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FERTILISATION. Fertilisation is the term applied to the union to form a zygote which contains the typical nu
The meeting of the gametes and their union or middle part of the uterine tube.
The details of the process are unknown in many animals it has been noted that as the the latter shows signs of excitement, and a attraction, appears on its surface. At the changes of form. As the two gametes meet which surrounds the ovum and passes throu of the ovum.
In some cases apparently only the he effect an entrance, but in others the whole
After the entrance occurs and before
lie ex-centrically in the
cle, centrally, by the sur-
does not appear until the
Gebryonic Area. The area where mer vesicles lie in apposition with es the region of the zygote from Se embryo will be formed; it is
therefore, the embryonic area, and at med its definition it consists of three
caderm, primary mesoderm, and em It is uncertain whether the sem which is present in the area at
od takes part in the formation of
ecto-mesodermal vesicle; the
two inner vesicles (Fig. 32).
primary mesoderm on the outer surface
in later, in the formation of the umbilical cord, which
e placenta (p. 54). of the Embryonic Area.-As the embryonic area is the the ecto-mesodermal and the entodermal vesicles it is, at
As growth continues the area becomes oval, and a linear streak, appears in that part of the oval which becomes the
urea (Fig. 31). ve the position of the mesodermal elements of the wall of the
sicle is revealed, for the primitive streak is a thickened ridge of
from the ecto-mesoderm and projects against the entoderm in the of the embryonic area, pushing aside the primitive mesoderm which netween the adjacent parts of the walls of the ecto-mesodermal and the
vesicles. The deeper cells of the ridge, those next the entoderm, are dermal elements of the primitive ecto-mesoderm, and, by proliferation, on the larger part, if not the whole, of the embryonic mesoderm and also an
- the notochord. The mesoderm produced from the primitive streak
rmed the secondary mesoderm. iediately after the formation of the primitive streak a groove, the neural appears in the anterior I the embryonic area.
Embryonic area - formed by the longi
Plasmodial trophoblast, 'nal folding of a thickened Chorion Cellular trophoblast
Mesoderm lining site of ectoderm, the neural
of trophoblast plate, which is the rudiment
Mesoderm of amnion ut the whole of the central and peripheral portions of the
Neural fold nervous system, except perhaps the olfactory nerves, and
Ectoderm of amnion the end organs of the sensory Neurenteric canal nerves. From it also are derived the cells of the primitive
Primitive streak sheaths of the nerve-fibres
Mesoderm of body and the chromaffin cells of the
(allantoic) stalk supra-renal glands and other chromaffin bodies.
The lateral walls of the Fig. 31.—SCHEMA OF DORSAL SURFACE OF EMBRYONIC AREA OF neural groove are called the neural folds.
Almost from the first the anterior ends of the neural folds are united together a short distance posterior to the anterior end of the embryonic area. Their posterior ends, which remain separate for a time, embrace the anterior part of the primitive streak. In the meantime, however, a groove, the primitive groove, has appeared on the surface of the primitive streak. The anterior end of the primitive groove deepens, until it forms a perforation which passes, through the anterior end of the streak and the subjacent entoderm, into the cavity of the entodermal vesicle. As this perforation passes from the floor of the posterior part of the neural groove into that part of the entodermal vesicle .which afterwards becomes the primitive enteron or alimentary canal, it is called the neurenteric canal. The neurenteric canal is but a transitory passage, and it disappears in man and other mammals before the neural groove is converted into a closed neural tube.
After the appearance of the primitive groove and the neurenteric canal the posterior ends of the neural folds converge, across the anterior part of the primitive streak and groove, and fuse together posterior to the neurenteric canal. The primitive streak is thus divided into two portions. (1) An anterior portion, which lies at first in the floor of the neural groove, and, later, in the floor or ventral wall of the posterior end of the spinal medulla ; and (2 a posterior portion, which remains on the surface and takes part in the formation of the median portion of the posterior end of the body, forming the perineum, and the median part of the ventral wall of the body, from the perineum to the umbilicus. It is through the
perineal section of the posterior part of the primitive streak that, at a later period
The Formation of the
Notochord and the Secondary
[blast Mesoderm. The notochord
formed from the primitive
streak; the notochord from its an-
terior extremity, and the second-
As soon as the primitive
streak is established its anterior
end becomes a node or centre
of growth by means of which
extent, the breadth of the body
are increased. The portion of Fig. 32.-SCHEMA OF SAGITTAL SECTION OF ZYGOTE ALONG
the body formed by the activity LINE A in Fig. 31.
