The embryos of several types of animals look very similar and develop in almost the same manner especially in the early stages what do these physical similarities indicate

Embryos of many different kinds of animals: mammals, birds, reptiles, fish, etc. look very similar and it is often difficult to tell them apart. 
This shows that the animals are similar and that they develop similarly, implying that they are related, have common ancestors and that they started out the same, gradually evolving different traits, but that the basic plan for a creature's beginning remains the same.