What is the Haeckel-Müller law and why is it only partially manifested?

A fetus does not immediately become like a little man. There is an opinion that at first it resembles a fish, then a lizard, finally a man. German scientists Haeckel and Müller even formulated the law: in individual development, the body repeats the history of the development of its species. This is not so. Indeed, at a certain stage, the human fetus has gill arches, the same as those of fish larvae, there is a period when the fetus, like a reptile, has a tail, which subsequently disappears. But it is difficult to name a stage of the embryo or fetus at which it would look like a fish or a reptile, because the development of organs proceeds at different rates

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