View these stages as they occur in amphibians such as Xenopus laevis, the South African clawed frog. In time, the cells of the embryo differentiate to form the specialized structures and functions that they will have in the adult. They form neurons, blood cells, skin cells, muscle cells, etc., etc. These are organized into tissues, the tissues into organs, the organs into systems. After all the systems are formed, most animals go through a period of growth.
Growth occurs by the formation of new cells and more extracellular matrix.
The genome of the zygote contains all the genes needed to make the hundreds of different types of cells that will make up the complete animal. Dolly is the sheep that was formed by inserting a nucleus from a single cell of an adult sheep into an enucleated sheep egg. She proves that the cell from the adult had lost none of the genes needed to build all the tissues of a sheep. How Dolly was made Spemann's egg-tying experiments. Many years earlier, the German embryologist Hans Spemann demonstrated the same truth.