The nuclear genetic factors, or genes, remaining dormant throughout the period of cleavage, begin to manifest themselves during gastrulation, and in ever-increasing measure control the process of development from this stage onward. This does not mean that the process taking place during cleavage are not affected by genes but the effect of the genes is then indirect- it is an action through the mRNA molecules and egg organization which are synthesized during oogenesis by maternal genes.
Various indirect evidences (such as the observation of cleavage without the participation of nuclei and hybridization experiments) have suggested that (i) a nucleus containing maternal and paternal genes, if, removed from the egg, the non-nucleated egg or its fragment continues to divide up to morula stage. This shows that nucleus is necessary for development to go beyond cleavage.
(ii) The paternal genes cause the production of some entirely new kinds of proteins during gastrulation which make the blastomeres biochemically different from each other (chemo-differentiation).
(iii) During gastrulation many lethal mutations make their phenotypic expression into death of the gastrula.
(iv) Hybridization in amphibians often leads to developmental arrest, and actinomycin arrest development in frogs, sea urchins and ascidians.