are produced by the ovaries ( ) and expelled from them during ovulation.

The of the uterine tubes sweep the into the , where it may be fertilized.

Usually only one is expelled at ovulation.

Sperms are produced in the ( ) and are stored in the .

Ejaculation of semen results in the deposit of millions of sperms in the vagina.

Several hundred sperms pass through the and enter the .

When an is contacted by a sperm, it completes the division.

As a result, a mature and a are formed.

The nucleus of the mature constitutes the .

After the sperm enters the oocyte, the of the sperm separates from the tail and enlarges to become the ( ).

is complete when the male and female pronuclei unite and the maternal and paternal chromosomes intermingle during of the division of the .

* pro-phase -> meta-phase -> ana-phase -> telo-phase.

As it passes along the uterine tube toward the uterus, the zygote undergoes cleavage (a series of mitotic cell divisions) into a number of smaller cells, or .

Approximately 3days after fertilization, a ball of 12 or more (a ) enters the uterus.

A forms in the morula, converting it into a consisting of the , a , and the .

The encloses the and and later forms extraembryonic structures and the embryonic part of the .

At 4 to 5 days after fertilizations, the is shed and the trophoblast adjacent to the attaches to the .

The trophoblast at the embryonic pole differentiates into two layers, an outer and an inner .

The invades the endometrial epithelium and underlying connective tissue.

Concurrently, a cuboidal layer of forms on the deep surface of the embryoblast.

By the end of the first week, the is superficially implanted in the .

of the blastocyst in the uterine endometrium begins at the end of the first week and is completed by the end of the second week.

The cellular and molecular events relating to are complex.

The (day 5). Its disappearance results from enlargement of the blastocyst and degeneration caused by enzymatic lysis.

The are released from the of sperms that surround and partially penetrate the zona pellucida.

The adheres to the endometrial epithelium.(day 6)

The trophoblast differentiates into two layers, the and the .(day 7)

The erodes endometrial tissues and the blastocyst begins to embed in the endometrium. (day 8)

appear in the syncytiotrophoblast. (day 9)

The sinks beneath the and the defect is filled by a closing plug. (day 10).

form by fusion of adjacent lacunae (day 10, 11)

The erodes , allowing maternal blood to seep in and out of lacunar networks, thereby establishing a uteroplacental circulation. (day 11, 12)

The defect in the is repaired. (day 12, 13)

develop. (day 13, 14)

Rapid proliferation and differentiation of the occurs as the blastocyst completes implantation in the uterine endometrium.

The endometrial changes resulting from the adaptation of these tissues in preparation for implantation are known as the .

Concurrently, the primary umbilical vesicle forms and develops.

The extraembryonic coelom(cavity) forms from spaces that develop in the .

The coelom later becomes the .

Concurrently, the forms and extraembryonic mesoderm develops.

The extraembryonic ( ) forms from spaces that develop in the extraembryonic mesoderm.

The later becomes the chorionic cavity.

The amniotic cavity appears between the and .

The embryoblast differentiates into a embryonic disc consisting of , related to the amniotic cavity, and , adjacent to the blastocystic cavity.

The develops as a localized thickening of the hypoblast, which indicates the future region of the embryo and the future site of the ;

the is also an important organizer of the region.

The bilaminar embryonic disc is converted into a during .

These changes begin with the appearance of the , which appears at the beginning of the third week as a thickening of the epiblast at the of the embryonic disc.

The results from migration of epiblastic cells to the median plate of the disc.

Invagination of epiblastic cells from the primitive streak give rise to cells that migrate ventrally, laterally, and cranially between the and .

As soon as the primitive streak begins to produce , the epiblast is called .

Some cells of the epiblast displace the hypoblast and form .

produced by the primitive streak soon organized into a third germ layer, the

, occupying the area between the former hypoblast and cells in the epiblast.

Cells of the migrate to the edges of the embryonic disc, where they join the covering the amnion and umbilical vesicle.

At the end of the third week, the embryo is a flat ovoid embryonic disc.

Mesoderm exists between the ectoderm and endoderm of the disc everywhere except at the , in the median plane occupied by the , and at the .

Early in the third week, mesenchymal cells from the primitive streak forms the between the embryonic ectoderm and endoderm.

The notochordal process extends from the primitive node to the .

Openings develop in the floor of the , and they soon coalesce, leaving a .

This plate infold to form the , the primordial axis of the embryo around which the forms (eg. vertebral column).

The appears as a thickening of embryonic ectoderm, induced by the developing notochord.

A longitudinal develops in the neural plate, which is flanked by .

Fusion of the folds forms the , the primordium of the .

As the neural folds fuse to form the neural tube, neuroectodermal cells form a between

the surface ectoderm and neural tube.

The mesoderm on each side of the notochord condenses to form longitudinal columns of , which, by the end of the third week, give rise to .

