In 1829 Baer discovered the human egg, and later the chorda dorsalis. In an address delivered in 1834, entitled 'The Most Universal Law of Nature in all Development,' he explained that only from a most superficial point of view can the various species be looked upon as permanent and immutable types; that, on the contrary, they can be nothing but passing stages, or series of stages, of development, which have been evolved by transformation out of common ancestral forms. JOHANNES MUELLER, born at Coblenz in 1801, established himself as _Privat-docent_ at Bonn, where in 1830 he became Professor of Physiology. In 1833 he accepted the Chair of Anatomy and Physiology at Berlin, where he died in 1858. He was one of the most distinguished physiologists and comparative anatomists. By summarizing the labours and discoveries already made in the field of physiology, by reducing them to order, and abstracting the general principles, he became the founder of modern physiology. But he was scarcely less distinguished by his researches in comparative anatomy. His 'Vergleichende Anatomie der Myxinoiden,' in _Abhandlungen der Berliner Akademie_, 1835-45, and 'Ueber die Grenzen der Ganoiden' (_ibid._, 1846), are standard works of lasting value. Mueller exercised a stimulative influence as a teacher. Many well-known men--such as Helmholtz, Gegenbaur, Bruecke the physiologist, Guenther the zoologist, Virchow the pathologist, Koelliker and Haeckel--have been his pupils. RUDOLPH VIRCHOW was born in 1821 at Schievelbein, a small town in Eastern Pomerania. He studied medicine in Berlin as a pupil of Johannes Mueller, and went in 1849 to Würzburg, where, under the influence of Koelliker, and Leydig the pathologist, he laid the foundation of an entirely new branch of medical science--that of 'cellular pathology.' Since 1856 he has filled the principal Chair of Pathology at Berlin. In 1892 he received the Copley medal of the Royal Society. 'His contributions to the study of morbid anatomy have thrown light upon the diseases of every part of the body; but the broad and philosophical view he has taken of the processes of pathology has done more than his most brilliant observations to make the science of disease. 'In pathology, strictly so called, his two great achievements--the detection of the cellular activity which lies at the bottom of all morbid as well as normal physiological processes, and the classification of the important group of new growths on a natural histological basis--have each of them not only made an epoch in medicine, but have also been the occasion of fresh extension of science by other labourers' (Proc. Royal Soc., 1892).