Mechanism of Cleavage
Such as the mitotic division in any cell, cleavage is the result of two events: mitotic nuclear division (Karyokinesis) followed via cytoplasmic division (Cytokines).The two events include numerous metabolic processes. The egg cytoplasm consists of all the needs for these processes before the egg leaves the ovary. They are synthesized and stored in its cytoplasm throughout oogenesis as a result of the activity of maternal genes present in the oocyte. Fertilization by sperm activates the metabolic procedures in the egg cytoplasm and initiates cleavage according to the progamme already set through the maternal genes during oogenesis.
There is much more evidence to prove that cleavage is guided by the genetic information received by the egg cytoplasm from the mother during oogenesis. There is little or no transcriptional activity in the zygotic nucleus during early cleavage. Hence, the effects of paternal genes that come into the egg along with sperm nucleus are manifested only later. The regulative factors for such type of biphasic segmentation are said to lie in the egg cytoplasm itself.