Roux in 1888 performed an experiment which may be considering as the beginning of the experimental embryology. He took a frog egg at the two cells, egg of cleavage and destroy one of the blastomere of the frog with red hot niddle. The embryos that was derived from the surviving blastomere was defective in many respects that is the surving blastomere devlope into a half embryo.
So Roux concluded that certain areas of the eggs are already destined in the ovary to develop into special region of the body. The pigment cytoplasm of frogs egg develop into the head region into the future animals. The yolky cytoplasm of the vegetal pole of the egg develop into future blastopore. During cleavage these predestined region are segregate into blastomere that develop into specific tissue & organs. Before fertilization the egg is radily symmetrical but after fertilization the egg is radily symmetrical as cleavage continues. The blastomere gradually become smaller & smaller and more areas for specific tissue and organs and demarketed in the blastula, as a result finally it comes to resemble a mosaic structure with regards potentialities of its blastomere .
Thus Roux prove that the Preformation theory is correct the mosaic theory state that every part of the egg is predetermined to develope into a particular organ.