal cells and an external layer of splanchnic mesoderm.
Free communication between the yolk-sac and the primitive alimentary canal