At the end of cleavage, the insect embryo consists of a single epithelial layer (the blastoderm) surrounded a central yolk (as opposed to a blastocoel cavity). As opposed to gastrulating at a single blastopore point, the insect embryo gastrulates along the ventral midline.

The mesoderm arises from invaginating cells of the ventral furrow, and the endoderm arises from cells at each end of the invaginating mesoderm. Fog, concertina and DRhoGEF are the three genes with identified roles in the cell constriction movements of gastrulation.

Sea urchin gastrulation begins with flattening of the vegetal plate on the side of the blastula opposite the apical tuft. Thus, the embryo already has a polarity prior to onset of the morphogenetic changes of gastrulation. A group of blastula vegetal pole cells ingress into the blastocoel cavity; these ingressing cells are called the primary mesenchyme, forming the mesoderm and eventually the skeleton.

After the sea urchin vegatal plate flattens, it invaginates to form an inner pouch called the archenteron. The archenteron gives rise to the endoderm and eventually forms the digestive tract. The opening of the archenteron to the outside is the blastopore, a term reused in amphibian gastrulation.

Next, the archenteron elongates via convergent extension and then ingresses. The number of cells in the archenteron does not increase much during convergent extension, but instead become stretched out to transform a short and fat invagination into a longer and narrower one. Following convergent extension, secondary mesenchyme cells at the tip of the archenteron extend filipodia between the ectoderm and endoderm (primary mesenchyme cells do the same) to form the mesoderm, which forms muscle.

The fish embryo is telolecithal and contains lots of yolk. Cleavage results in a flat mass of cells on one side of the embryo. Gastrulation follows, consisting of two major processes: epiboly and involution. In epiboly, cells at the animal pole (termed the blastoderm or blastodisc in fish) spread out and expand to cover the yolk at the vegetal pole (in amphibians, yolk-filled cells are covered instead).

During epiboly, cells at the edge of the blastoderm began to involute (fold in) and later form the endoderm and mesoderm, with non-involuted cells forming the ectoderm. During epiboly and involution, blastodermal cells converge on one side of the embryo (the future dorsal region) and intercalate via convergent extension to form the axis.

Like fish eggs, amphibian eggs are telolecithal and have bulky yolk at the vegetal pole which prohibit the simple invagination seen in sea urchins. In fish and amphibians, the blastopore is located just above the largest of the yolk. As animal pole cells increase in area and spread over the embryo (via epiboly), the lip of the blastopore widens. Beginning at the dorsal side, cells at the edge of the blastopore lip begin moving inward via involution.

The archenteron — which gives rise to the endoderm — is now formed from the vegetal yolky cells that were surrounded via epiboly, and by the cells involuted from the dorsal lip. Other cells that involute over the dorsal lip form the intervening mesoderm. Convergent extension is responsible for the capacity for a small ring of cells just above the dorsal lip to lengthen along the anterior-posterior axis and form the internal endodermal and mesodermal sheets.

Gastrulation in amphibians requires fibronectin, a glycoprotein in the extracellular matrix that provides a substrate for cell migration. Fibronectin binds to integrins (receptors for extracellular matrix proteins). By binding extracellular fibronectin and intracellular actin, integrins link the external environment to the cytoskeleton. In amphibians, the surfaces of cells in the blastocoel roof become covered with oriented fibronectin fibrils. Blastocoels injected with fibronectin antibodies do not undergo gastrulation; the cell layer near the dorsal lip folds repeatedly without going inward.

Bird eggs are extremely telolecithal. After cleavage, the blastodisc is a small group of cells atop the yolk. The blastodisc consists of two layers: the epiblast (surface layer, from which the developing embryo derives) and hypoblast (below the epiblast). Cells converge at one edge of the epiblast, forming a line (the primitive streak) that defines the anterior (aka Hensen’s node) and posterior of the embryo.

A slight depression forms in the midline of the primitive streak, and cells ingress through the depression into the space between the epiblast and hypoblast. Cells which ingress toward the anterior (near the node) form the notochord (a mesoderm derivative) and some endoderm; cells which ingress toward the posterior form more mesoderm and endoderm. The head end develops first. The node and the primitive groove, as the passages for cells on there warm to form internal layers, are homologous to the blastopore.

Mammalian gastrulation begins with formation of a primitive streak in the epiblast and continues much like birds. Mammalian eggs have little or no yolk, but mammalian gastrulation is nonetheless similar to bird gastrulation due to evolutionary remnants from ancestral reptiles that laid very yolky eggs.

Identical (monozygotic) twins can arise in mammals by formation of two primitive streaks in the epiblast; Siamese twins occur when two primitive streaks do not completely separate.

Invagination, involution, ingression and convergent extension are the types of cell movement during gastrulation. Invagination and involution maintain epithelium.

