Embryos are composed of cells that are organized into layers. These layers are called germ layers, and they are the building blocks of all the tissues and organs in the body.

Here's a breakdown of the key components of an embryo:

1. Cells:

* Totipotent cells: These cells are found in the very early embryo and have the potential to develop into any cell type in the body, including the placenta.

* Pluripotent cells: These cells are found in the inner cell mass of the blastocyst and can develop into any cell type in the body, but not the placenta.

* Multipotent cells: These cells are found in later stages of development and have a more limited developmental potential, but can still differentiate into multiple cell types within a specific lineage (e.g., blood stem cells).

2. Germ Layers:

* Ectoderm: The outer layer, which forms the skin, nervous system, and sense organs.

* Mesoderm: The middle layer, which forms the muscles, bones, blood, and circulatory system.

* Endoderm: The inner layer, which forms the digestive system, respiratory system, and various internal organs.

3. Other Components:

* Amniotic sac: A fluid-filled sac that surrounds and protects the developing embryo.

* Yolk sac: A sac that provides nourishment to the embryo in the early stages of development.

* Placenta: An organ that connects the embryo to the mother's body and provides oxygen and nutrients, and removes waste products.

Note: The composition of an embryo changes drastically throughout development. The early embryo is a simple ball of cells, but as development progresses, the cells specialize and organize into complex tissues and organs.