An embryo contains the basic building blocks of a future organism. It's essentially a collection of cells that have begun to differentiate and organize themselves into specific tissues and organs. Here's a breakdown of what's inside:

1. Cells:

* Embryonic Stem Cells: These are undifferentiated cells with the potential to become any type of cell in the body. They are the foundation for all tissues and organs.

* Germ Layers: The embryo develops three primary germ layers:

* Ectoderm: Forms the outer layer, giving rise to skin, nervous system, and sensory organs.

* Mesoderm: Develops into the middle layer, forming muscle, bone, blood, and connective tissue.

* Endoderm: Creates the inner layer, leading to the digestive system, respiratory system, and certain glands.

2. Structures:

* Amnion: A membrane that encloses the embryo in a fluid-filled sac, providing protection and cushioning.

* Yolk Sac: Provides nutrients to the developing embryo, especially in birds and reptiles.

* Allantois: In mammals, it helps with waste removal and becomes part of the umbilical cord.

* Chorion: The outer membrane that helps with gas exchange and, in mammals, becomes part of the placenta.

3. Genetic Material:

* DNA: The embryo contains the genetic blueprint inherited from both parents, which determines the organism's traits.

Important Note: The specific contents and developmental stages of an embryo vary depending on the species.

Let me know if you have any more questions about a specific aspect of embryo development!