A heat engine gives greater efficiency when run with a large temperature difference. In the oceans the temperature difference between surface and deep water is greatest in the tropics, although still a modest 20 to 25 Â°C.
A blastocyst is a cellular mass that forms early in the process of embryo development in mammals. In humans, this stage of embryogenesis occurs five days after fertilization, when there are fewer than 100 cells in the mass.
Blastocyst: hollow ball of cells after five days. Trophoblast: forms early embryo, fetal membranes and placenta after five to seven days. Embryo: the developing human from fertilisation to the eighth week of pregnancy. Fetus: the developing human from nine weeks of pregnancy to birth at around 40 weeks. Neonate: newborn baby from birth to 28 days old.