In amphibians, the dorsal lip of the blastopore invaginates, involutes and then spreads out between the ectoderm and endoderm to form the mesoderm.

Sea urchin embryos invaginate to form mesoderm. Birds and mammals’ blastodisc ingresses at the primitive groove to give rise to mesoderm.

In invagination, the epithelium buckles inward like a finger poking into a soft ballon, thus forming an invagination. More technically, groups of contiguous epithelial cells actively constrict at their apical pole by contraction of the band of actin microfilaments located there. Thus, the epithelial sheet folds in forming a tubular (or vesicular) endoderm with its apical surface facing a lumen.

If invagination happens passively, for example in the wake of a neighboring population of cells that actively invaginates, it is called involution. This movement can also be seen in sea urchin embryos. Most morphogenetic movements in which cells withdraw from the surface to form inner tubular structures are achieved by a combination of invagination and involution.

In ingression, tight and adherens junctions are lost, epithelial cells become mesenchymal and this mesenchyme reforms a polarized epithelium. This occurs in higher vertebrates, including birds and mammals. Birds form a disc-shaped blastula whose cells migrate toward the midline when gastrulation begins. These cells pile up to form a visible structure called the primitive streak. The primitive streak ingresses inward; ingressed cells spread out laterally and reorganize into an epithelium (endoderm) that gradually spreads around the yolk.

Epithelia change in length and width by convergent extension and individual cells slide past each other. Cells that start out positioned beside each other in one row intercalate. Thereby, the shape of an epithelium that was wide and short before the convergent extension changes to narrow and long afterwards. More technically, a cell layer elongates in one axis via intercalation (interdigitation) of cells along a perpendicular axis. Two types of convergent extension are meso lateral intercalation and radial intercalation.

Cellular intercalation during convergent extension involves degradation and reformation of tight and adherens junctions. Many organisms, including amphibians like Xenopus, use convergent extension to convert the embryo from a sphere to an elongated rod more like the final shape of the mature organism.

Epiboly is the process in which the layer of cells spread out and expand to cover the yolk or yolk-filled cells (in fish and amphibians) at the vegetal pole. This spreading of the cells occurs through an increase in area due to a flattening of individual cells and an intercalation of cells.

Experiments Demonstrating Role
1	Marginal zone cells do not form mesoderm when isolated and cultured until after the 64-cell stage.
2	Removing the marginal zone and recombining animal and vegetal caps, dorsal mesoderm arises from animal cap cells nearest the vegetal cap, and vegetal cells opposite the SEP.
3	Recombining animal caps with various vegetal blastomeres: dorsal vegetal blastomere cells induced dorsal mesoderm; ventral vegetal blastomere cells induced ventral mesoderm. Also, dorsal mesoderm induced ventral mesoderm to become lateral mesoderm. Thus, signaling must be involved in dorsal and central cell fates.
4	Fertilized eggs did not form a D/V axis when irradiated at the vegetal pole, but could be restored by a single vegetal- and dorsal-most cell. Thus, this vegetal- and dorsal-most structure must induce other regions to become dorsal mesoderm. This vegetal-most and dorsal-most region first induces the primary organizer (which then induces other tissues) and was called the Nieuwkoop center.
5	To determine which part of the embryo acts to organize the mesoderm into dorsal structure and to induce neural tube formation, the dorsal lip of the blastopore of an early gastrula from a light-colored newt was transplanted into an early gastrula of a dark-colored newt. The donor tissue formed a second embryonic axis. The notochord of this second embronic axis was composed entirely of graft (donor) tissue, while the neural tube and somites were composed only partly of graft (donor) tissue and the kidney tubules and gut of the new axis were composed entirely of host tissue. Spemann and Mangold concluded that the graft tissue induced a new embryonic axis. This structure is named Primary organizer.

The Nieuwkoop Center is the dorsal- and vegetal-most cell of the early blastula. It gives rise to the Spemann Organizer, which is the dorsal lip of the blastopore. The Spemann Organizer has a dorsalizing effect, and together with the Sperm Entry Point (SEP) gives rise to the dorsal/ventral axis.

The first signals are the vegetal maternal mRNAs Veg1 and VegT. The second signal is the dorsal-most protein β-catenin, which accumulates via cortical rotation. This induces formation of the Spemann Organizer and the Nieuwkoop Center. The Spemann Organizer is responsible for the third signal by expressing Noggin, Chordin and Follistatin, which bind and inhibit the ventralizing factors BMP-4 and Frzb.

Drosophila Dpp is homologous to BMP-4. Dpp activity is highest at the dorsal region. Drosophila Sog is homologous to Chordin. [Sog] is highest at the ventral region, where it binds and inactivates Dpp. These Drosophila genes can be replaced by their Xenopus homologs, but their activity along the dorsal-ventral axis is inverted. This switching occurred in a vertebrate ancestor. Also, Drosophila Wingless is homologous to Wnt-8.