After fertilization, the zygote divides by mitosis into a number of smaller cells called blastomeres. This process of division, known as cleavage, is in a sense the opposite to the process of oogenesis: cleavage is a period of intense DNA replication and cell division in the absence of growth, whereas oogenesis is a period of growth without replication or division. Early cleavages are often synchronous, but sooner or later synchrony is lost. The blastomeres become organized in layers or groups, each group having a characteristic rate of cleavage. Although cleavage may be considered a mitotic process as found in adult somatic tissues, there is one important difference: in adult tissue the daughter cells grow following each division and are not able to divide again until they have achieved the original size of the parent cell. The cells in a somatic population thus maintain an average size. During cleavage this is not the case: with each division the resulting blastomeres are approximately half the size of the parent blastomere. As cleavage progresses, the embryo polarizes and differences arise between the blastomeres. Such differences may result from the unequal distribution of cytoplasmic components as already laid down in the oocyte during oogenesis, or from changes occurring in the blastomeres as a result of new embryonic gene transcription during development. Each blastomere nucleus will be subjected to a different cytoplasmic environment, which, in turn, may differentially influence the genome activity.