Protostomados and deuterostomas

The blastopore can give rise to the mouth or anus. When it gives rise to only the mouth or both the mouth and the anus, animals are called protostomies (proto = first). This is the case of worms, mollusks and arthropods.

When the blastopore gives rise to the anus, animals are called deuterostomies (deutero = later). This is the case of the echinoderms and chordates.

Amphioxus Organogenesis

The third stage of embryonic development is organogenesis, which is characterized by organ differentiation from embryonic leaflets formed in gastrulation. The following scheme represents the early phase of organogenesis: neurulation. After neurulation, the embryonic leaflets continue to differentiate, leading to specialized adult tissues.

From the ectoderm is distinguished the neural tube, which presents inside the neural canal. The endoderm gives rise to the digestive tract. The mesoderm gives rise to somites and notochord. Somites are laterally arranged cell blocks on the back of the embryo, and the notochord is a massive structure located just below the neural tube.

The mesoderm delimits cavities called celomas.