Cleavage this stage happens in the oviduct, with the first division being completed by 30 hrs. Cleavage is mitotic divisions to increase cell number without an increase in size, as opposed to growth which involves cell divisions with an increase in size. The developing embryo moves down the oviduct towards the uterus. Implantation 3-4 days after fertilization the morula reaches the uterus where implantation will occur. Before implantation the round ball of cells forms a two-layered blastocyst.
Cell fate is determined by placement in the embryo .
HCG is made to contain the corpus luteum and the pregnancy, so the corpus luteum will continue to make progesterone and estrogen, and the uterine lining will continue to thicken. Gastrulation occurs during the second week of development- by day 7, the embryo is partially embedded in the uterine wall. These layers have developmental fates as outlined in p. 308 of the text. Induction is normally the result of an individual cell's placement within the developing embryo V. Organ Development and Development of the Placenta by the third week of development the heart and nervous system begin to form by this time the placenta has begun to form. Nutrients travel from the mother to the developing child, but blood never passes.