The Stages of Prenatal Development

• Induction– During prenatal development differences in

cytoplasmic composition trigger changes in genetic activity

• Gestation periods– Three trimesters

Embryonic and Fetal Periods

SECTION 29-4 The First Trimester

• Cleavage – Zygote becomes a preembryo then a blastocyst

• Implantation – Blastocyst burrows into uterine endometrium

• Placentation – Blood vessels form around blastocyst and placenta develops

• Embryogenesis – Formation of a viable embryo

The First Trimester

• A series of cell divisions that subdivides the cytoplasm of the zygote– Trophoblast – outer layer of cells – Inner cell mass – cluster of cells at one end of

blastocyst

Cleavage and blastocyst formation

Figure 29.2

Cleavage and Blastocyst Formation

• Occurs about 7 days after fertilization• Trophoblast enlarges and spreads

– Maternal blood flows through open lacunae

• Gastrulation– Embryonic disc composed of germ layers

• Endoderm• Mesoderm• Ectoderm

Implantation

Stages in Implantation

Figure 29.3

Figure 29.4

The Inner Cell Mass and Gastrulation

• Gastrulation – By day 12 surface cells move toward the

primitive streak – A third germ layer forms

• The three germ layers are:– Ectoderm – superficial cells that did not migrate– Endoderm – cells facing the blastocoele– Mesoderm – migrating cells between ectoderm

and endoderm

Germ layers

• Four extraembryonic membranes:– Yolk sac – Amnion– Allantois– Chorion

Extraembryonic Membranes

Figure 29.5a, b

Extraembryonic Membranes and Placenta Formation

Figure 29.5c, d

Extraembryonic Membranes and Placenta Formation

Figure 29.5e

Extraembryonic Membranes and Placenta Formation

• Yolk sac– Important site of blood cell formation

• Amnion– Encloses fluid that surrounds and cushions

developing embryo

• Allantois– Eventually becomes bladder

• Chorion

Embryo Anatomy