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
sele. The larger and more
se ecto-mesodermal vesicle.
- ze trophoblast, peripherally,

cle, centrally, by the sur-
Siderm.
mance of the mesoderm in the
sation at so early a period be-
and the entoderm are peculiar-
man subject. In most mammals

does not appear until the
and its primitive streak are

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
elryo or is replaced at a later period
sanserm derived from the cells of the

ecto-mesodermal vesicle; the
latter certainly forms a large part
of the mesoderm of the embryo.

The Extra-Embryonic
Colom. - The extra-embryonic
coelom is a space which appears
as two clefts, one on each side of
the embryonic area, in the primary
mesoderm Fig. 30). The clefts
fuse together round the periphery
of the embryonic area, and the
single space so formed expands
rapidly until the mesoderm which
originally filled the greater part
of the larger vesicle becomes con-
verted into a thin layer which
lines the inner surface of the
trophoblast and covers the outer
surfaces of the epithelial walls of
the extra-embryonic parts of the

two inner vesicles (Fig. 32).
not extend into the embryonic area, and
mesodermal vesicle from the inner surface

primary mesoderm on the outer surface
its continuity with the mesoderm on the
the termination of intrauterine life, and

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

Chorion

perineal section of the posterior part of the primitive streak that, at a later period
of embryonic life, the anal and urogenital orifices of the body are formed.

The Formation of the
Anterior end of neural fold
Plasmodial trophoblast

Notochord and the Secondary
Cellular trophoblast
Amnion cavity

[blast Mesoderm. The notochord
Mesoderm lining of tropho- and the secondary mesoderm are
Mesoderm of amnion

formed from the primitive
Ectoderm of amnion

streak; the notochord from its an-
Allantoic diverticulum
of entoderm vesicle

terior extremity, and the second-
Body stalk mesoderm ary mesoderm from its lateral
Extra-embryonic celom margins and posterior end.
Entoderm

As soon as the primitive
Mesoderm covering of

streak is established its anterior
entoderm vesicle

end becomes a node or centre
Neurenteric canal

of growth by means of which
the length and, to a certain

extent, the breadth of the body
Cavity of entodermal vesicle

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
Plasmodial trophoblast Neural groove

is the dorsal portion, from the
Cellular trophoblast
Mesoderm lining of trophoblast

back part of the roof of the nose,
Amnion cavity

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.

Chorion
Mesoderm lining of

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

amnion

amnion

Lateral

32).

amnion

Notochord

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

Paraxial
Neural crest

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

Mesoderm
Entoderm

of chorion

streak mesoderm (Fig. 34). Cavity of

Trophoblast

The superficial cells form part entoderm vesicle

of chorion

of the surface ectoderm of the

embryo.
Fig. 35.- TRANSVERSE SECTION OF A ZYGOTE, showing early stage
of embryonic secondary mesoderm before the appearance 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

toelom

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

Paraxial mesoderin

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.

35).

of amnion

colom

Yolk-sac