Gaetrula (Gr. Gaster - Stomach)

Gaetrula (from the Greek "gaster" - stomach) is an early stage of embryonic development that occurs after the blastula. At this stage, the body consists of two layers - ectoderm and endoderm, and contains two cavities - the blastocoel and the cavity of the primary intestine. The cavity of the primary intestine is formed by invagination of the ectoderm and communicates with the external environment through the blastopore.

During this stage of development, further formation of the organism occurs. The ectoderm gives rise to the nervous system, skin, and some other tissues, while the endoderm forms internal organs such as the liver, lungs, and stomach. Gaetrula is an important stage in the development of the embryo, since the further formation of the organism depends on it.

During the development of the haetrula, three germ layers are formed - ectoderm, mesoderm and endoderm. The mesoderm forms between the ectoderm and endoderm and gives rise to many tissues and organs such as muscles, the circulatory system and the kidneys.

It is important to note that the haetrula is a common developmental stage for all multicellular animals, including humans. This means that all animals go through this stage of development before further formation of their organisms begins.

In conclusion, the haetrula is an important developmental stage of embryonic development. At this stage, the organism is formed from two layers - ectoderm and endoderm, and two cavities are formed - the blastocoel and the cavity of the primary intestine. The further formation of the body and the formation of three germ layers - ectoderm, mesoderm and endoderm, which will give rise to many tissues and organs in the body, depend on the haetrula.