Embryology

Studies of embryology date back no farther than the day when naturalists firmly overturned the hypothesis that living beings were entirely contained in their primitive germ, that all their transformations consist of growth of its parts, and that organs that are initially invisible gradually become more apparent over time. For many years, this sterile hypothesis, to which the great names of Swammerdam, Malebranche, Leibnitz, Haller, Bonnet, and even Cuvier were attached, resisted all efforts to overturn it. As late as the first half of this century, its partisans were still struggling against the growing evidence. As early as 1652, Harvey had posed the principle of embryology in its basic form when he affirmed that all living beings arise from an egg. In truth, his insight was a simple but ingenious stroke of intuition. The aphorism: ‘Omne vivum ex ovo’ [all life comes from the egg], could be shown to be valid only if one can establish the nature of the egg and find such eggs in all living creatures. Régner de Graaf, who died in 1673, was the first to recognize the eggs of mammals in the fallopian tubes of the womb and to find the part of the ovary where the egg is formed, but he did not identify the actual presence there of the egg itself. It was not until a hundred and fifty years later, that von Baer established that it is precisely in the Graafian follicle that the eggs of mammals are created, but the parts of this egg were not satisfactorily correlated with those of the eggs of birds until Coste did so in 1834.