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.
The blastocyst consists of an outer layer (trophoblast) which is attached to the inner cell mast. A pre-implantation embryo consisting of a thin-walled hollow sphere of 16â40 cells, which appears in early mammalian development, the wall of which is the trophoblast.