The within the embryo arises as isolated spaces in the lateral mesoderm and cardiogenic mesoderm.

The subsequently coalesce to form a single, horseshoe-shaped cavity that eventually give rise to .

first appear in the wall of the umbilical vesicle, allantois, and chorion.

They develop within the embryo shortly thereafter.

develop from different hematopoietic precursors.

Blood vessels first appear in the wall of the , , and .

They develop within the embryo shortly thereafter.

Fetal erythrocytes develop from different hematopoietic precursors.

The is represented by paired endocardial heart tubes.

By the end of the third week, the heart tubes have fused to form a that is joined to vessels in the embryo, umbilical vesicle, chorion, and connecting stalk to form a .

The premordial haert is represented by .

By the end of the third week, the heart tubes have fused to form a tubular heart that is joined to vessels in the , , , and to form a primordial cardiovascular system.

Primary chorionic villi become as they acquire mesenchymal cores.

Before the end of the third week, capillaries develop in the secondary chorionic villi, transforming them into .

Cytotrophoblastic extensions from the stem villi join to form a that anchors

the chorionic sac to the endometrium.

Primary chorionic villi become secondary chorionic villi as they acquire .

Before the end of the third week, develop in the secondary chorionic villi, transforming them into .

Cytotrophoblastic extensions from the stem villi join to form a that anchors

the chorionic sac to the endometrium.

At the beginning of the fourth week, folding in the median and horizontal planes converts the flat trilaminar embryonic disc into a C-shaped, .

The formation of the head, caudal eminence, and lateral folds is a continuous sequence of events that results in a constriction between the and .

As the head folds , part of the endodermal layer is incorporated into the developing embryonic head region as the .

Folding of the head region also results in the oropharyngeal membrane and heart being carried and the developing brain becoming the most cranial part of the embryo.

As the caudal eminence folds , part of the endodermal germ layer is incorporated into the caudal end of the embryo as the .

The terminal part of the hindgut expands to form the .

Folding of the caudal region also results in the cloacal membrane, allantois, and connecting stalk being carried to the surface of the embryo.

Folding of the embryo in the horizontal plane incorporates part of the endoderm into the embryo as the .

The remains attached to the midgut by a narrow ( ).

During folding of the embryo in the horizontal plane, the primordia of the lateral and ventral body walls are formed.

As the expands, it envelops the connecting stalk, omphaloenteric duct, and allantois, thereby forming an epithelial covering for the .

The three germ layers differentiate into various and , so that by the end of the embryonic period, the beginning of the main organ systems have been established.

The external appearance of the embryo is greatly affected by the formation of the , , liver, somites, limbs, ears, nose, and eyes.

Because the beginning of most essential external and internal structures are formed during the to weeks, this is the most critical period of development. Developmental disturbances during this period may give rise to major

Reasonable estimates of the age of embryos can be determined from the day of onset of the , the estimated time of , ultrasound measurements of the chorionic sac and embryo, and examination of external characteristics of the embryo.

The period begins 8weeks after fertilization (10 weeks after the LNMP) and ends at birth.

It is characterized by rapid body growth and differentiation of tissues and organ systems.

An obvious change in the fetal period is the relative of head growth compared with that of the rest of the body.

At the beginning of the 20^th week, (fine downy hair) and head hair appear and the skin is coated with (a fatty cheesy substance).

The eyelids are closed during most of the fetal period, but they begin to reopen at approximately 26weeks.

At this time, the fetus is usually capable of , mainly because of the maturity of its respiratory system.

Up to 30weeks, the fetus appears reddish and wizened(wrinkled) because of the thinness of its skin and the relative absence of subcutaneous fat.

usually develops rapidly at 26 to 29 weeks, giving the fetus a smooth, healthy appearance.

The fetus is less vulnerable to the effects of drugs, viruses, and radiation, but

these agents may interfere with growth and normal functional development, especially of the and .

The physician can determine whether a fetus has a particular disease or birth defect by using various diagnostic techniques, such as amniocentesis(CVS), ultrasonography, and .

In selected cases, treatments can be given to the fetus, such as drugs to correct or thyroid disorders. Surgical correction of some birth defects in is also possible. (e,g, ureters that do not open into the bladder can be surgically corrected).

The placenta consists of two parts:

a larger fetal part derived from the and a smaller maternal part developed from the .

The two parts are held together by stem chorionic villi that attach to the cytotrophoblastic shell surrounding the chorionic sac, which attaches the sac to the decidua basalis.

The principal activities of the placenta are (synthesis of glycogen, cholesterol, and fatty acids), gas exchange (oxygen, carbon dioxide, and carbon monoxide), transfer of nutrients (vitamins, hormones, and antibodies), elimination of waste products, and endocrine secretion(e.g. ) for maintenance of pregnancy.

The fetal circulation is separated from the maternal circulation by a thin layer of extrafetal tissues, the .

