Embryonic Stage

Gastrulation

Late in the second week after gastrulation

Gastrulation

Occurs when a blastula folds inwards and enlarged

3 germ layers of gastrula

1. Ectoderm

2. Mesoderm

3. Endoderm

Final phase of gastrulation

Formation of the primitive gut

Complete gastrulation

With a functioning digestive tube

Endoderm

Inner layer

Mesoderm

Middle layer

Ectoderm

Outer layer

Endoderm

Forms the cells of internal glands and organs (lungs, intestines, thyroid, pancreas, and bladder)

Mesoderm

Forms the cells of the heart, blood, bones, muscles, and kidneys

Ectoderm

Forms the cells of the epidermis, nervous system, eyes, inner ears, and many connective tissues

Ectoderm

Gives rise to the:

• Epidermis

• Glands of the skin

• Some cranial bones

• Pituitary and adrenal medulla

• Nervous system

• Mouth

• Anus

Mesoderm

Gives rise to:

• Connective tissues

• Bone

• Cartilage

• Blood endothelium of blood vessels

• Muscles

• Synovial membranes

• Serous membranes

• Kidneys

• The lining of gonads

Endoderm

Gives rise to:

• Lining of the airways and digestive system

• Digestive, endocrine, and adrenal cortex glands

Neurulation

Occurs during weeks 3-4 after fertilization

Neurulation

Embryo develops strucutres that will eventually become the nervous system

Organogenesis

Organs develop within the newly formed germ layers

Organogenesis

Start to develop during the third to eigth weeks following fertilization

Heart

First functional organ to develop in the embryo

Week 5

• Embryo measures about 4 mm (0.16 in). In length

• Has begun to curve into a C shape

Week 5

• Pharyngeal arches form (develop into face and neck)

• The inner ears begin to form

• Arm buds are visible

• The liver, pancreas, spleen, and galbladder start to form

Week 6

• Embryo measures about 8 mm (0.31 in.) in length

Week 6

• The eyes and nose start to develop

• Leg buds form and the hands form as flat paddles at the ends of the arms

• The precursors of the kidneys begin to form

• The stomach starts to develop

Week 7

• The embryo measures about 13 mm (0.51 in.)

Week 7

• Lungs begin to form

• The arms and legs have lengthened; have started to develop digits

• The lymphatic system starts to develop

• The primary prenatal develoment of the sex organs begins

Week 8

• Final week of the embryonic stage

• Embryo measures about 20 mm (0.79 in.)

Week 8

• Nipples and hair follicles begin to develop

• External ears start to form

• The face takes on a human appearance

• Fetal heartbeat and limb movements can be detected

• All essential organs have at least started to form

Extraembryonic structures

1. Placenta

2. Chorion

3. Yolk sac

4. Amnion

Placenta

Temporary organ that provides a connection between a developing embryo and the mother

Placenta

Conduit from the maternal organism to the offspring for the transporr of nutrients, oxygen, antibodies, hormones, and other needed substances

Plancenta

Passes waste products from the offspring to the mother’s blood for excretion

Placenta

Starts to develop after the blastocyst has implanted in the uterine lining

Placenta

Consists of both maternal and fetal tissues

Maternal portion of placenta

Develops from the endometrial tissues lining the uterus

Fetal portion of the placenta

Develops from trophoblast, which forms the chorion

Chorionic villi

Allows the exchange of substances between the fetal blood and maternal blood without the two sources or blood actually intermixing

Chorion

Membrane formed by extraembryonic mesoderm and trophoblast

Chorion

Undergoes rapid proliferation and forms the chorionic villi

Chorionic villi

Invades the uterine lining and help form the fetal portion of the placenta

Yolk sac

Membranous sac attached to the embryo and formed by cells of the hypoblast

Yolk sac

Provides nourishment to the early embryo

Amnion

Membrane forms from extraembryonic mesoderm and ectoderm

Amnion

Creates the amniotic sac around the embryo