The floor of the amniotic cavity

Amniotic cavity

The amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid. The amniotic cavity is formed by the fusion of the parts of the amniotic fold, which first makes its appearance at the cephalic extremity, and subsequently at the caudal end and sides of the embryo...

 is formed by the embryonic disk (or embryonic disc) composed of a layer of prismatic cells, the embryonic ectoderm

Ectoderm

The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...

, derived from the inner cell-mass and lying in apposition with the endoderm

Endoderm

Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...

.

In humans, it is the stage of development that occurs after implantation and prior to the embryonic folding (eg. seen between about day 14 to day 21 post fertilization). It is derived from the epiblast

Epiblast

In amniote animal embryology, the epiblast is a tissue type derived either from the inner cell mass in mammals or the blastodisc in birds and reptiles. It lies above the hypoblast.-Mammals:...

 layer, which lies between the hypoblast

Hypoblast

The hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.Extraembryonic endoderm is derived from hypoblast.-External links:...

 layer and the amnion

Amnion

The amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish , which are consequently termed “Anamniota”. The primary role of this is the protection of the...

. The epiblast layer is derived from the inner cell mass

Inner cell mass

In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus...

. Through the process of gastrulation

Gastrulation

Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...

, the bilaminar embryonic disc becomes trilaminar. The notochord

Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...

 forms thereafter. Through the process of neurulation

Neurulation

Neurulation is the stage of organogenesis in vertebrate embryos, during which the neural tube is transformed into the primitive structures that will later develop into the central nervous system....

, the notochord induces the formation of the neural tube

Neural tube

In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...

 in the embryonic disc.

External links

• Diagram at manchester.ac.uk

Category:Embryology