4. We have already pointed out the wonderful impulse given to the study of morphology and phytotomy, of embryology and cells about the year 1840; it was shown also that this was due in a great measure to discarding the errors of the nature-philosophy and the idea of vital force, and requiring in the place of such speculations exact observation and systematic induction, and how Schleiden’s ‘Grundzüge’ soon after 1840 vigorously met the demands of the newer time in these respects, but without satisfying them by the positive results obtained. The rapid progress made by phytotomy and the doctrine of cells in the hands of von Mohl and Nägeli proved specially favourable to vegetable physiology, by making it possible to follow the processes of fertilisation in the interior of the ovule. The formation of the pollen-tube from the pollen-grain had been observed long before 1840, and Schleiden in 1837 had proposed the view that the embryo of Phanerogams was formed at the end of the pollen-tube by free cell-formation after it had entered the embryo-sac. But Amici in 1846 and Hofmeister in 1849 showed that this notion was erroneous, and that the germ-primordium is in existence in the embryo-sac before the arrival of the pollen-tube and is excited by it to further development, to the forming the embryo. Similarly Hofmeister’s further observations on the embryology of Vascular Cryptogams and Mosses left no doubt, that the spermatozoids of these groups of plants discovered by Unger and Nägeli serve to fertilise the germ-cell or egg-cell previously formed in the female organ and to excite it to further development (1849, 1851). Soon after the sexual act was observed in various Algae, and these afforded the best opportunity for solving by the aid of the microscope the questions which experiment had still left open. Thuret showed in 1854, how the large egg-cells in species of Fucus are surrounded and fertilised by spermatozoids, and he even succeeded in producing hybrids by fertilising the egg-cells of one species with the spermatozoids of another; but it was still uncertain whether simple contact of the male and female organs was sufficient, or whether fertilisation is due to the mingling of the substance of the spermatozoid and the germ-cell; the question was settled by Pringsheim in 1855; he saw the male organ of fertilisation of a fresh-water alga penetrate into the substance of the egg-cell and be dissolved in it, and this proceeding was afterwards observed in higher Cryptogams and is represented in its simplest form in the sexual act of the Conjugatae, which De Bary described at length in 1858 and like Vaucher regarded as a sexual process.