Gastrulation is the rearrangement process that forms the three germ layers and sets the stage for tissue formation and organogenesis (formation of the organs).

• During gastrulation the endoderm and mesoderm move to the inside of the embryo where they will give rise to the internal organs.
• The ectoderm is spread on the outside surface and it will give rise to the epidermis and nervous system.
• Thus, the three germ layers consisting of the ectoderm, mesoderm, and the endoderm are first seen during gastrulation.

• Just before gastrulation begins, the blastula consists of a single layer of cells surrounding a central fluid-filled bastocoel.
• The future mesoderm occupies the most vegetal region with the future endoderm adjacent to it.
• The rest of the embryo gives rise to the ectoderm.

These mesenchymal cells become detached from each other due to loss of cadherin and migrate into the blastocoel as single cells.

• This is followed by a process referred to as invagination or the infolding of a region of cells.

The endodermal cells invaginate into the blastocoel to form the embryonic gut which is also referred to as the archenteron

• The mesenchymal cells form long filopodia which make contact with the blastocoel wall and they assist in pulling the elongating gut across the blastocoel until it eventually comes in contact and fuses with the mouth region.

At this stage the embryo is referred to as a gastrula and the formation of the ectoderm, the interstitial mesoderm or future skeleton of the sea urchin, and the endoderm or the digestive system can be visualized.

• The main processes of amphibian gastrulation are involution and epiboly.

• Involution is the inward movement of an expanding outer layer so that it spreads over the internal surface.
• Epiboly is the movement of epithelial sheets or the ectoderm to enclose the embryo.

• In the late Xenopus blastula gastrulation is initiated by the formation of bottle cells in the blastopore region which is followed by involution of mesoderm.
• At the same time, the animal cap spreads downward.
• The mesoderm continues to involute as the blastocoel shrinks and the endoderm now forms the roof of the forming archenteron or primitive gut.
• This process continues until the ectoderm covers the outside of the gastrula to converge at a plug of yolky vegetal cells.
• Once the gastrula is formed, the embryonal tissues are in their proper positions with the ectoderm on the outside, the mesoderm in the middle, and the endoderm on the inside.

• Otherwise the process of avian gastrulation is similar to mammals.
• Cleavage in the chick embryo results in the formation of a disc of cells referred to as the blastodisc which is the chick's blastoderm.
• A layer of cells referred to as the hypoblast from the blastoderm forms over the yolk mass.

This will give rise to the extra-embryonic structures such as the stalk of the yolk sac.

• The upper layer of the blastodisc is lined with epiblast cells.

These remaining blastoderm cells will form the embryo proper.

• The onset of gastrulation is marked by the development of the primitive streak.

• This is the forerunner of the antero-posterior axis.
• It arises from a thickening of the epiblast cells and is similar to the blastopore of amphibians.

• Future mesodermal and endodermal cells migrate through the primitive streak into the interior of the blastoderm through a process referred to as ingression.

Ingression is the migration of individual cells from the surface layer into the interior of the embryo.

• The cells of the epiblast continue to ingress and then move outward again to give rise to the mesoderm and the endoderm.
• The mesoderm and endoderm move inward and a yolk stalk is formed.
• Cells begin to condense above the yolk stalk and form what is referred to as Henson's node.
• Cells from Henson's node give rise to the notochord and contribute to the somites.

• The notochord is mesoderm that runs from the head to the tail of the early embryo and lies beneath the future central nervous system.
• Somites are segmented blocks of mesoderm on either side of the notochord.

They give rise to the body and limb muscles, the vertebral column, and the dermis.

• As the notochord is formed, the neural tube begins to develop to form the future brain and spinal cord.

• Thus, the mammalian embryo obtains nutrients directly from the mother and does not rely on stored yolk.
• This has required the development of a fetal organ referred to as the placenta which is derived primarily from embryonic trophoblast cells.
• At implantation, the trophoblast invades the uterine wall.

The inner cell mass divides into two layers, the epiblast and the hypoblast.

• This formation of two cellular layers from one is referred to as delamination.
• The epiblast is the primitive ectoderm of the embryo and the hypoblast will form the yolk sac.

• A chorion also develops from the surrounding trophoblast cells and will fuse with the uterine wall to form the placenta.
• An amnion and the amnionic sac provide mechanical protection by providing a protective chamber for the embryo during development.
• Thus, the embryo is encased in amnion and is further shielded by the chorion.

Numerous villi extend from the outer surface of the chorion and these villi allow the chorion to have a large area exposed to the maternal blood by diffusion.

• Thus, the mother provides the fetus with nutrients and oxygen and the fetus sends its waste products into the maternal circulation.

These waste products consist primarily of carbon dioxide and urea.

• The chorion can also act as an endocrine organ.

The syncytiotrophoblast portion produces chorionic gonadotropin, chorionic progesterone, and chorionic somatomammotropin.

• The gonadotropin hormone stimulates other cells in the placenta and ovary to produce progesterone that keeps the uterine wall thick and full of blood vessels.
• The chorion itself also produces the progesterone steroid and somatomammotropin promotes maternal breast development during pregnancy.

• The chorion also produces proteins and certain types of lymphocytes that suppress the potential maternal immune reaction against the fetus.
• Thus, the placenta provides nutrients and physical support for the fetus and also regulates hormones and the immunological aspects of embryo development.