EMBOLY -

The embolic morphogenetic movements are concerned with the inward migration of prospective endodermal and mesodermal blastomeres from the external surface of blastula.

The movement of mesodermal and endodermal blastomers may take place by any of the follwoing methods -

(i) Invagination -

1. It is the easiest way to gastrulation. It is the process in which the cells from the vegetal side of blastula sinks in or move downwards or are pushed into the interior of blastocoel.
2. Thus blastocoel becomes smaller and smaller as the invagination gets higher, and a new cavity is formed which is called Archenteron cavity. This archenteron cavity forms the mesenteron of alimentary canal.
3. The apperture of archenteron cavity is called blastopore.
4. This blastopore gives rise to mouth in protostomia animals e.g. Helminthes Annelida, Mollusca tec.
5. In some animals blastopore gives rise to the anus. e.g. Deuterostomians.
6. Complete gastrulation in amphioxus takes place by invagination.
7. Invagination is also found in frog.
8. The megameres of blastula of frog undergo invagination and become bottle-shaped and protrude inside the blastocoel.

(ii) Involution -

1. It is the Rolling movement of the blastomeres from the surface to the inner side of the embryo.
2. In frog, mesoderm & endoderm are formed by involution.
3. All the micromeres divide and reach the dorsal-lip of blastopore.
4. The collective movement of blastomeres towards common point is termed as convergence.
5. From here the blstomeres enter inside the blastocoel cavity while performing the rolling movement.This is termed as involution.
6. As a result of involution, the blastocoel cavity is completely obliterated and a new cavity archenteron is formed.