Fig.13. The ways of gastrulation: a – invagination; b – epiboly; c – immigration; d – delamination.

In forms with free swimming larvae (Anamniota) the whole blastula is transformed into the embryo, whereas in terrestrial forms (Amniota) some of the blastula forms extra-embryonic membranes.

In Amphioxus gastrulation proceeds by invagination: part of the blastula wall sinks below the surface so that a double-walled cup – the gastrula – is formed. The central cavity of the cup is a primitive gut – gastrocele, or archenterons, and the opening to the exterior is termed blastopore. Further rearrangement of the gastrula cells gives rise to a third layer within the double wall (Fig.14).

Fig.14. Gastrulation in Amphioxus.

In Amphibian gastrulation occurs through combination of invagination and epiboly(Fig.15).

Fig.15. Gastrulation in Frogs

In reptiles, birds and mammals gastrulation proceeds by combination of delamination and immigration.

For practical reasons most of experiments on the mechanisms of vertebrate development have been carried out on amphibia. At the end of cleavage the amphibian blastula consists of a hollow sphere of cells enclosing a cavity – the blastocele. At the animal pole the cells are small and pigmented; at the vegetative pole they are large, pale and laden with yolk. If small regions of the surface of the blastula are stained with a vital dye, their fate in subsequent development may be followed. It is thus possible to say, for instance, that the cells of the animal pole normally give rise to ectoderm. From the results of many such experiments a map indicating presumptive fates may be drawn .

Gastrulation begins when a small indented crease, the dorsal lip of the blastopore, appears on one side of the blastula. The crease is formed by cells changing shape and pushing inward from the surface (invagination). Gastrulation entails spreading on the animal pole cells (the future ectoderm) over the vegetative cells (a process called epiboly). Sheets of cells at junction of vegetal and animal pole begin migrating in along the blastocoele. The site of this invagination is the blastopore(primitive mouth), which forms on the dorsal side of the embryo. .The presumptive (future) endoderm, notochord and mesoderm of the marginal zone move inside at this site through the dorsal lip of the blastopore. (Fig.15). These cell movements result in obliteration of the blastocele and the formation of another cavity – the gastrocele (archenteron) –which will become the gut. The gastrocele communicates with the exterior through the blastopore. The roof of the gastrocele consists of the chorda-mesoderm material, which now lies in contact with the overlying presumptive nerve tube.

The region of the blastopore through which invagination of chorda-mesoderm is occurring, is its dorsal lip. General body mesoderm proliferates from the lateral lips of the blastopore which approach each other and fuse, forming the primitive streak. The original blastopore is thus divided by the primitive streak into dorsal and ventral orifices (foramens).

Expressed in general terms, gastrulation is a complex series of morphogenetic movements of cell groups leading to the establishment of the primary germ layers and bringing the presumptive organ rudiments (notochord, nerve tube, somites, and primitive gut) into their definitive position. Other major changes during gastrulation:

1. Heavy RNA transcription using embryonic genes;

2. Cells starts major differentiation processes losing their pluripotentiality.

Gastrulation is regulated by homeobox genes and their proteins – morphogens.