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The bucco-pharyngeal membrane disappears about the third week, and about the twenty-first day a diverticulum from the stomatodæum is projected into the caudal surface of the head, from the point
where that surface originally joined the
dorsal end of the external surface of the Prosencephalon bucco-pharyngeal membrane. The diver
ticulum is Rathke's pouch.
The Separation of the Stomatodæum
into Nose and Mouth.-In the cephalic FIG. 64.-ANTERIOR VIEW OF boundary of the stomatodæal space lies
the ventral end of the head, which is called the fronto-nasal process.
STOMATODEUM BEFORE COMPLETION OF PRIMI-
In the fronto-nasal process, on each side of the median plane, is situated a shallow pit, the olfactory pit, and by the pits the process is divided into a median part, the median nasal process, and two lateral parts, the lateral nasal processes. Further, the margin of the median process is divided by a median cleft into right and left globular processes (Fig. 64).
The orifices of the olfactory pits are directed laterally, therefore the lateral nasal processes lie dorsal to the median nasal process in the cranial boundary of the stomatodæal space, and as their margins increase in height the pits deepen (Fig. 69). At this period the cranial boundary of the stomatodæum is divided by the median sulcus and the olfactory pits into four projections-the two globular processes,
each of which lies between the median sulcus and an olfactory pit, and the two lateral nasal processes, which form the dorso-lateral borders of the olfactory pits. The lateral boundaries are formed by the maxillary processes and the dorsal parts of the mandibular bars, and the caudal boundary is formed by the medially turned and conjoined ventral parts of the mandibular bars. Immediately cranial to the maxillary process, on each side, is the projecting eye; and leading from it, between the maxillary process and the lateral nasal process, is the naso-lacrimal sulcus.
As growth proceeds and each maxillary process grows ventrally, its ex
FIG. 65.-SCHEMA OF ANTERIOR VIEW OF THE HEAD tremity fuses with the caudal or pos
terior border of the lateral nasal process, and then, carrying the lateral nasal process with it, it fuses with the globular process of the same side.
After the fusion of the maxillary processes, and the posterior or caudal borders of the lateral nasal processes, with the globular processes has occurred, the olfactory pits are completely separated, for a time, from the stomatodæum, and they lie in the ledge which now forms the cranial boundary of the stomatodæum. This ledge consists of the two globular processes, fused into a single mass, and the two maxillary processes, the caudal or posterior edges of the lateral nasal
1 Inferior in erect posture.
processes being shut off from the margin of the ledge by the maxillary processes (Fig. 65).
After the ledge is completed the dorsal ends of the olfactory pits are separated from the stomatodæum by a thin membrane, but this soon disappears, and the pits open again into the stomatodæal space, through apertures which are called the primitive choanæ.
After the formation of the primitive choanæ a ledge grows from the medial surface of each inaxillary process towards the median plane, caudal to the choana. These ledges, the palatine processes, meet and fuse during the third month of foetal life, the fusion commencing ventrally and being completed dorsally in the region of the uvula. As the ledges meet and fuse, the stomatodæum is separated into a
FIG. 66. PORTION OF THE HEAD OF A HUMAN
EMBRYO ABOUT 2 MONTHS OLD (His). The lips
common dental germ is visible in the latter. The
cranial and a caudal portion. The cranial part is the nasal cavity; it is soon divided into two lateral halves by a septum which passes caudally from the base of the cranium. The caudal portion of the stomatodæum blends with the ventral part of the primitive pharynx and it forms the vestibule of the mouth and its derivatives, and the gums and teeth.
The details of the process by which the primitive lips are separated into the permanent lips, and the gums are defined, are described in the section dealing with the digestive system.
The Derivative of the Proctodæum. The proctodæum is a surface depression which owes its origin to the elevation of the surface round the margin of the anal membrane. It forms the lowest portion of the pars analis recti of the adult. Urino-genital System.-The formation of the internal parts of the urino-genital system from the intermediate cell tract, the urino-genital chamber, and the differentiation of the external genitals in the region of the cloacal membrane are described in the account of the urino-genital system.
The development of the auditory organ is so intimately associated with the development of the pharyngeal portion of the primitive gut that a short consideration of the chief phenomena may with advantage be introduced here; but for the details of the development of the internal, middle, and external portions of the ear the student must refer to the account of the development given in association with the description of the auditory organ.
AND THE EXTERNAL EAR.
In the human subject, as in other mammals, the auditory organ consists of the internal ear or labyrinth, the middle ear or tympanum, with which is associated the auditory tube (O.T. Eustachian); and the external ear, which consists of the external acoustic meatus with the auricle at its lateral end.
The internal ear itself consists of two parts-the cochlea, which is the true organ of hearing, and the vestibule and the three semicircular canals connected with it, which are associated with the recognition of alterations in the position of the head, and, therefore, with the recognition and maintenance of equilibrium.
The whole of the internal ear is lined with ectodermal epithelium, the auditory epithelium, which is derived from the surface of the head of the embryo. It is recognisable in embryos of about 26 mm. (Fig. 67) as a thickened and slightly depressed plate of ectodermal cells which lies on the surface of the head, in the rection of the hind-brain, dorsal to the second branchial cleft. As development
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.
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
FIG. 68.-TRANSVERSE SECTION THROUGH THE HEAD
Showing the rudiments of the three parts of the ear and their
1 See note 3, p. 79.
of the recessus labyrinthi, first expands and then becomes compressed and
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 to
wards 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 cœlom 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