in the same direction. The third member of the group, made professor in 1849, was Rudolf Virchow, a young teacher then in his best years. It was Virchow who did most to bring practical medicine into line with histology. As the vital processes in the human body seemed to him, with his strict histological outlook, to be traced back always to the tissue-building cells, he concluded that disease also, or the pathological condition of the body, and therefore the proper field of the medical man, was a process in these cells. Man seemed to him to be a “cell-state”: the tissues were the various active social strata in this state: and disease was, in its ultimate source, a conflict in the state between the citizens, the tissue-forming cells, that normally divide the work amongst them for the common good. Pathology must be cellular pathology. The science was already being taught by Virchow at Würtzburg, and the dry bones of it were covered with flesh for his hearers. But his ideas were not published until a few years afterwards (1858). In the first three terms Haeckel studied chiefly under Kölliker and Leydig. They taught him animal and human embryology, as it was then conceived. Embryology was the science of the development of the individual animal or man, the description of the series of changes that the chick passes through in the egg or the human embryo in the womb. This science, also, had been profoundly affected by the invention of the microscope. Firstly, the spermatozoa, the active, microscopically small particles in the animal and human sperm, had been discovered. Then, in the twenties, Karl Ernst von Baer had discovered the human ovum. The relation of these things to the cell-theory was clear. It was indubitable that each of these male spermatozoa and each female ovum was a cell. They melted together and were blended into a new cell in the act of procreation, and from this, by a process of repeated cleavage of cells, the new individual was developed with all his millions of cells and all the elaborate tissues that these cells united to form. A whole world of marvellous features came to light, but the key to the unriddling of them was still wanting.