An embryo is defined as any organism in a stage before birth or hatching, or in plants, before germination occurs. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.
All triploblastic animals (Three germ layers / all Bilaterians animals ) can be further divided into Protostomes and Deutereostomes based on the fate of first opening of the development. If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome and if the first pore becomes the anus then it is a deuterostome.
The protostomes include majority of invertebrate animals, such as insects, worms, annelids and molluscs, while the deuterostomes includes animals such as echinoderms, urochordates, hemichordates, cephlaochordates, xenoturbella and vertebrates.
In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, whereas protosomes show spiral cleavage. Another difference in protosomes is that solid mass of mesoderm split to form coelom whereas folds of archenteron leads to coelom formation in deuterostomes.
In due course, the blastula changes into a more differentiated structure called the gastrula.
The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:
– The innermost layer, or endoderm, gives rise to the digestive organs, lungs and bladder.
– The middle layer, or mesoderm, gives rise to the muscles, skeleton and blood system.
-The outer layer of cells, or ectoderm, gives rise to the nervous system and skin.