- Step 1: contact of the sperm and egg. This results in the binucleate (two nuclei) zygote.
- Step 2: cleavage, or the first cell division.
- Step 3: the blastomeres (identical results of step 2) split every 18 hours or so. At the eight-cell stage, it is apparently safe to remove one cell for chromosomal analysis.
- Step 4: the cells, now numbering 16, start to specialize.
- Step 5: the now-four-day-old cells differentiate. Some form a pre-placenta (here's an interesting tidbit about that) and the total embryo is called a blastocyst.
- Step 6: the now-120-cell blastocyst implants itself, becoming properly attached. More specialization happens.
- Step 7: the precursors of organs form in gastrulation. At this step, there is no more possibility of twins forming.
- Animal husbandry scientists treat bovine embryos (for example) in ways still prohibited for humans, thankfully. One of these uses is cloning, often resulting in one or more abnormally large calves.
- "A year after Dolly's [the famous sheep] birth, several more cloned farm animals were in existence, and Dolly herself was pregnant. But the yield of healthy cloned animals was still, on balance, minuscule." Perhaps that had something to do with this?