The problem of embryonic development

On the basis of these observations, Aristotle concluded that in a developing embryo organs not only increase in size, as Hippocrates had said, but also in number. Embryonic development, according to Aristotle, is an epigenesis, a chain of one genesis after another, where new structures and new functions appear at various steps. During embryonic development, in short, the complexity of the system increases. 

Almost 2000 years later, around 1660, Marcello Malpighi repeated Aristotle s experiment, but with an important difference. He was the first man to watch a developing embryo under a microscope, and what he saw led him to a very different conclusion. The area where blood vessels are destined to appear, for example, is apparently empty to the naked eye, but under the microscope is full of capillaries. Aristotle had concluded that blood vessels appear ex novo, but according to Malpighi he had been betrayed by his own eyes. Could he have used a microscope, he would have realised that organic structures are present even when they are not yet visible. Malpighi therefore reached the conclusion that an embryo s development is 

With the advent of the cell theory, embryonic growth was immediately accounted for by a sequence of cell divisions. A fertilised egg becomes 2 cells, and then 4, 8, 16, 32, 64 and so on. With 10 divisions the cell number is about a thousand, with 20 is a million, with 30 is a billion, with 40 is a thousand billion, and so forth. For the 

The discoveries of cell growth, histological differentiation and morphogenesis, in short, gave a precise answer to the problem of embryonic development in cellular terms. Embryonic development is a true epigenesis and consists of three fundamental processes growth, differentiation and morphogenesis. 

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At the very centre of biology there are two complementary problems How does an organism produce an egg (the problem of generation), and How does an egg produce an organism (the problem of embryonic development). These questions have been debated since... 

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