|MadSci Network: Development|
Hi Leonie Quadruplets can arise in the same way that twins form, either from fertilization of four eggs or one embryo splitting twice. Obviously to have four ova present at one time is particularly rare and the most common cause of quads is a double split. Of course it is also possible for 3 ova to be fertilized and then one splits to give a total of 4, giving two identical and 2 non-identical babies; but again this would be quite rare. It is not fully understood why an embryo splits but it occurs when the casing of the embryo (the Zona Pellucida) splits with the first cellular (bastomere) split. Normally these blastomeres split repeatedly within the confines of the zona to form one individual. It is really an intact zona pellucida which denotes a singular embryo. Eventually the cells fill the zona so much that it hatches and releases the blastocyst. At this point the cells are so organized (differentiated) that only one fetus can arise from them. Hope this answers your question and keep asking, Steve
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