Question

Primary germ layers are
A. Trophoblast, ectoderm, mesoderm and endoderm
B. Endoderm, trophoectoderm and trophomesoderm
C. Trophomesoderm and trophoectoderm
D. Ectoderm, mesoderm and endoderm

Answer 1

A germ layer is a group of specialized cells in a developing embryo. The germ layers contribute to the formation of all organs and tissues of the living organism. Through the process of gastrulation, the germ layers develop early during the embryonic life. The different layers of cells in an embryo interact with each other and give rise to the different organ and tissue system.

Complete step by step answer: A germ layer is a collection of cells. This layer is generated during animal embryogenesis. Germ layers are only found in vertebrates. However, all animals which are more complex than sponges build two or three primary tissue layers that are called primary germ layers.
The germ layers generate during the process of gastrulation when the hollow ball of cells that construct the blastula and start to differentiate into more-specialized cells that become layered across the developing embryo. The germ layers represent some of the first lineage-specific stem cells in embryonic development. Finally, the germ layer gives rise to certain tissue types in the body.
There are three primary germ layers and they are the endoderm, the ectoderm, and the mesoderm. The endoderm is the inner layer, the ectoderm is the outer layer, and the mesoderm is the middle layer. The endoderm is the innermost of the three germ layers. The cells derived from the endoderm composed several internal linings of the body, including the lining of the lungs, the liver, the pancreas, and certain other organs, like the upper urogenital tract.
On the other hand, the ectoderm, outermost layer which is responsible for the formation of certain outer linings of the body, including the epidermis and hair. The ectoderm also is the precursor to mammary glands and the central and nervous systems.
The mesoderm layer lies between the endoderm and the ectoderm. Cells derived from this layer give rise to all other tissues of the body, including the dermis of the skin, the heart, the muscle system, the bones, and the bone marrow. The higher life-forms are distinguished from lower life-forms on the basis of the mesodermal layer.
Therefore, the correct option is option D.

Note: A germ layer is the collection of cells produced during the process of gastrulation or animal embryogenesis. There are three primary germ layers in all the higher animals and these are the endoderm which is the innermost layer, the ectoderm or the outermost layer, and the mesoderm which is the middle-layer. But in the animals with radial symmetry, like cnidarians, only two layers present called ectoderm and endoderm.