Gastrulation in animals
Gastrulation is the process through which the three primary germ layers- ectoderm, mesoderm and endoderm are ultimately segregated. The blastula possesses polarity, and bilateral symmetry. It contains predetermined cell area, which would become the future ectoderm, mesoderm, endoderm, notochord and the neural plate.
GASTRULATION IN AMPHIOXUS
The animal pole of the blastula corresponds roughly to the front end of the future embryo. The animal and vegetal poles are the future ectoderm and endoderm respectively. In between, is a girdling zone, which is subdivided into prospective mesoderm, notochord and neural plate.
Invagination of vegetal cells is followed by involution of cells, around the margin of the double walled cup, thus being formed. The continuation of these movements carries endoderm, mesoderm and notochord to the interior and obliterates the original blastocoel.
The central cavity now formed is archenteron and its mouth at the vegetal pole is blastopore. At this period, the young embryo is termed a gasturla.
The margin around which involution took place is the lip of the blastopore. Backward growth of this lip-region, lengthens the embryo and upward growth of the ventral lip elevates the blastopore, thus tilting up the mesodermal portion.
Now, the roof of the archenteron consists of a median strip of notochordal cells, flanked on each side by a strip of mesodermal cells (which were tilted up by the elevation of the ventral lip of the blastopore). The entoderm then closes the dorsal defect caused by the loss of notochord and other mesoderm and thus produces the definitive, tubular gut.
The cells left on the external face on the gastrula form the ectoderm. Dorsally they constitute the neural plate while the rest of the area forms the general covering of the embryo, which will become epidermis.
GASTRULATION IN CHICK
Gastrulation in chicks is confined to the blastoderm, which contains the cells of all the three future germ layers. The process, as a whole, is accomplished in two stages.
The gastrula is formed by a process involution of the blastoderm. The caudal part of blastoderm rolls and tucks itself under the blastodermal cells. It divides until a new inner layer, the endoderm and an outer layer, the ectoderm are formed. The region where the ectoderm and endoderm meet represent dorsal lip of the blastopore of amphioxus. The space between the endoderm and the yolk is the archenteron.
At the end of gastrulation a median band appear posteriorly on the surface of the blastoderm. It is a linear thickening of ectoderm and acquires an anterior knob called primitive knot. The thickening is called as the primitive streak which encloses a primitive groove and close to the primitive knot the primitive pit.
The major function of the primitive streak is the formation of the third germ layer, the mesoderm. The adjacent cells on the surface of the blastoderm move medially enter the primitive streak and leave it as mesodermal cells.
Soon the germinal disc is comprised of an outer ectoderm and an inner endoderm and the mesoderm, which is between the ectoderm and endoderm. Due to the migration of cells from the primitive streak on both the sides, a middle pole like invagination occurs on the median line of the streak, giving rise to the primitive groove folded by the primitive folds. From the primitive knot, a column of cells arise from the head process which becomes the notochord.
GASTRULATION IN MAMMALS
The inner cell mass delaminates into an inner layer of ectoderm. The endoderm forms the roof of the yolk sac and the two layers together are spoken as embryonic disc.
The primitive streak appears caudally on the upper face of the disc and proliferation of the cells in this area produces the cells of the mesoderm, which spread out in all directions between the other two germ layers. This forms the middle germ layer or mesoderm and this differentiates into the axial cord, the notochord and two lateral sheets.
The sheets split tangentially into two layers, the somatic and splanchnic layers to enclose the coelom or body cavity. The ectoderm and the somatic mesoderm together is the somatopleurae or body wall and endoderm and the splanchnic mesoderm is the splanchnopleurae or gut wall.
