The movements are called morphogenic movements. All movement in gastrulation is driven by one or a combination of changes in cell shape and affinity for neighboring cells or the extracellular matrix. They include Epiboly – Descending of dividing cells to cover other cells. Emboly – Upward movement of dividing cells underneath the other cells, Involution – Inward migration of blastomeres to go into the blastocoel. Invagination – It is the inward folding of an epithelium (of blastula wall) caused by a change in shape without a change in affinity. The blastopore is created by invagination. Delamination – Separation of a sheet of cells from blastula. During gastrulation a cavity called archenterone occurs inside the gastrula, which is the future alimentary canal.
It opens to the outside through blastopore which later on closes. Blastopore is absent in amnions. In some vertebrates especially mammals the gastrula is formed by delamination. The three germ layers ectoderm, mesoderm and endoderm are formed by the differentiation of cells of blastula (blastocyst).
Endoderm in a mammalian embryo is formed as the second layer by delamination of cells from the inner cell Trophoblast Embryonic disc mass. The endodermal cells line the lumen of primitive gut (or archenterone). After the detachment of endoderm, the remaining inner cell mass is called embryonic disc. Embryonic disc is formed by the stretching knob like inner cells due to which its cells become columnar and come to lie as a regular layer. Formation of mesoderm initiates only after the formation of endoderm as a distinct layer.
Mesoderm proliferates from caudal end of embryonic disc. After the emergence of the mesodermal layer, the rest of cells of embryonic disc orient themselves in layer to form ectoderm. Besides these, other events occur as well during gastrulation like – i. Notochord is formed (neurulation) ii. Embryo elongates somewhat in the cranial caudal direction and shows a marked expansion of the blastoderm in the cranial end. In this, formation of primitive streak is the next change in the blastoderm which is usually formed in place of blastopore in amniotes. Primitive streak is represented by an opaque band of presumptive ectoderm in blastodisc (reptiles and birds) and inner mass of blastocyst in mammals.
It is involved in mesoderm movement. The primitive streak elongates at its caudal end by the addition of cells. The cranial end thickens to form primitive knot (Hensen’s node).