This permeable membrane allows water, oxygen, nutritive substances, hormones, and noxious agents to pass from the mother to the embryo/fetus.

Excretory products pass through the from the fetus to the mother.

The fetal membranes and placentas in multiple pregnancies vary considerably, depending on the derivation of the embryos and the time when division of embryonic cell occurs.

The common type of twins is DZ twins, with two , two , and two that may or may not be fused.

MZ twins, the less common type, represent approximately one third of all twins;

they are derived from one zygote. MZ twins commonly have chorion, amnions, and placenta.

Twins with one amnion, one chorion, and one placenta are always monozygotic, and their are often entangled.

Other types of multiple birth (e.g. triplets) may be derived from one or more zygotes.

The umbilical vesicle and allantois are vestigial structures; however, their presence is essential to normal embryonic development.

Both are early sites of and both are partly incorporated into the embryo.

also originate in the wall of the umbilical vesicle.

The and are vestigial structures; however, their presence is essential to normal embryonic development.

Both are early sites of blood formation and both are partly incorporated into the embryo.

Primordial germ cells also originate in the wall of the umbilical vesicle.

The amnion forms and amniotic sac for amniotic fluid and provides a covering for the .

The amniotic fluid has three main functions:

to provide a for the embryo/fetus, to allow room for ,

and to assist in the regulation of fetal .

The is bounded laterally by pharyngeal arhes.

Each arch consists of a core of mesenchyme covered externally by ectoderm and internally by endoderm.

The original mesenchyme of each arch is derived from .

Later, migrate into the arches and are the major source of the connective tissue components, including cartilage, bone, and ligaments in the oral and facial regions.

Each arch contains an artery, cartilage rod, nerve, and a muscular component.

The disappear except for the first pair, which persists as the external acoustic meatus. The also disappear, except for the first pair, which becomes the tympanic membranes. The forms the tympanic cavity, mastoid antrum, and pharyngotympanic tube.

The is associated with the development of the palatine tonsil.

Externally, are separated by pharyngeal grooves.

Internally, the arches are separated by evaginations of the pharynx ( ).

Where the ectoderm of a groove contacts the endoderm of a pouch, are formed.

The thymus is derived from the pair of , and the parathyroid glands are formed from the and pairs of pouches.

The develops from a down growth from the floor of the primordial pharynx in the region where the tongue develops.

The parafollicular cells(C cell) in the thyroid gland are derived from the , which are derived mainly from the fourth pair of pharyngeal pouches.

Cervical cysts, sinuses, and fistulas may develop from parts of the second pharyngeal groove, the , or the second pharyngeal pouch that fail to obliterate.

An results when the gland fails to descend completely from its site of origin in the tongue.

The may persist, or remnants of it may form and ectopic thyroid tissue masses.

Infected cysts may perforate the skin and form thyroglossal duct sinuses that open anteriorly in the

median plane of the neck.

is a common birth defect.

Although frequently associated with cleft palate, cleft lip and cleft palate are etiologically distinct defects that involve different developmental processes occurring at different times.

Cleft of the upper lip results from failure of mesenchymal masses in the medial nasal and maxillary prominences to merge, whereas cleft palate results from failure of mesenchymal masses in the palatal processes to meet and fuse.

Most cases of cleft lip, with or without cleft palate, are caused by a combination of genetic and environmental factors　(multifactorial inheritance; see Chapter 20)

Cleft of the upper lip results from failure of mesenchymal masses in the and to merge, whereas cleft palate results from failure of mesenchymal masses in the to meet and fuse.

Birth defects are any type of structural abnormalities that are present at birth.

The defect may be macroscopic or microscopic and on the surface or within the body.

The four clinically significant types of birth defect are , , , .

Approximately % of neonates have an obvious major defect.

Additional defects are detected after birth; the incidence is approximately 6% among 2-year-old children and 8% among 5-year-old children.

Other defects (approximately 2%) are detected later (e.g during surgery, dissection, autopsy)

Birth defects may be single or multiple and have minor or major clinical significance.

defects occur in approximately 14% of neonates. Theses defects have no serious medical consequences, but they alert clinicians to the possibility of an associated major defect.

Ninety percent of infants with multiple minor defects have one or more associated major defects.

Of the 3% of neonates with a major birth defect, multiple major anomalies are found in 0.7%.

Major defects are more common in (up to 15%) than they are in neonates(up to 3%)

Some birth defects are caused by (e.g. chromosomal abnormalities, mutant genes),

and a few defects are caused by (e.g. infectious agents, environmental chemicals, drugs),

but most common defects result from a complex interaction between .

The cause of most birth defects is unknown.

During the of development, teratogenic agents usually kill the embryo or have no effects.

During the , teratogenic agents disrupt development and may cause major birth defects.

During the , teratogens may produce morphologic and functional abnormalities, particularly of the brain and eye.