An embryo doesn't have separate organs for respiration, nourishment, and excretion in the same way a developed organism does. Instead, these functions are fulfilled by the placenta in mammals, and by the yolk sac in other amniotic vertebrates like reptiles and birds.

Here's a breakdown:

Placenta (in mammals):

* Respiration: The placenta allows for the exchange of oxygen and carbon dioxide between the embryo's blood and the mother's blood.

* Nourishment: The placenta transports nutrients from the mother's blood to the embryo.

* Excretion: The placenta removes waste products from the embryo's blood, which are then filtered out by the mother's body.

Yolk sac (in amniotic vertebrates):

* Nourishment: The yolk sac contains a nutrient-rich yolk that nourishes the developing embryo.

* Respiration: In some species, the yolk sac may also play a role in gas exchange.

* Excretion: While the yolk sac does not directly handle excretion, it may contain some waste products.

Important Note: The embryo itself does not have a developed respiratory, digestive, or excretory system until later in its development. The placenta and yolk sac act as temporary substitutes for these systems.