The first known study of comparative developmental anatomy was undertaken by Aristotle in the fourth century b.c.e. He noted the different ways that animals are born: from eggs (oviparity, as in birds, frogs, and most invertebrates), by live birth (viviparity, as in eutherian mammals), or by producing an egg that hatches inside the body (ovoviviparity, as in certain reptiles and sharks). Aristotle also identified the two major cell division patterns by which embryos are formed: the holoblastic pattern of cleavage (in which the entire egg is divided into smaller cells, as it is in frogs and mammals) and the meroblastic pattern of cleavage (as in chicks, wherein only part of the egg is destined to become the embryo, while the other portion—the yolk—serves as nutrition). And should anyone want to know who first figured out the functions of the placenta and the umbilical cord, it was Aristotle.
After Aristotle, there was remarkably little progress in embryology for the next two thousand years. It was only in 1651 that William Harvey concluded that all animals—even mammals—originate from eggs. Ex ovo omnia (“All from the egg”) was the motto on the frontispiece of his On the Generation of Living Creatures, and this precluded the spontaneous generation of animals from mud or excrement. This statement was not made lightly, for Harvey knew that it went against the views of Aristotle, whom Harvey still venerated. (Aristotle had thought that menstrual fluid formed the material of the embryo, while the semen acted to give it form and animation.) Harvey also was the first to see the blastoderm of the chick embryo—that small region of the egg that contains the yolk-free cytoplasm that gives rise to the embryo—and he was the first to notice that “islands” of blood cells form before the heart does. Harvey also suggested that the amnionic fluid might function as a shock absorber for the embryo.