of the anterior end of the streak
is the dorsal portion, from the
back part of the roof of the nose,
anteriorly, to the posterior end Extra-embryonic cælom
of the trunk. The perineum Mesoderm of amnion
and the ventral wall of the Ectoderm of amnion
body, from the perineum to the Mesoderm covering
umbilicus, are formed from the entoderm
posterior part of the primitive Entoderm
streak. Nevertheless, the Cavity of entodermal
primitive streak undergoes little vesicle
or no increase in length; indeed, as growth continues, it
becomes relatively shorter as Notochord
contrasted with the total length
of the embryonic region, for the Fig. 33.-SCHEMA OF TRANSVERSE SECTION OF ZYGOTE ALONG
new material, formed by its LINE B IN FIG. 31.
borders and its anterior ex
tremity, is transformed into Primitive streak Primitive groove
the tissues of embryo as rapidly Plasmodial trophoblast Cellular trophoblast
as it is created.
The Notochord.— The trophoblast
notochord or primitive skeletal Extra-embryonic colom axis is formed by the prolifera
tion of cells from the anterior Mesoderm of amnion
end of the primitive streak. Ectoderm of amnion
On its first appearance it is a
narrow process of cells, the head Amnion cavity
process, which projects forwards Mesoderm covering from the anterior boundary of entoderm
the neurenteric canal, between Entoderm
the ectoderm and the entoderm. Cavity of entodermal vesicle Shortly after its appearance the
head process wedges its way FIG. 34.-SCHEMA OF TRANSVERSE SECTION OF ZYGOTE AI
between the entoderm cells, and LINE C IN FIG. 31.
from that period onwards, as
the posterior parts are formed, by continued proliferation from the front end of the primitive streak, they are at once intercalated in the dorsal wall of the entodermal sac, where they remain, forming a part of the dorsal wall of the entodermal cavity (Fig. 33), for a
considerable time. At a later period the notochordal cells are excalated from the entoderm, and then they form a cylindrical rod of cells which occupies the median plane, lying between the floor of the ectodermal neural groove and the entodermal roof of the primitive alimentary canal, which, in the meantime, has been more or less moulded off from the dorsal part of the entodermal sac (Fig. 37). For a still longer time the caudal end of the notochord remains connected with the anterior end of the primitive streak, and its cephalic end is continuous with the entoderm of a small portion of the embryonic area, which lies immediately in front of the anterior end of the neural groove and which becomes bilaminar by the disappearance of the primary mesoderm. This region, because it afterwards forms the boundary membrane between the anterior end of the primitive entodermal canal and the primitive buccal cavity or stomatodæum, is called the bucco-pharyngeal membrane (Fig. 55, p. 42). It disappears about the third week of embryonic life, and immediately afterwards the anterior end of the notochord separates from the entoderm, but the posterior end remains continuous with the primitive streak, until the formation of the neural tube is completed.
After a time the cylindrical notochordal rod is surrounded by secondary mesoderm which becomes converted into the vertebral column of the adult. As the vertebral column is formed the notochord is enlarged in the regions of the intervertebral tibro-cartilages and for a time assumes a nodulated appearance (Fig. 60).
Ultimately the notochord disappears, as a distinct structure, but remnants of it are believed to exist as the pulpy centres of the intervertebral fibro-cartilages. The extension of the notochord into the region of the head is of interest from a morphological, and possibly also from a practical point of view. It extends through the base of the cranium from the anterior border of the foramen magnum into the posterior part of the body of the sphenoid bone. Its presence in the posterior part of the skull suggests that that region was, primitively, of vertebral nature. As the notochord passes through the occipital portion of the skull it pierces the basilar portion of the occipital region first from within outwards and then in the reverse direction. It lies, therefore, for a short distance, on the ventral surface of the rudiment of the occipital bone, in the dorsal wall of the pharynx, and it is possible that some of the tumours which form in the dorsal wall of the pharynx are due to the proliferation of remnants of its pharyngeal portion.
