This great work, published in 1651, begins by describing the hen’s egg and its development, the doctrine being enunciated that all animals as well as plants are produced from ova. Incidentally, as well as subsequently, observations of great merit and value on reproduction in all kinds of animals are given, and it is clearly shown that instead of containing, from the first, excessively minute but complete animals, eggs at first include extremely simple structures, which by successive and gradual changes come to be like the adults from which they have sprung. It is true that Harvey, with Aristotle, believed that the germs of lower animals could arise out of non-living matter; but it is only in the most recent days that the most elaborate microscopical investigations seem finally to have disposed of this view. The doctrine that the simply constructed germ grows by feeding on non-living matter, converting it into living matter, and gradually transforming it into the form characterising the parent, was a great innovation in Harvey’s age, and it hung fire till Caspar Wolff, in 1759, securely established it. But this has remained till the present century to be made fruitful. Throughout Harvey’s treatise it is evident how greatly the lack of powers such as those of the microscope crippled the entire investigation, although it is truly wonderful how much was accomplished without its aid. Incidental remarks show the acute mind everywhere tending towards sound procedure, as in tying the main artery of a tumour he wished to destroy; arriving on the brink of a discovery even when its full perception did not come, as when in regard to the lungs he says, “Air is given neither for the cooling nor the nutrition of animals,” contrary to the prevailing notion. But the absence of chemical knowledge in that age prevented his going farther. His published works only represent a portion of Harvey’s life-work. We find allusions to his “Medical Observations” and “Medical Anatomy,” which, if written, were probably destroyed in the College of Physicians at the Great Fire. In one place Harvey states that in his medical anatomy he meant, “from the many dissections he had made of the bodies of persons worn out by serious and strange affections, to relate how and in what way the internal organs were changed in their situation, size, structure, figure, consistency, and other sensible qualities, from their natural forms and appearances, such as they are usually described by anatomists, and in what various and remarkable ways they were affected. For even as the dissection of healthy and well-constituted bodies contributes essentially to the advancement of philosophy and sound physiology, so does the inspection of diseased and cachectic subjects powerfully assist philosophical pathology.” Thus it appears that, had we possessed Harvey’s pathological observations, he would also have merited the title of founder of pathology.