The Differentiation of the Secondary Mesoderm. - It has already been noted that a portion of the inner mass of the human zygote becomes converted directly into mesoderm which may be called, for convenience, primary mesoderm. It was stated also that the wall of the larger of the two inner vesicles of the zygote consists of ecto-mesoderm, that term being intended to convey the idea that the cells of the wall of the larger inner vesicle were the progenitors of both ectodermal and mesodermal cells.
As soon as the larger of the two inner vesicles is formed two areas of its wall are defined : (1) the part in contact with the smaller inner or entodermal vesicle and (2) the remainder. As future events prove, the cells of the larger area, which is not in contact with the entodermal vesicle, simply produce ectodermal descendants which line the inner surface of a sac-like covering of the embryo termed the amnion; they are, therefore, the predecessors of the amniotic ectoderm.
The cells of the larger inner vesicle, which lie adjacent to the smaller entoderm vesicle, and are merely separated from the entoderm by a thin layer of primary mesoderm, take part in the formation of the embryo; forming, with the entoderm, the embryonic area from which the embryo is evolved. These cells are the forerunners of both ectoderm and mesoderm, and as the mesoderm developed from them is differentiated after the formation of the primary mesoderm it may be termed secondary mesoderm or primitive streak mesoderm; the latter term being applied because it is differentiated in a linear region called the primitive streak (p. 23). It is the formation and fate of this primitive streak mesoderm which is now to be considered.
At first the embryonic area is circular in outline, at a later period it becomes ovoid, and in the narrower or caudal portion of the ovoid area a linear thickening
appears; this is the primitive
streak (Figs. 31, 34). It is formed Mesoderm of
Amnion cavity by the proliferation of the ectoEctoderm of
mesodermal cells of the wall of
nesoderm the larger inner vesicle. The
deeper cells of the streak, which Roof-plate
plate displace the primary niesoderm
mesoderm Lateral wall of
from the median plane, and thus neural groove
come into contact with the Floor-plate
Notochord entoderm, are the rudiments Mesoderm of entoderm vesicle
of the secondary or primitive
streak mesoderm (Fig. 34). Cavity of
The superficial cells form part entoderm vesicle
of the surface ectoderm of the
At the anterior end of the embryonic parts of cælom.
primitive streak the mesodermal elements of the streak fuse
with the subjacent entoderm Trophoblast of
and through the fused mass chorion Mesoderm of
Amnion cavity a perforation, the neurenteric chorion
Paraxial canal (p. 23), is formed (Fig.
Anesoderm Mesoderm of amnion
Embryonic The canal itself soon disEctoderm of
appears, but the cells of its Neural crest
walls form a nodal growing Roof-plate Lateral wall of
point, and by their proliferation neural groove
the length and breadth of the Floor-plate
embryonic area are increased. Entoderm
The mesoderm cells proliferated Mesoderm of entoderm vesicle
from the cephalic border of the Cavity of
nodal point are the rudiments entoderm vesicle
of the notochord, which has FIG. 36.- TRANSVERSE SECTION OF A ZYGOTE, showing early
already been considered (p. stage of developinent of embryonic coelom and differentiation 24). of mesoderm.
It is uncertain whether or
the mesodermal cells Trophoblast of chorion
budded off from the nodal point Neural crest
blend with the cells of the Neural tube
„Intermediate cell tract
primary mesoderm, but there Ectoderm
Splanchnic and can be little doubt that they Mesoderm
somatic layers of form by far the greater part, of amnion
lateral plate meso.
if not the whole, of the permanent mesoderm of the embryo.
Either by displacement or Embryonic
by union with the primary
mesoderm the secondary mesoAmnion
derm forms a continuous sheet cavity
of cells, in the embryonic area, on Extra-embryonic each side of the median plane. colom
Each of the lateral sheets Notochord
is thickest where it abuts
Alimentary canal Meso. of yolk-sac
against the notochord and the Entoderm of yolk-sac
wall of the neural groove, and Fig. 37.-TRANSVERSE SECTION OF A ZYGOTE, showing union of
thinnest at its peripheral intra- and extra-embryonic parts of cælom and separation of margin, where it is continuous embryonic mesoderm into paraxial bars, intermediate tracts, with the primary mesoderm of and lateral plates, with separation of lateral plates into the extra-embryonic area (Fig. somatic and splanchnic layers by the intra-embryonic part of the colom.